Observing the Night Sky During an Airplane Flight

During a flight back home from a recent business trip, I was lucky to get a window seat. I was thinking of the possibility to see some stars of the night sky above the clouds and I wasn’t disappointed. The flight was held entirely during night time with cabin lights turned off. This was crucial because cabin lights are an additional light source which make unwanted reflections visible. The reflections usually overcast faint light sources such as stars. I took some photographs with my smartphone Samsung Galaxy S22 at exposure duration of 20-30 seconds and ISO 3200. The exact location of the photograph is unknown, it must have been somewhere in a part of south-eastern Algeria over the Sahara desert while looking to the east. I would estimate the position as follows: 23° North, 8° East, flight attitude 11 km (36 000 ft).

Result of a shaky picture during long exposure duration (20 – 30 seconds). The stars appear “smeared”

I made about a dozen pictures of the night sky but due to minor turbulence and slight flight course corrections, most of the images were shaky and had to be discarded. However, I was able to get 1-2 pretty decent pictures of the night sky which turned out to be OK. I was pressing my smartphone against the window to avoid shaking motion and put a pillow over it to block most of the residual (emergency) cabin lights.

Beautiful night sky picture taken with a smartphone from an airplane window. 20 second exposure at ISO 3200

The taken pictures show some prominent stars from the constellations Cygnus and Lyra: Vega, Deneb, Albireo and others. I was able to identify some stars by using an web-based star map application such as Stellarium Web. During the flight I could observe meteor flashes occasionally (no picture taken though) and lightning of distant thunderstorms, which were perhaps >200 km away. There were virtually no city lights visible during the flight over Sahara desert. Just imagine being there and watching the night sky without any light pollution

Identified stars from the constellations Cygnus and Lyra
Stellarium Web

The 8-hour-flight was very tiring because of lack of space to move and stretch. Sitting for hours in an uncomfortable position wasn’t very pleasant but it worked out somehiw. However, I was able to catch some nice pictures of the world 11 km below my feet. #worthit

Tektronix 7104 – 1 GHz analog oscilloscope

I’m really happy of being a new owner of a Tektronix 7104 oscilloscope – one of the fastest analog oscilloscopes made by man.

Tektronix 7104
Tektronix 7104, 1 GHz analog oscilloscope
Last time I saw one of those on eBay, I was hesitating to buy it and it went away for dirt cheap. Anyways, I didn’t want to miss a chance this time. The unit is working and in a very good condition. The horizontal and vertical plug-ins delivered with the scope are meant for the full bandwidth of 1 GHz.

The Microchannel Plate CRT display is still in a good shape – no burn-ins except the typical wear-out areas (horizontal trace, annotation areas).

Measured rise time of Tektronix 7104 with Leo Bodnar pulse generator on a 7A29 single channel vertical amplifier plugin: approx. 300 ps
Sine wave at 1.0 GHz, approx. -13 dBm into 50 Ohm

Bandwidth test with Leo Bodnar pulser on one of the 7A29 plug-ins gave me a rise time of 300 ps with an estimated bandwidth of approx. 1.16 GHz. This is my fastest oscilloscope now. I’ll check the innards in a couple of weeks.

Voltnuts will hate this: Improve your DMM Resolution With One Simple Trick…

Measuring voltages accurately is a basic task for technicians, engineers and scientists. The voltages to be measured range from perhaps few picovolts to several megavolts – a dynamic range of 18 orders of magnitude! But every modern digital multimeter is kinda limited in resolution. The multimeter’s resolution can be stated in the number of digits it can resolve. A 6 digit multimeter can resolve six decimal places going from 0 to 9. In a decimal number system, this corresponds to \(10^6\) numbers or 1 million digitizing steps. Without going much into detail and exposing my limited knowledge on this matter, I’ll just link to Keysight’s Website where everything is well explained.

Older multimeter models such as the shown MeraTronik V543 can resolve only \(2 \cdot 10^4\) numbers in the selected measurement range (e. g. ±1 V = 2 V full scale:  \(2~V/2 \cdot 10^4 \rightarrow 100~µV\) resolution).

MeraTronik Type V543 (PRL T-189), 4.5 digit multimeter with Nixie tube display

One of the best and most accurate digital multimeters in present time – the HP 3458A – has “only” a resolution of 8.5 digits, which hasn’t improved for about 35 years.

HP/Agilent/Keysight 3458A 8.5 digit multimeter

The so-called “Voltnuts” (crazy electro-fanatics) buy those HP multimeters on a second hand market for $3k to $7k. This surely is crazy and overpriced and in my opinion just not worth it. How about buying a couple of cheaper 6.5 digit multimeters (e. g. HP 34401A for about $200 to $400) and combining them in a serial configuration? I’ve achieved a total of 19.5 digit resolution this way*. I was able to display 10 volts up to 18 decimal places, e. g. attovolts resolution and saved a lot of money.

Two HP/Agilent/Keysight 34401A 6.5 digit multimeters in an unusual configuration showing exactly 10 Volts on April 1st…

*If you don’t believe this nonsense, it’s fine. I can live with that. It’s April Fools’ day anyway 😉

Tauntek LogICTester – A TTL IC Tester

Introduction

About a year ago I saw one of CuriousMarc’s YouTube videos on repairing a Xerox Alto computer. CuriousMarc published shortly afterwards an episode, which compared the Tauntek LogICTester with the TL866II+ EEPROM programmer which has some transistor-transistor logic (TTL) testing capabilities. 1970’s and 1980’s era test equipment used alot of those intergrated circuits (IC) in order to operate. Servicing or repair of older test equipment like oscilloscopes, multifunction calibrators, power supplies etc. will be much easier with tools such as a logic tester. After reading a very positive review from a friend at the Wellenkino, I thought it would be very nice to own such a tester for servicing or repairing my (broken) equipment.

Tauntek LogIC Tester

I contacted Bob Grieb of Tauntek and ordered one of his kits back in March 2022. The $35 kit contained the printed circuit board and two pre-programmed PIC microcontrollers. The shipment to Germany cost me about $19 with additional 10 EUR for customs and fees. The total cost for the kit ans shipment was in the order of $64 or approx. 60 EUR. The shipment from USA to Germany took about week and a half.

20220403_114751.jpg
Shipment received from Bob Grieb of Tauntek.

After receiving Bob’s package, the project got delayed because I’m always kinda busy. I ordered most of the remaining parts via Mouser in December 2022. The ordered parts would cost additional 69 EUR. Many of the components on the bill of materials (BOM) list could have been easily skipped (e. g. resistors and 2x PIC microcontrollers, RS232 level shifter) resulting in 45…48 EUR price range. Nevertheless, I ordered few extra parts just to be sure if anything fails. Extra parts can be always used for different projects or experimenting with other circuits.

Two of needed parts on the BOM weren’t available at the time: the 74HC138 3-to-8 line decoder and LM358AN dual opamp. According to Mouser, the lead times for the decoder were four weeks and for the opamp approx. 1 year (which has been reduced to May 2023 in the meanwhile). So yeah, I had to order both parts on eBay instead. Waiting a year for an opamp wasn’t acceptable. I also ordered an USB to TTL FTDI Serial Adapter on Amazon (2 pieces for 8 EUR) so I could use the Tauntek LogICTester via USB instead of RS232. Everything arrived last week so I had to find few “quiet hours” for the soldering job.

