Tektronix

Recharging Ni-Cd batteries for Tektronix THS 720 STD portable oscilloscope

Tektronix, Uncategorized / /

Merry day after Christmas everyone! I hope everyone is having a good time and doing fun projects during holidays and vacation. I’m trying to write a blog post again, so today’s blog entry will be about some of my efforts to recharge a battery! Can’t be that hard, can it?

Tek THS 720 with batteries
Tektronix THS 720 with rechargeable batteries. The original battery pack is labeled THS7BAT.
The Tektronix THS 720 is a portable and battery-operated 2 channel 100 MHz analog bandwidth, 500 MS/s oscilloscope with isolated BNC inputs and an integrated digital multimeter (DMM). It is a very useful tool for troubleshooting electronics or servicing (switch-mode) power supplies due to its isolated input channels. It can perform “floating” measurements and measure voltage differences safely on arbitrary potentials (similar to a handheld DMM). When testing switch-mode power supplies (SMPS, having a galvanic isolation and line voltages, e. g. 230 V) with an “ordinary” oscilloscope, keep in mind that the probe ground connector (GND) provides a direct and low impedance path to the oscilloscope chassis (mains earth referenced). This is potentially very dangerous, because if you connect your GND to a “floating” Device Under Test (DUT) at a higher voltage potential, the GND connector creates an electrical short between the DUT and the oscilloscope chassis. If the DUT and chassis are somehow connected through protective earth (PE), it can short out line voltages directly via your oscilloscope and perhaps ruin your day, too (danger of being electrocuted). In order to avoid these electrical safety dangers, the serviced DUT needs to be powered through an isolation transformer as a safety measure – or alternatively –  it has to be probed by either using a high-voltage differential probe (for example Tektronix P5205A) or via a battery-powered, isolated (“floating”) oscilloscope (e. g. Tektronix THS 700 Series or Fluke ScopeMeter). There is an excellent explanation video on this matter from Dave Jones of EEVblog on YouTube.

 

Isolated BNC inputs and DC supply input of the Tektronix THS 720
Tektronix THS 720 battery compartment. The (+) electrode contact can be seen as a black square on the bottom left (around 7 o’clock)
While powered on, the THS 700 Series oscilloscopes have quite a high current demand, which can be only supplied reliably by a Nickel-Cadmium (Ni-Cd) type of rechargeable batteries. Unfortunately, the Ni-Cd batteries have two disadvantages: they self-discharge over longer periods of time (at a rate of approx. 1% per day, it takes about 3 months for a complete discharge, inside of the scope they may discharge even faster within a couple of weeks) and they suffer to the memory effect (capacity deterioration due to charging/discharging cycles). I bought this oscilloscope for a very fair price (~200 EUR), however, with a dead Ni-Cd battery. The EEVblog users have investigated modern-era replacement possibilities such as NiMH batteries with special adapters which will fit inside of the oscilloscope battery compartment (length and diameter-wise). The alternative to this method is to buy certain Ni-Cd battery cells and combine them together into a battery pack.
I was lucky to find an offer on Kleinanzeigen from a guy who already premade this kind of battery pack and I bought it from him for about ~50 EUR. This saved me a lot of time and tinkering because I didn’t have a spot welding machine in order to attach metal sheets to the electrode. So here is a comparison between the original THS 700 Series oscilloscope batteries and the DIY-type.

 

Comparison between THS7BAT and a DIY Ni-Cd battery pack
Ni-Cd battery spot welding detail image
Four C-type cells (Panasonic Cadnica, model N-3000CR, Ni-Cd 1.2 V, 3000 mAh, Flat Top (non extending), diameter 26 mm, single cell length 50 mm, ~7 EUR/piece) were used and contacted in series and assembled with Kapton tape so they form a single battery unit. Unfortunately, the THS 720 oscilloscope demands the positive electrode (cathode) to be placed at a certain position along the battery axis – you can’t simply use the both ends of the batteries unless you want to modify your scope for this purpose. In order to meet the requirements, the positive battery electrode needs to be extended by spot-welding a thin metal sheet and connect it to a metal ring, located about 40 mm above the negative electrode (anode) of the bottom cell. The metal ring provides the positive battery supply connection to the oscilloscope. This is a very odd construction but that’s how the oscilloscope was made back in the mid 1990s. It’s important to notice that the metal sheets must be spot-welded to the battery electrodes in order to provide reliable electrical connections. Leads cannot be soldered to the battery electrodes directly for different reasons – if they come off, they can cause power loss or even shorts, the heat applied during soldering can also damage the battery cell. The metal sheet needs also to be sufficiently isolated with suitable non-conducting tape in order to prevent (potentially dangerous) battery shorts.