Building the IC Tester

One starts with the printed circuit board and two PIC microcontrollers (U1 as Master and U2 as Slave) as shown in the picture below.

The unboxed Tauntek LogICTester.

I started populating the resistors and diodes in the first place. After soldering and cutting excess wires, I moved on to capacitors.

20230305_125434.jpg
Starting the population with passive components is always a good idea….

20230305_140448.jpg
Population continues, only few more parts…

20230305_173457.jpg
Chaos unfolds when doing a soldering project…

After hours and hours, I finally got it finished. The pins in the top right corner needed to be removed. Also JP1 and JP2 (top right above the IC socket) were shorted by a solder blob in order to use the FTDI to USB adapter instead of the RS232 level shifter. The level shifter 16 pin socket on the top right remains unpopulated. The final assembly can be seen in the picture below.

20230305_202548.jpg

After assembly I checked for cold joints or shorts. Everything looked fine and I turned it on.

20230305_184215.jpg
Tauntek LogIC Tester: finally assembled and ready for IC testing.

After plugging in the USB cable of the FTDI adapter into the Windows 10 machine, the driver installation started automatically. No additional drivers were needed to be installed. The settings for communication via Serial connection were by default “8-N-1”, which corresponds to 9600 baud, 8 data bits, no parity bit, 1 stop bit and no flow control. The connection needs to be established via a terminal emulator such as PuTTY. After establishing a connecting to the tester, one has to press ENTER to get the welcome screen and voilà, We’re In Like Flynn.

The usage of the IC tester is pretty much straight-forward. Power up the tester, establish a PC connection and put the IC into the socket as shown in the picture above. Enter the IC model inside of the console.  If the IC is found inside of the database, type t and press enter to start the IC test. The results are presented in a very comprehensive way. Typing v and pressing enter displays the pin voltages.

Tauntek LogICTester menu via terminal emulator PuTTY.

That’s it – there isn’t much more to it. Very simple and effective when it comes to debugging the 1970s to 1980s era logic ICs. Few additional hints and comments:

  • Unfortunately the IC tester can not be used in order to recognize part numbers of ICs under test. The part numbers have to be looked up manually inside of the terminal emulator (or Bob’s list)
  • Bob maintains the firmware and the list of supported ICs. Perhaps by writing him an email and by sending him the missing or unsupported IC, Bob may update his firmware in the future and add support for the missing IC
  • The logic tester certainly isn’t free of bugs – some tests may result in a “FAIL” although the IC is still fine according to the datasheet specifications. Please, don’t blindly trust the machine. Take your time and try to sanity-check or interpret the results
  • The logic tester can’t check memory chips for faulty memory cells (such as Retro Chip Tester Professional from the 8Bit-Museum.de)
  • I’ve added a picture gallery to my Piwigo website if you want to check out some additional pictures

Have a nice Sunday and happy IC testing!

Having Fun with 75 Ohm Technology

If one wants to capture analog images or analog videos (e. g. images coming from a CCTV camera), those signals need to be digitized, decoded and displayed with a device called framegrabber. A framegrabber is basically a video capture card with fast video processing capabilities and onboard memory which is useful for image processing.

I wrote in my previous post how to make good quality screenshots of the Anritsu MS2661N front display. However, there is another ancient method how to acquire images of frequency spectra. In modern test equipment, the video signals are transmitted digitally via DisplayPort or HDMI. Some 20 to 40 years ago, the manufacturers offered instruments with an optional analog video output which could be connected to a television screen or to a video recorder for viewing or documentation purposes.

75 Ohm composite output can be seen on the back side of the Anritsu MS2661N spectrum analyzer

I’ve got myself a framegrabber card NI PCI-1405 for different projects. I wanted to test the card for its functionality. The quickest access to an analog video was via the composite output of my spectrum analyzer as shown in the previous picture.

NI PCI-1405 Video Framegrabber Card

The NI PCI-1405 framegrabber card has two inputs: a 75 Ohm video input and a 75 Ohm trigger. The video input is connected via a 75 Ohm cable (type RG-59) to the composite output of the spectrum analyzer.

The 75 Ohm coaxial cable (RG-59) needs to be connected to the video input of the framegrabber card

After installing the drivers from National Instruments, the framegrabber card was successfully detected and I was able to grab some frames inside of NI MAX (Measurement & Automation Explorer).

Framegrabber user interface inside of NI MAX

I didn’t bother to improve the image quality – this task will be relevant for future projects as soon as I get my Tektronix C1001 camera running 🙂  So far, it’s been a successful test!

Grabbed frame in a 640×480 resolution. The analog source signal was received in greyscale NTSC format at approx. 30 frames per second

Tektronix 2456B with attached Tektronix C1001 video camera

Anritsu MS2661N Spectrum Analyzer Readout

Introduction

A spectrum analyzer (SA) is a very useful tool when it comes to measure spectra of radio frequency signals. I recently acquired a 2004 era spectrum analyzer. It’s from a Japanese test equipment manufacturer Anritsu and the model number is MS2661N. Luckily there are operating manuals available online but I wasn’t able to find service manuals for this type of spectrum analyzer on the internet. There are some service manuals available for similar models of spectrum analyzers (e. g. MS2650/MS2660) which would allow troubleshooting but I would be lost if the instrument breaks.

Anritsu MS2661N Spectrum Analyzer (100 Hz – 3 GHz). Those blue handles totally aren’t butchered from Rohde & Schwarz test equipment… Sacrilege! Don’t ask!

However, I’ve been looking for a decent SA for a longer time and stumbled upon the Anritsu MS2661N. It had a bunch of very nice and useful features: frequency range 100 Hz to 3 GHz, 30 Hz resolution and video bandwidth, oven controlled crystal oscillator (OCXO), GPIB interface, 10 MHz reference IN/OUT and a tracking generator ranging from 9 kHz to 3 GHz. I was looking for a similar SA from HP/Agilent 8590 Series or Tektronix but there were no attractive offers at the time. Either the SA frequency range was too low for modern ages (1 GHz) or outside of my measurement capabilities (26 GHz), the price was either too high or it was partially broken. There were also 75 Ohm spectrum analyzers which aren’t very useful for what I’m doing. On the other side, the documentation for HP/Tek hardware is the real deal so leaving this kind of test equipment ecosystems was a tough decision.

Long story short: I wasn’t disappointed and the SA works perfectly fine. I don’t want to write a lengthily blog about it. One of the first experiments was connecting my GPS disciplined oscillator to the signal generator and spectrum analyzer simultaneously in order to provide the same external reference for both instruments and checking if the frequency (1.5 GHz) and the amplitude  (-35 dBm) are accurate. Acquiring measurements was super easy and the operation of the SA is very straight-forward.

Agilent E4432B Signal Generator. Note that the EXT REF is on and the output signal is referenced to a 10 MHz GPS disciplined oscillator.

10 MHz reference signal distribution from a GPS Disciplined Oscillator (GPSDO).

Back side of the Anritsu MS2661N. The 10 MHz signal is fed into the REF IN.

Documentation of Measurements

I would consider the somewhat cumbersome recording of readings as a minor disadvantage of this SA. Taking a photograph of the display may be “quick and dirty” but you have to deal with bad image quality due to reflections, visible RGB pixels and picture alignment. It is possible to take screenshots in bitmap format (BMP) but one needs a special type of a Memory Card (basically a PCMCIA or PC Card) in order to save the screenshots on an external storage. That’s really unfortunate but measuring instruments of that era were either equipped by a floppy disk or Memory Carc. I was always afraid of damaging the fragile pins while pushing the PCMCIA card in its slot although it is rated for 10k mating cycles. The MS2661N type SA even has a 75 Ohm composite out – it’s possible to record video stills in the NTSC format. However, there are two elegant methods which I would like to show how to transfer the readout from the instrument to the personal computer (PC) by modern means.

A photograph taken of the frequency spectrum. The image shows LCD pixels, scratches on the surface of the front panel and reflections due to bad light conditions.

Method 1: Sending a Hard Copy from SA to a PC

Back in the days, the measurement results such as frequency spectra would be printed on a piece of paper as a part of the documentation. A device called printer or plotter was needed and the process was called “hard copy”. The difference between a printer and plotter is how the drawing is generated: while the printer generates text and images line by line, a plotter can draw vectors in a X-Y-coordinate system. HP developed its own printer control language back in 1977 for this purpose – the HP-GL or Hewlett-Packard Graphics Language. HP-GL consists of a set of commands like PU (pen up), PD (pen down), PAxx,yy (plot absoute) and PRxx,yy (plot relative) in order to control a plotter, which is basically an electro-mechanically actuated pen. The commands are transmitted in plain ASCII via GPIB or RS-232C interfaces. If we were somehow able to capture the HP-GL ASCII code, it should be possible to generate a lossless vector graphics instead of a lossy bitmap.

An example of the acquired HP-GL code in a text editor.

Hardware Requirements

Besides the already mentioned spectrum analyzer one needs either a GPIB/USB or GPIB/Ethernet adapter. I have tested it successfully with a National Instruments GPIB-ENET/100 on a Windows 10 machine with NI 488.2 v17.6 drivers. It should also work with a NI GPIB-USB-HS+ (Chinese clone) adapter.

Software Requirements

I was looking for a quick solution how to acquire hard copies. Thanks to einball on a certain Discord channel 😉 for showing me the KE5FX 7470.EXE HP-GL/2 Plotter Emulator. John Miles, KE5FX, already wrote a software back in 2001 which does emulate a HP 7470A pen plotter. The 7470.EXE is still maintained by John and supports popular spectrum analyzers from HP and Tektronix. His software is able to fetch the HP-GL ASCII via GPIB and render the hard copy image on the screen. The image may be saved in a bitmap format (BMP, TIFF, GIF) or in a vector format (PLT, HGL). I have tested John’s software with Anritsu MS2661N and it worked perfectly fine. I suppose this could work on similar Anritsu spectrum analyzer models, such as MS2661C.

Setting up the Spectrum Analyzer

Here is a brief summary how to obtain a hard copy from an Anritsu MS2661N spectrum analyzer:

  • Connect the spectrum analyzer to the GPIB adapter and boot up the device
  • Go to the Interface menu and use settings as followed → GPIB My Address: 1, Connect to Controller: NONE, Connect to Prt/Plt: GPIB, Connect to Peripheral: NONE
    The SA wants to send its data via GPIB to a plotter. It’s important to disable the “Connect to Controller” option, otherwise it won’t be possible to select GPIB as “Connect to Prt/Plt”. The GPIB address is set arbitrarily to 1
  • Go to Copy Cont menu (Page 1) → Select Plotter
  • Copy Cont menu (Page 2) → Plotter Setup → Select following options: HP-GL, Paper Size: A4 Full Size, Location: Auto, Item: AllPlotter Address: 2
    It’s important to set the “Plotter Address” value to a different number than the “GPIB My Address“. If both addresses share the same number, the hard copy will result in a timeout error

Install John’s 7470.EXE software and start the HP 7470A Emulator. There is no need to change the settings of the GPIB controller, it works out of the box. Click on GPIBPlotter addressable at 2. The selected address in 7470.EXE should be identical as the previously set Plotter Address. In order to obtain a hard copy, press the button w and the 7470.EXE should display a message like shown in the screenshot below. Once you press the Copy button on the spectrum analyzer, a data transfer progress should be visible. It takes about 10-15 seconds to transfer the data (approx 7-10 kb) from the SA to the PC. Once it’s complete, an image of the current frequency spectrum is shown on the display. That’s it.

Creating Publication-Quality Vector Images

At this point, it’s possible to save the acquired hard copy in a bitmap image format. If one needs a publication quality images – which should be free of compression artifacts – one should save the images in a vector format such as PLT/HPGL. This workflow proved to be a little bit inconvenient but it’s perfectly doable. Save the hard copy as .PLT and open the image in a HP-GL supported viewer. John suggests few of them on his website – I’ve tried CERN’s HP-GL viewer. It’s distributed free of charge and still maintained by the developers. Download their viewer and load the PLT-image. If the colors seem wrong, there is a setting where you can change the pen colors. Once done, it’s possible to export the PLT image as PostScript (PS) or Encapsulated PostScript (EPS) or print as PDF. EPS files can be embedded in LaTeX documents or can be imported in a vector graphics editor such as Inkscape.

The results turned out to be really good. Especially the vector images are crisp and sharp. One can zoom in without any image quality losses. The printouts on my laser printer are perfect. A little downside would be few breaks in the workflow: one has to use three different applications in order to obtain, convert and process the images. But it’s worth it 😉

Method 2: Readout Data via pyvisa and Plot it via Matplotlib

A different method to plot the frequency spectra would be by downloading the acquired raw data via GPIB and plot it directly. This is exactly what I’ve done. I’ll share the Python code down below. It’s possible to refine the plot by automating more stuff: one can generate annotations directly from queried instrument settings. Just put enough time in it and you’ll get superb results. The plotted image can be saved directly in a Scalable Vector Graphics (SVG) or any supported bitmap/compressed format.