 

Ni-Cd battery cathode ring – detail image

Anyways, the Panasonic Cadnica replacement batteries turned out to work exceptionally well and provided enough power for a certain amount of time (depends on oscilloscope usage, perhaps 1 day with intermittent usage). I didn’t use the oscilloscope very much in the past and the battery was discharged as expected. In order to recharge the battery, I originally used the oscilloscope’s internal charging circuit, which “kinda” did the job. The charging at supplied voltage of 12 V and 1 A took around 24 hours. I noticed a significant heat-up of the battery packs up to ~45 °C, maybe 50 °C so I wasn’t quite sure about the unsupervised safety of this charging procedure. The max. charging temperature according to the datasheet is specified at +45 °C so I really hit the temperature limit. To avoid this in the future, I was looking for a fast Ni-Cd charger where I could quickly recharge the batteries without a significant heat build-up. There is a Tektronix THS7CHG Battery Charger on the 2nd hand market (e. g. eBay), however, it’s too rare and too expensive and not a viable option. An EEVblog user has constructed a similar charger with off-the-shelf components and 3D-printed housing. For a single Ni-Cd cell, the rapid charging time should be in the order of 1 … 2 hours at a nominal Voltage of 1.2 V and maximum currents of up to 4500 mA – compared to the 16-24 hours at standard charging currents (300 mA).

Robbe Power Peak Infinity 2 battery charger
Robbe charger, sitting on top of a switch-mode power supply during a charging operation

For this purpose, I bought a 2nd hand discontinued universal charger, which are very common in the fields of radio-controlled toys and RC models (drones, cars, boats etc.). It was manufactured by a German company called Robbe (type: Power Peak Infinity 2), which is capable of charging and discharging different types of batteries (NiMH, Lead Acid, Ni-Cd). In order to operate it, one needs a (switch-mode) power supply which provides the necessary voltages and currents for operation of the charger (e. g. 13.8 V and 0.1 … 5 A). The battery charger’s microcontroller monitors and regulates the output voltages and currents, which suits the charging profile of the batteries being recharged. In case of 4 Ni-Cd batteries, we need at least 4x 1.2 V = 4.8 V with a current limitation of 4.5 A (according to the Cadnica datasheet). The charging speed is limited by the battery temperature, the build-up of internal pressure and the so-called charge rate C. Typical charge rates are around 0.1 C while “fast” charge rates are around 0.5 C up to 1 C. Due to limitations of Ni-Cd batteries (memory effect, ~500 recharging cycles), they should be discharged first, then recharged at low rates (e. g. 0.1 C) to increase their longevity. For long-term storage of Ni-Cd batteries, Robbe user manual recommends to discharge them first and store them in a cold and dry place (e. g. in a fridge at 4 °C) in order to mitigate the performance deterioration due to the memory effect.

My experimental setup for battery charging and monitoring: Robbe charger, switch-mode power supply, Tektronix TDS 3034B oscilloscope with a P6139A probe and TCP202 current probe. In the center part of the image, the charging fixture for the Ni-Cd battery is shown. Not shown in picture: digital multimeter HP 34401A. Top left: Rubidium Precision Time Base (not used for this experiment, just casually laying around)
Quick-and-dirty assembled Ni-Cd battery fixture from optics parts. The electrode contacts are just standard 4 mm banana plugs inserted into 4 mm banana couplings.

My charging/discharging setup looks as follows: the switch-mode power supply (JAMARA Germany DC Regulated Power Supply, Output: 13.8 V & 0-20 A) is connected to the charger. The charger output is then connected to the battery pack fixture. For my battery fixture, I used some spare optics parts I had at hand. The spring-loaded electrode helps to keep the battery in place and also helps to provide a reliable electrical contact. The recharger was powered on and set up to the “DISCHARGE -> CHARGE” mode, which is recommended by the Robbe manual. The battery under test had a remaining voltage of about 2.4 V and was discharged with a current of about 0.1 to 0.3 A. As soon as the battery was discharged to a certain threshold voltage (e. g. 1 V open circuit voltage), the charging cycle began by a slow ramp-up of the voltages and currents. After few minutes, the rapid charging is reached at approx. 6 V and 4.5 A (about 30 W), which corresponds to a charge rate of 1.5 C. I could observe some kind of a charge duty cycle, where the current flow stopped for a certain amount of time before continuing again – perhaps for thermal management purposes. The charging process took about 60 minutes, the estimated restored battery capacity (= transferred charge) was in the order of 3000 mAh. The temperature of the setup was monitored during the charging process with a thermal camera, however the battery pack didn’t heat up significantly in comparison to the mentioned THS 720 internal battery charger. The voltages were monitored by a digital multimeter HP 34401A and an oscilloscope Tektronix TDS 3064B, the current was monitored by a Tektronix TCP 202 current probe. This allowed me to estimate the power demand during the charging process.

Charging ramp-up shortly after charging begins

I was surprised how well and how fast this recharging process went. The DIY battery pack proved to be suitable alternative to the original battery pack Tektronix THS7BAT. Unfortunately, the Ni-Cd batteries age and need to be replaced over time. However, an inexpensive alternative with modern-day parts for a reasonable amount of money (~30 … 40 EUR) can be used to replace the original battery pack and extend the lifetime of the oscilloscope usage, at least until the internal electronic parts start to fail (e. g. the opto-couplers of this scope).