Spectrum analyzer data plotted via Python’s library Matplotlib

# -*- coding: utf-8 -*-
"""
Created on Tue Jan  3 16:45:39 2023

@author: DH7DN
"""
import numpy as np
import pyvisa
import pandas as pd
import matplotlib.pyplot as plt

#%% Open the pyvisa Resource Manager
rm = pyvisa.ResourceManager()
print(rm.list_resources())

#%% Create the Spectrum Analyzer object for Anritsu MS2661N at GPIB address 13
sa = rm.open_resource('GPIB0::13::INSTR')

# Print the *IDN? query
print(sa.query('*IDN?'))

#%% Take a measurement
# Set frequency mode to CENTER-SPAN
sa.write('FRQ 0')

# Set the center frequency in Hz
cf = 1.5E9
sa.write('CF ' + str(cf) + ' HZ')

# Set span in Hz
span = 10000
sa.write('SP ' + str(span) + ' HZ')

# Take a frequency sweep (TS)
sa.write('TS')

# Select ASCII DATA with 'BIN 0' according to Programming Manual
print(sa.write('BIN 0'))

# Create a Python pandas Series
data = pd.Series([], dtype=object)

# Fetch data, convert string to float, print the power level values
for i in np.arange(501):
    data[i] = float(sa.query('XMA? ' + str(i) + ',1')) * 0.01
    print(data[i])

#%% Plot the results
# Generate the frequency values for the x-axis
f = np.linspace(cf-span/2, cf+span/2, 501)

# Plot the results, set a title and label the axes
plt.plot(f, data)
plt.xlabel('f in Hz')
plt.ylabel('Power Level in dBm')
plt.title('CF: 1.5 GHz, Span: 10.0 kHz, RBW: 100 Hz, VBW: 100 Hz, \n Peak at 1.5 GHz and -35.85 dBm')
plt.grid(axis='both')
plt.minorticks_on()
plt.show()

Few things to consider when using Python to obtain data from the spectrum analyzer:

  • Anritsu MS2661N acquires only 501 data points per sweep
  • The frequency axis values need to be generated manually. I used numpy‘s linspace method. It was a bit tricky because you one has to change the generation of frequency step values depending on whether parameters “Center Frequency & Span” or “Start/Stop Frequency” are used
  • Fetching the data takes quite some time (approx. 30 seconds). This is due to the fact that every single data point needs to be queried with the XMA? command in a for-loop. This is at least how it’s done in an example from Anritsu’s Programming Manual. I haven’t figured out yet how to fetch a block of data

Summary and Conclusion

I was clearly impressed how easy it was to obtain good quality frequency spectra images from a 20 year old instrument. I’ll refine the workflows and do further testing in Python. It should be possible to do all of this “automagically” via one little Python script. So far I’m really happy with the results where I don’t have to rely on smartphone pictures anymore. Thanks to einball for his help (basically googling for me) and to John (KE5FX) for writing his plotter emulator which helped me a lot to obtain hard copies from my SA. That’s it for today! Happy measurements! 😉

73 de Denis, DH7DN

Happy New Year 2023

Hi everyone,

let’s just forget 2022 and move on. Covid, inflation, wars and conflicts – it doesn’t get better. Nevertheless, I had many positive moments in 2022, too! I had some lucky test equipment acquisitions, I managed to sell unused test equipment and free up some space, did some Python and GPIB programming, did a bike tour, met lots of nice and interesting people throughout the year, finally published some decent blog posts and successfully reduced my weight from 132.9 kg (290 lbs) to 115.5 kg (254 lbs) in 365 days.

There is a lot more to do in 2023 and I’ll try to keep up with publishing interesting blog posts. My new year resolution in 2023? No new resolutions! They are useless – at least for me.  I have a bunch of interesting projects on my list and I hope they will happen some day… perhaps this year. If you take on too much, you will end up doing nothing at all.

Happy New Year 2023, stay positive, safe and sound 🙂

73 de DH7DN

Doesn’t always go as intended…

Calibration of a Piezoelectric Accelerometer by Comparison to a Reference Transducer

Introduction

Today’s modern technology is full of sensors. Sensing temperature, motion, force, humidity, sound, electricity, radiation – basically every imaginable physical quantity – necessary to measure and understand our environment. This is usually done with so-called transducers (sometimes abbreviated as Xducers) which convert the measured physical quantity (e. g. temperature or any kind of a signal) into a different physical quantity (e. g. resistance, voltage) or any other type of signal. In many cases, the transducer converts the measured physical quantity into an electrical signal which is used as an input to a digital voltmeter or an analog-to-digital converter. The conversion into electrical quantities is highly practical in order to be able to connect the transducer with our measurement instruments or our microcontrollers (which are basically small computers). A special kind of transducers I want to talk about here today are piezoelectric accelerometers.

Figure 1: Small selection of newly acquired accelerometers. The accelerometers in the front row are obviously damaged and will not be tested/calibrated. The accelerometers in the back row have no visible defects although they may have some issues. The previous owner labeled them as “replaced due to repair”. Whether they work or not has to be determined.

Just recently I’ve acquired a huge batch of piezoelectric accelerometers in an unknown condition which need to be tested for functionality. The goal of this project is to develop a small prototype calibration device in order to be able to calibrate piezoelectric accelerometers by comparison method.

Piezoelectric Accelerometers

Working Principle

Piezoelectric (PE) accelerometers are basically “acceleration-to-charge” transducers. They rely on the piezoelectric effect which – in simple words – converts mechanical energy into electrical energy. A piezoelectric transducer consists of a piezoelectric material (e. g. quartz, lithium niobate) and a small seismic mass.

Figure 2: Elements of a single-ended compression-type piezoelectric accelerometer, presumably from manufacturer Endevco (Model 213E, 233, 2272 or similar). The accelerometer housing was cut open in order to see the seismic mass, piezoelectric disks and electric connections.

As soon as dynamic forces act on the spring-mass-system along the acceleration-sensitive axis, mechanical stress is introduced on the piezoelectric material which is mounted inside of the accelerometer housing (see Figure 2). The resulting deformation of the PE material causes a polarization which in return generates a change in surface charge density. The change in surface charge density is directly proportional to the mechanical stresses (e. g. force or pressure) and therefore proportional to the acting force or acceleration (if you remember the Newton’s 2nd law \( F = ma \longrightarrow a = F/m \)). The resulting change in surface charge density can be detected, amplified and converted into a measurable voltage with a proper signal conditioner or so-called “charge amplifier”. For simplicity’s sake I’ll refer to “charges generated by the accelerometer” instead of “polarization and change in surface charge density of the PE material”.

Harmonic Oscillator

From a mechanics point of view, the basic construction of a PE accelerometer can be approximated as a spring-mass-system with a low dampening as shown in Figure 3.

Figure 3: Analogy of a piezoelectric accelerometer to a harmonic oscillator.

The harmonic oscillator is a very basic physical model of a spring-mass-dampener system. The huge advantage of this model is its simplicity: the harmonic oscillator equations contain the Newton’s laws of motion (\(F = ma\)) and Hooke’s law (\(F = kx\)) which can be solved analytically using so-called differential equations. While I’m skipping the mathematics part here and just want to mention that in reality things are more complex, some of the results of the differential equation for a driven harmonic oscillator are shown in Figure 3. Applying oscillations on a spring-mass-dampener system leads to the curves (Bode plots) showed in Figure 3. One of critical parameters of a harmonic oscillator is the natural frequency \(\omega_\mathrm{n}\). It’s a particular frequency where the spring-mass-system is oscillated (or “shaken”) in resonance, e. g. the mechanical system responses with very large displacement amplitudes while being excited by very small amplitudes. Resonance phenomena can be experienced in everyday situations like music instruments, swinging bridges, vibrations in cars driving at certain speeds, tuning forks etc.

For example, sinusoidal excitations of an accelerometer at its resonance frequency can lead to damage or change of its specified properties, e. g. sensitivity. A high resonance frequency is achieved by using stiff material (spring constant \(k\) should be high) and small seismic mass. In case of an accelerometer, the resonance frequency should be as high as possible, usually in the order of 30…50 kHz for high-frequency or shock measurements. Brüel & Kjaer suggests in [1] that the typical useable frequency range of an accelerometer is specified to approx 30% of the natural frequency.

The equation \( \omega_\mathrm{n} = \sqrt{k/m} \) for the undamped natural frequency suggests that using no seismic mass would lead theoretically to an infinite natural frequency! In reality, we need a small seismic mass – it has to be just big enough so it can compress or tension our spring (which is basically the PE material) through its inertia. We need to create mechanical stresses on the PE material in order to generate our precious charges. Basically a larger seismic mass leads to a larger signal output which is exploited in the low-frequency range (\(f \ll 10 ~ \mathrm{Hz}\)) and in seismometers.

Measuring Accelerations with a PE Accelerometer

The amounts of charge generated by an PE accelerometer are very minuscule. In order to get a measureable amount of charge, the PE elements are stacked in parallel as seen in Fig. 1. We’re talking about tens to hundreds of femto-Coulombs (fC) per m/s² of acceleration up to few pico-Coulombs (or pC) per m/s². Typical values are in the order of few pC where \(1~\mathrm{pC} = 10^{-12}~\mathrm{A} \cdot \mathrm{s}\). Just imagine charging a small capacitor with a capacitance of C = 100 pF and a voltage of U = 0.1 V and you will get according to the capacitor equation \(Q = C \cdot U\) a value of \(Q = 10~\mathrm{pC}\). High-intensity accelerations in the order of 1 … 100 km/s² – which are found in crash or shock testing – may generate few nano-Coulombs of charge. Measuring such minuscule quantities requires a somewhat specialized test equipment: ultra low noise coaxial cables with limited length and a signal conditioner for impedance matching, signal amplification and filtering.

Figure 4: Measurement setup consisting of a PE accelerometer (Brüel & Kjaer 8305), coaxial cable (Brüel & Kjaer AO0038) and a conditioning amplifier (Brüel & Kjaer 2525) and its equivalent circuit diagram. The PE accelerometer can be described as a charge source. The PE element acts as a capacitor in parallel with a very high internal leakage resistance. Images taken from [1].
The use of PE accelerometers is pretty much straight-forward. The accelerometer needs to be attached to a vibration source which can be virtually anything: electric motor, mountain bike, structure of a bridge, washing machine, car armature, rocket engine etc. In order to perform vibration measurements properly, one has to consider many experimental issues such as mounting, temperature influences, cable fixture, grounding loops, amplifier settings and few more. Informations on this topic can be gathered from instruction manuals and application notes from different manufacturers. In order to perform accurate measurements, the instruments needs to be calibrated.

Calibration of a Piezoelectric Accelerometer

As soon as one buys (very expensive) acceleration measurement equipment, the new instruments will be factory calibrated and the manufacturer will provide calibration certificates to the customer. A calibration certificate contains important information how to establish the relationship between the input quantity (acceleration) and the output quantity (charge or voltage). This information is usually called sensitivity of an accelerometer. The sensitivity of an accelerometer is determined during a process called calibration. According to JCGM:200 (2012), the International Vocabulary in Metrology (VIM), a calibration is

[…] operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication.

In other words: a calibration is a comparison between input and output quantities of any kind. The input quantity is provided by a well-known standard, the output quantity is provided by a device under test (DUT). In case of an PE accelerometer, the input quantity is an acceleration, the output quantity is charge (or voltage if using a conditioning amplifier).

Unfortunately – when buying surplus stuff – there is always a risk of getting either a defective or incomplete unit. The provided calibration certificates may be either wrong or got lost. Some PE accelerometers are well over 50 years old and may have drifted over time. For my purposes, I’ll have to skip the “measurement uncertainties” part for now because I want to test the accelerometers for their qualitative condition and functionality. I’ll return to the metrology part in a future project.

Description of the Calibration

The calibration process is shown in a block diagram (Figure 5). In order to generate an acceleration \(a(t)\), we need to set our Accelerometer Standard (REF) and the Device Under Test (DUT) in an oscillating motion. This is usually done with an electrodynamic exciter – a technical term for “shaker” or “loudspeaker”. The working principle of an electrodynamic exciter is identical to the principle of the well-known loudspeaker. We’re generating a low distortion sinusoidal signal with a function generator which is fed into a power amplifier. The amplified signal drives the coil of the moving part inside of the exciter which in return creates the oscillating motion. Amplitude and frequency of acceleration are set by the function generator, which are typically in the range from 10 Hz to 10 kHz and 1 m/s² to 200 m/s². The frequency and amplitude ranges depend strongly on the construction of the electrodynamic shaker and the total weight of the DUT and REF accelerometers.

Figure 5: Experimental setup (block diagram) for back-to-back calibration of a piezoelectric accelerometer. The comparison method according to ISO 16063-21 is described in [2] and [3].
The generated motion is applied to both accelerometers, which are physically connected to each other. In this case, the DUT is mounted or screwed on the REF accelerometer in so-called back-to-back or piggy-back configuration. Our goal is now to establish the relationship between the input and output quantities by calculating the acceleration and measuring the output voltage of the DUT measuring chain. Basically, the charge sensitivity \( S_\mathrm{qa,DUT} \) of the DUT can be calculated as follows:

\( S_\mathrm{qa,DUT} = \cfrac{q_\mathrm{DUT}}{a} = \cfrac{u_\mathrm{DUT}}{u_\mathrm{REF}} \cdot S_\mathrm{qa,REF} \cdot \cfrac{G_\mathrm{uq,REF}}{G_\mathrm{uq,DUT}} \)

The shown equation might look scary and complicated but it’s pretty straightforward: we’re measuring the output voltages of both measuring chains and multiplying their ratio with the  charge sensitivity of our reference accelerometer (\(S_\mathrm{qa,REF}\)). Afterwards we’re multiplying the resulting expression with the ratio of transfer functions of our charge amplifiers (\(G_\mathrm{uq}\)), which have to be determined by a different type of calibration. For the sake of completeness, I would like to mention that the sensitivities and transfer functions in general are complex values (e. g. \( \underline{S}_\mathrm{qa} = |S_\mathrm{qa}| \cdot \exp{(\mathrm{j}\omega t + \varphi_\mathrm{qa}}) \)) and we’re dealing with the magnitude \( |S_\mathrm{qa}| \) of the complex transfer function. I’ll try to cover this in a future blog post.

Experimental Setup

Since we need to perform the measurements over a wide set of frequencies, it is highly recommended to automate the task as much as possible. The instrument control, data acquisition and data analysis can be done with a PC. I’m using Python 3.9 with pyvisa, pandas and numpy on a Windows 10 machine. My accelerometer reference standard is a Kistler 8076K piezoelectric back-to-back type accelerometer. For the purpose of this experiment, I’ve tested two different PE accelerometers: Brüel & Kjaer 4371 and Endevco 2276, which are so-called “single-ended” accelerometers. Single-ended type accelerometers can be mounted on the top of a back-to-back type accelerometer and therefore calibrated by comparison method. The vibrations are generated by a Brüel & Kjaer 4809 electrodynamic exciter which is connected to a Brüel & Kjaer Type 2706 Power Amplifier and an Agilent 33250A frequency generator. I’ve used two charge amplifiers  for the accelerometers, basically Brüel & Kjaer Types 2650 (REF) and 2635 (DUT). They were connected to HP 34401A digital multimeters. AC voltage measurements were performed in “ACV mode” which outputs the root mean square (RMS) voltage of the respective measurement chain signal output. An oscilloscope can be used to monitor the output waveforms in order to detect unwanted noise and distortions. This is a small downside of RMS measurements: the DC offsets and noise fully contribute to the measurement result.

Setting up the devices wasn’t very difficult. The electrodynamic exciter needs a stable and massive base along with an adequate vibration isolation. If the vibration isolation is neglected, the vibrations are coupled into the desk and into the building. Using hard foam between the desk and granite block proved being very inexpensive and efficient. The support for low noise cables are also improvised. The cable mounting is a major source of experimental errors. Due to the triboelectric effect, a bending or vibrating coaxial cable also generates charges which are superimposing the measured accelerometer signal. In short words: the reference accelerometer is measuring a slightly higher acceleration than expected. Therefore the sensitivity drops due to \( S = q/a\). This can be seen in the measurement results at frequencies below 25 Hz. At higher frequencies (e. g. > 25 Hz) the displacement amplitude of the vibration becomes very small and the triboelectric effect becomes negligible. I’ve used a torque wrench with 2.0 Nm and the contact surfaces were slightly lubricated in order to prevent deviations at higher frequencies (>5 kHz).

Measurement Results

 

Figure 6: Frequency response (magnitude) of Endevco 2276 piezoelectric accelerometer in comparison with datasheet specifications.

The calibration result can be seen on the left hand side in Figure 6. The top curve represents the charge sensitivity of the DUT plotted vs. excitation frequency. There are some deviations in the frequency response which are really annoying but I’m really satisfied with the overall result. Calculations of the relative deviation of the charge sensitivity at a reference frequency of 160 Hz can be compared with the data provided by the manufacturer. A 5% deviation at 6 kHz is in a good agreement with the specifications! My measurements show even higher deviations at frequencies f > 6 kHz so there must be some kind of systematic error which has to be investigated. Nevertheless, the measurement is automated and it takes approx. 5 minutes for a “sweep” of 31 discrete frequencies in the range from 10 Hz to 10 kHz. I’ve used standardized frequencies which are known as Third Octave Series according to ISO 266. The bottom graph shows the acceleration amplitude over the frequency range. I’m ramping up slowly in order to minimize distortions. A limit of 20 m/s² is set for noise reasons in my apartment – the generated sine tones can be very annoying and I don’t want to wear ear protection all the time.

Summary and Conclusion

This project clearly is a success! It took much time and effort in order to get the experiments straight and to automate the measurements. I was able to perform a calibration of a piezoelectric accelerometer with decent quality equipment. The results are “not bad” although I see much room for future improvements. I’ll have to improve the measuring chains and eliminate noise sources. I’d like to improve the Python code and generate a Graphical User Interface (GUI) for calibration purposes. Playing with a HP 3562A Dynamic Signal Analyzer was also very fun! I was able to dump the FFT measurement data (Thanks to Delrin for his Python hint!) via GPIB and didn’t rely on photographs of the display. A little downside of this instrument is its loudness and electricity consumption in the order of 400 W. I’ll certainly use the signal snalyzer during the winter months in order to heat my apartment 😉 I’m literally scratching on the surface in the fields of vibration measurements and the future will bring more interesting projects. The ultimate goal is to build a laser interferometer as an acceleration reference standard and to estimate the uncertainties of the built calibration devices.

References

[1] Serridge and Licht, Piezoelectric Accelerometer and Vibration Preamplifier Handbook, Brüel & Kjaer Naerum, Denmark, 1987
[2] Methods for the calibration of vibration and shock transducers – Part 21: Vibration calibration by comparison to a reference transducer, ISO 16063-21:2003
[3] Richtlinie DKD-R 3-1, Blatt 3 Kalibrierung von Beschleunigungsmessgeräten nach dem Vergleichsverfahren – Sinus- und Multisinus-Anregung, Ausgabe 05/2020, Revision 0, Physikalisch-Technische Bundesanstalt, Braunschweig und Berlin. DOI: 10.7795/550.20200527

 

Agilent E4432B Signal Generator Repair

Introduction

About a year ago, I bought an Agilent E4432B ESG-D Series Signal Generator on eBay. Those signal generators are used in the area of analog and digital wireless communications system testing. The frequency range of my model goes from 250 kHz up to 3.0 GHz, the maximum radio frequency (RF) power output into a 50 Ω termination is in the range from +17 dBm  to -136 dBm. This instrument will be useful in a different project of mine where I need a good spectral quality sinewave at frequencies of about 80 MHz for mixing RF signals. Another very useful feature is the ability to modulate the carrier frequency (AM or FM) which can be demodulated by a Software Defined Radio (SDR). It can be automated via GPIB and synchronized to a 10 MHz reference. Due to its high broad frequency coverage and level accuracy, it would be useful to test my (currently broken) spectrum analyzers, too. Unfortunately, my eBay score had a flaw which I want to talk about.

Fig. 1: Agilent E4432B ESG-D Series Signal Generator

Problems with the Step Attenuator

The unit I received is about 15-20 years old and had already approx. 61000 operating hours (equals to ~7 years of 24/7 operation) and 770 power cycles. It was overall in a good shape although one of the external input BNC jacks was loose and had a slightly damaged thread. After turning the signal generator on, I ran the diagnosis and no faults or errors occurred. I tried to set up an RF signal output at different frequencies and I noticed a very uncommon behavior: there was a sinewave at set frequencies in the range from 250 kHz up to 400 MHz (max. analog bandwidth of my oscilloscope) but the signal wasn’t present at certain amplitude levels. I checked the different amplitude levels noticed some kind of a strange pattern (see Table 1).

Table 1: Tested amplitudes at 10 MHz. The plus sign (+) indicates a present signal, minus sign (-) represents “no signal”
Amplitude (dBm) -60 -55 -50 -45 -40 -35 -30 -25 -20 -15 -10 -5 0
Signal present? ? + + + + + +

The signal at -60 dBm and lower amplitudes was too weak for my oscilloscope to detect but the pattern indicated a problem. Luckily I’ve seen a repair video by Shahriar of TheSignalPath  some time before acquiring this signal generator where he had a similar unit on the healing bench with identical symptoms. This kind of problem seems to occur in HP/Agilent test equipment (e. g. Types HP 8648A or E4400 Series signal generators or HP spectrum analyzers) where mechanical attenuators are actuated by solenoids. The detailed construction and working principle of the mechanical step attenuator is well explained in Shahriar’s video. There is also a very good repair video from YouTube user idpromnut where I got many useful repair tips.

Fig. 2: Aging O-rings as error source inside of the Agilent 33322-60014 step attenuator. Image credit: YouTube/TheSignalPath

While changing the attenuation range in steps of 5/10/20/40/60 dBm, the corresponding solenoid exerts a force on very small rubber O-rings which are used as an interface between a metal plate and plastic plungers (see photos). The plastic plungers are connected to metal strips which are pushed onto resistor traces in order to make electrical contact with a corresponding attenuation resistor. Over time, the O-rings lose their flexibility, become brittle and are either crushed by the actuating mechanics or just fall off their spot. This impairs the ability to switch the attenuator levels properly. Checking the instrument diagnosis revealed over 660k attenuator cycles which supports the faulty O-ring assumption. Luckily, this fault is  easily repairable to a certain degree without having to disassemble the delicate RF parts. Buying a new attenuator wasn’t an option because the current prices on eBay are in the order of $500 per attenuator. The signal generator is worth $700 – $1200 depending on measurement capabilities/installed options so replacing the attenuator would hurt the hobby budget.

Fig. 3: Schematic drawing of the attenuator mechanics and O-ring

In order to repair the attenuator, the O-rings have either to be replaced or glued with a flexible epoxy according to TheSignalPath video. I’ve chosen the replacement method since I don’t have proper epoxy glue. However, idpromnut showed in his attenuator repair video the replacement procedure of O-rings on a similar unit. He suggested to buy “Wrist watch clock Crown-O-Rings” on eBay from RUIHUA No. CO-12. It’s a set of O-rings in different sizes used by watchmakers. I ordered mine from XIAOJIA for approx 9 EUR. The replacement O-ring size is “2.0mm 1.0×0.5” (2.0 mm outer diameter, 1.0 mm inner diameter and thickness 0.5 mm).

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Fig. 4: XIAOJIA No. 0220 Watch Crown O-Ring set. I used the 2.0mm 1×0.5 O-Rings in order to replace the faulty ones inside of the Agilent 33322-60020 step attenuator

Repair Attempt

Fast forward to August 2022. Back in June 2021 I didn’t have the right lab equipment and spare time. It wasn’t a high priority repair so more than a year passed by. Meanwhile I acquired enough lab equipment to start a repair attempt. My basic tools were: screwdriver with hex/torx bits, flashlight, magnification glass, tweezers and a 1 Nm torque wrench for the SMD connectors. I was a bit paranoid concerning electrostatic-sensitive devices (ESD) so I used an ESD mat and ESD-compliant tools during repair. Wearing latex gloves proved to be useful because of ESD matters and avoiding contamination of sensitive parts and metal surfaces with grease or fingerprints. I had access to other tools like torque screwdriver and a stereo microscope for additional delicate work which weren’t necessary for this repair. I cleaned my lab prior to this repair attempt in order to remove as much dust as possible to mitigate the contamination of the (ESD-)sensitive electronics. I’ll post some pictures showing the repair steps. I’ve uploaded many more pictures into my Piwigo Photo Gallery. Check them out if you want to see more high-resolution images.

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Fig. 5: Agilent E4432B after removal of the covers

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Fig. 6: Photograph of the RPP (reverse power protection) module. The step attenuator is located just behind the RPP

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Fig. 7: After removing two cables, unscrewing the rigid coaxial connectors and removing two screws, the RPP/attenuator assembly can be lifted out very easily

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Fig. 8: Further disassembly of the RPP and Agilent 33322-60020 step attenuator

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Fig. 9: Screwdriver used to open up the Agilent 333222-60020 step attenuator. The +20 screws on the top (covered by the Agilent label) were not removed, only the screws on the left and right hand side covers

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Fig. 10: Agilent 333222-60020 step attenuator close-up. After removing the side cover, broken O-ring fragments fell out

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Fig. 11: Agilent 333222-60020 step attenuator after removing the shield/housing

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Fig. 12: Visual inspection of rubber O-rings and plastic plunger inside of a Agilent 333222-60020 step attenuator. One can see clearly a brittle O-ring in the center of the image which will fail somewhere in the future. However, this attenuator stage is still in a working condition

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Fig. 13: Close-up of the faulty attenuation stage Agilent 333222-60020 step attenuator). The yellow/brown-ish plastic plunger is moved by the metal plate up and down. At a certain point, the O-ring became brittle, opened up and got stuck to the metal housing. The other O-ring (not seen in the picture, hidden behind the silvery screw) already fell apart and crumbled away

At this point, the visual inspection showed one attenuator where O-rings were missing. Other stages were in a decent working condition. However, the impending failure was clearly visible. I decided not to disassemble the whole unit and try to insert new O-rings. This had to be done by fiddling around with tweezers and trial-and-error. It took me about two hours to insert two O-rings. In the end, they fitted perfectly.

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Fig. 14: I was trying to use sharp tweezers and push the O-ring into its place. The O-ring slipped many times and I had to start all over again

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Fig. 15: Photograph of the attenuator stage after a successful replacement of the failed O-ring. The newly installed O-ring can be seen on the top of the metal plate (the metal plate is “sandwiched” by two O-rings)

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Fig. 16: Faulty parts after repair of the Agilent 33322-60020 step attenuator. The rubber O-rings have dried out, become very brittle and failed. The outer diameter of the circular O-ring is approx. 2.0 mm

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Fig. 17: The assembly was done in the reverse order. It’s important to use a torque wrench for the SMA coaxial connectors. A torque of approx. 1 Nm or 9 in-lbs is necessary according to the service manual. Applying correct torque is needed to prevent damage of the connectors. It also affects the RF performance in the GHz range

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Fig. 18: Testing of the repaired unit. The signal at 0 dBm amplitude wasn’t present prior to this repair attempt. After repair, all amplitude settings worked without problems!

Summary and Conclusion

I would consider this repair as “easy” thanks to TheSignalPath and idpromnut‘s detailled repair videos. I couldn’t do it by myself because I’m not an RF test equipment expert and I haven’t done such repairs before. It’s just another hobby and I’m relying on some help from the outside. However, this repair was very rewarding and only temporary. The visual inspection showed the impending failure of the remaining O-rings. Servicing this unit will surely be necessary somewhere in the future. I’ll have to replace every O-ring to be sure they won’t fail again for the next few years.

Also one has to consider post-repair procedures as the service manual suggests. I haven’t checked the amplitude over the frequency range because of limited measurement capabilities. I just recently acquired a RF power meter which has to be calibrated first. My spectrum analyzers only cover the frequency range up to 1…1.8 GHz and aren’t very trustworthy. I’ll also have to check the post-repair performance of the step attenuator with a Vector Network Analyzer (e. g. NanoVNA V2 Plus4) in order to be sure the unit is working properly.

Bicycle Tour 2022

I really love bicycle touring. My last bike tour was back in July 2019 and took like 16 days and approx. 1000 km. The past two years have been very difficult for travelling due to COVID-19 restrictions. The camping sites either closed or the COVID-19 rules were very restrictive and differed from site to site. Travelling in 2020 and 2021 was really difficult and risky. Luckily, the situation changed in 2022 and due to vaccinations, declining COVID-19 pandemic situation and reopening of the tourism and travelling sectors in Germany, it was possible to travel again.

I took the chance and organized a little bike tour during my vacation. My original plan was something like this: travel by bike and tent for 6 days from Hanau to Braunschweig, Germany. It was coupled with a visit to my relatives in Hanau – a mid-sized town near Frankfurt am Main in the state of Hesse. The daily tour distance should be something like 60-80 km and the total distance from Hanau to Braunschweig should be approx. 400 km. Planning the route by OpenCycleMap.org was pretty easy but I missed two important factors: the terrain and total weight of me and my bike. While the terrain in Lower Saxony is mostly flat and very easy to manage – this isn’t the case in the hills of Hesse, especially when one has a 40 kg packed travelling bike and approx. 120 kg of muscles fat riding the bike. So yeah… I kinda underestimated the effort which was punished later 😉

However, I was preparing this tour for about a week. Riding approx. 30 km per day helped to build condition and to get used to sitting on a bike for 2+ hours.

Day 1: Hanau – Gelnhausen – Schlüchtern

The tour started on the hottest days of the month. This wasn’t planned at all. The temperatures were around 37-39 °C and there was not even one cloud from early morning until sunset. This was definitely not my cycling weather. Cycling the first 40 km was easy until the heat drained my powers. The last 20 km were exhausting but I managed to reach Schlüchtern. My first camping site was about 3 km outside of Schlüchtern – “piece of cake” I thought. The camping site was on a hill of approx 250 m height. My mistake was using the Google Maps guide. The suggested route was closed and I spent 2 hours in the woods and hills pushing my 40 kg bike at 12% upward slopes. I finally managed to reach the camping site Hutten-Heiligenborn at 8 pm and was wrecked. The camping site was very nice and I was welcomed by the site manager. Tent, shower, dinner, sleep, RIP.

Day 2: Schlüchtern – Fulda – Schlitz

The cycle route downhill was really nice! I was able to go at 40-50 km/h for about 7 km. 26 km later, I reached the city of Fulda. Fulda was very inviting due to very good cycling roads. I visited the central train station involuntarily because I got lost few times due to bad road signs. Nevertheless, the temperatures rose higher and higher up to 39 °C and my performance dropped steadily. In the village of Kämmerzell I got lost again but this was really bad. The route got me into a 7 km lasting agony of steep hills (remember? 12% upward slope at 39 °C and 160 kg of total mass?) which lasted for like 3 hours. Luckily, I was pushing my bike on forest roads where most places were in the shadows at 35ish °C but that’s it. This unintentional route killed my schedule and I was unable to reach my planned camping grounds.  My provisions have been spoiled by the hot weather and I had to resuppy it on the next day. Luckily, there was a small camping site in the city of Schlitz (where I got lost again, thanks Google Maps). Later at night, there was a thunderstorm but this was no problem for my Hilleberg Unna tent!

Day 3: Schlitz – Bad Hersfeld – Melsungen

I think this was the best cycling day I had in a long time. Due to the thunderstorm and colder weather, the temperatures dropped significantly from 39 °C to a cloudy 23 °C. This was like heaven for cycling – the heatwave was gone and the temperatures were ideal for cycling. My performance on this day was OK: my muscles and my butt didn’t hurt much and I was able to cycle 95 km total. I still wasn’t able to catch up to my schedule because I was like 40 km behind. When reaching the city of Melsungen at 7 pm, I thought I had enough power to make it the next 40 km but as soon as I saw the next camping site, I knew it was time to get some rest. Cycling at night in unknown terrain can be dangerous (you get lost very easily, bad sight during night). So I stopped on a small camping site near Melsungen and it wasn’t a bad decision at all.

Day 4: Melsungen – Kassel – Hann. Münden – Hemeln

Oh yeah, I overdid it on Day 3. The daily goal was set to 90-100 km but I managed to get only 78 km total. The weather was very good (cloudy, 25 °C) and I was progressing very well until I reached the city of Kassel. Here I got lost multiple times due to construction sites and spend like 2-3 hours cycling through Kassel. I stopped here and there to take photographs but wasn’t able to do a sightseeing tour. Kassel was very stressful because I had to take routes on busy roads for a while until I found the correct cycling route. This set me further back in my schedule. The route from Kassel to Hannoversch Münden was very nice and relaxing. I met another elder cyclist which cycled with me for about 12 km. Having company was nice because we were very fast and could talk about the usual stuff (small talk). Hann. Münden was a very nice city – this is the place where the rivers Fulda and Werra combine into the river Weser. I wish I could have stayed there longer. However, I left Hann. Münden at 2:30 pm and at about 3:30 pm, my performance started to decline due to muscle and rear pain. I didn’t want to ignore the pain signs and I visited a camping site in a village called Hemeln. Good decision (as usual). I needed some rest and as soon as my tent was up, I went inside and slept for like 2 hours. Then shower, dinner, sleep 8h.

Day 5: Hemeln – Höxter – Holzminden – Stadtoldendorf – Braunschweig (by train)

Unfortunately the final cycling day. I got up very early in the morning and left the camping site at about 8 am. All the muscle pain was gone and I was able to cycle at a fast pace again. The weather wasn’t very bad and this got me excited because I felt like I could do 100+ km on this day. My goal was Seesen, a small town near the Harz mountains. The distance was calculated at about 140 km. Since 100 km are no problem, an extra 40 km should be possible, too. Wrong! The first 60 km weren’t bad at all. My speed was OK but the temperatures started to rise again. By noon, the temperature was at 28-30 °C and my rear started to hurt every few kilometers. So I had to push the bike for a while until the pain went away but this set me back in my schedule. After visiting few cities such as Bad Karlshafen, Höxter and Holzminden, I left the Weser cycling route and set the route towards east (R1). At 80 km distance, I was really exhausted and had to take breaks every few kilometers. I reached a place called Stadtoldendorf at 4 pm and still had to cycle 50 km to Seesen. At 13 km/h, this would have taken 4 hours at least. I resupplied my water and food in Stadtoldendorf and I got lost again due to a major construction site. Unfortunately, Google Maps calculated a wrong route directing me into steep hills and I gave up.

I re-routed Google Maps to the next train station and started the adventure back home to Braunschweig by train. Luckily, I was able to travel with my 9-EUR-Ticket and the trains were not full at the time. After spending 3 hours in regional trains, I was able to reach home safely. My daily distance was 110.95 km, my new personal record (my highest was at 110.68 km in the year 2019). I must admit, I had to use Ibuprofen due to muscle pain.

Summary

It was a very interesting and adventurous cycling tour 2022 (as usual). I’ve seen many wonderful landscapes and places, cycled about 425 km total. The weather was good and bad and I got lost many times (as always). I learned a bit about planning routes and learned a lot about my cycling performance. Using petroleum jelly for skin lubrication was a winner. I packed the wrong stuff which I never used on tour (gas stove, blanket, accessories, food) but which added to the total weight. My provisions got spoiled by the hot weather so this is an important issue to consider for future routes. I hope to reduce my body weight by winter so I’ll be able to do another cycling tour at the North Sea. I’ve done this before and it was one of the best cycling tours I had so far.

I would recommend everyone to participate in such cycling tours. If you’re not a cyclist, go hiking instead! It’s a challenge to travel alone over the course of many days. For me it’s important to realize that travelling from A to B takes time and effort. Sure, one can drive into holidays or fly by airplane with little to none effort. This behavior has two negative effects: people just don’t realize how hard it is to move things from A to B and we take many modern things for granted. Cars and airplanes may be the foundation stone of our modern civilization but our civilization will have to change significantly in the next few decades or we’ll have really difficult times here on earth (climate *cough* change).