Tektronix THS 720, operational with a fresh recharged battery

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19″ Rack Mount Project

Engineering Stuff, Metrology, Tektronix / /

Yes, I have some kind of a Gear Acquisition Syndrome. Yes, I’m a Test Equipment Anonymous. Yes, I love Test Equipment! 🙂 However, this was bothering me for quite some time. The 19″ equipment was laying all over the place and I had some difficulties using it, since the instruments were bulky, scattered and there was no logical order as soon as I wanted to perform measurements. Luckily, a colleague of mine gave me a hint to buy a very cheap 19″ (42 HU) rack mount. This would solve few problems and also introduce new ones (more space for more equipment! Just kidding…). Anyways, I bought the rack mount and after few weeks of procrastinating, I finally equipped it with my 19″ sized test equipment. I’m pretty happy how it turned out. The most heavy parts are located at the bottom and lighter equipment is located at mid to top. It’s stable enough and won’t tip over and it can be easily moved around.

19″ Test Equipment rack mount.

Almost all instruments can be controlled remotely either via GPIB or RS232. An older Dell Workstation will be controlled via Windows Remote Desktop. Instruments from top to bottom are listed as follows:

  • HP 3325B – A 20 MHz frequency generator with some interesting features for audio testing
  • HP 3488A – Switch unit equipped with different modules. It’s used for automated, accurate and reliable signal switching or signal routing – it really depends on the used relay cards. I got few of them over the past years, e. g. types 44470A (10 CH multiplexer), 44471A (10 CH general-purpose relay module), 44472A (dual 4CH VHF switch module), 44473A (4×4 Matrix switch) and 44474A (16 bit digital I/O)
  • NI BNC-2090 is coupled with NI PCI-6110 (PCI multi-function-I/O-card with 4 analog inputs (12 bit, 5 MS/s per channel), 2x analog out, 8 digital I/O)
  • Tektronix TDS 644B, 4 CH, 500 MHz, 2 GS/s digital real-time oscilloscope. It’s a bit overkill for my audio-type applications – an excellent instrument for displaying waveforms, even RF stuff. A very useful feature is a GPIB interface for remote control and screenshot hardcopy (no need to use the disk drive)
  • Dell Precision T5400 workstation PC. It’s a 2007 era workstation which I bought probably around 2010 as a second hand PC. I used it like… forever. It was replaced by a faster workstation back in 2020. I haven’t dumped it because of the DVD burner and slots for 3x PCI and 3x PCIe cards. This makes it very valuable when it comes down to using data acquisition PCI cards from early to mid 2000’s era with a Win 7 or Win 10 operating system. Unfortunately this unit consumes a lot of power – it tends to heat up and it’s having thermal management difficulties during hot summers. Few of the ECC SD-RAMs failed over the past years but luckily they have been replaced easily. I tried to equip the three PCI slots with my oscilloscope cards, however, there were some serious thermal issues. I didn’t want to fry the cards so I put them inside of an external PCI expander system
  • Sun Microsystems Netra E1. After having thermal issues and space problems inside of the Dell T5400, I was looking for a better solution to put the cards in a separate chassis and operate them via a PCIe/PCI bridge. Few solutions exist today, however only few are viable due to hardware/driver compatibility or power delivery constrains. The cheap expansion cards (mostly from China) can’t always deliver the 25-75 W of power to the oscilloscope card. In my case, as soon as the oscilloscope card demanded more power, the PC system crashed. After browsing eBay and looking for National Instruments’ “PXI-like” external chassis solution, I found Sun Microsystems’ Netra E1 as a viable option – however, the price was hefty. In retrospective, I’m glad I bought it because it was ready for use and it saved me a lot of time and trouble. The installation was super easy as PCI/PCIe bridge drivers exist at least since Windows XP. A downside of Netra E1 are the really loud fans. They work relentlessly at full speed; however, they keep the power-hungry cards at acceptable temperatures around 50-60 °C in comparison to 75-80 °C inside of Dell T5400!

    Unfortunately, Netra E1 is obsolete technology and no replacement parts exist on the 2nd hand market. Those units were produced around the year 2001 as an expansion system for Sun’s Netra Servers. I might be wrong about this but IIRC their PCI expansion product line was acquired by a company called MAGMA somewhere around mid 2000’s. MAGMA was specialized in building PCI Expander Systems for the Pro Tools Digital Audio Workstations (Pro Tools by Avid Technology, famous tor their audio/video editing software). eBay has some offers with MAGMA expansion chassis up to 16 PCI card slots, however they come at an unrealistic price of $500 to $1000, at least for a hobbyist budget. Anyways, beside the three existing PCI slots inside of the Dell workstation, I was able to put all of my four “power hungry” PCI cards (full length) inside of the 19″ chassis

    • NI PCI-6110: multi-function I/O card
    • NI PCI-5105: 8 channel, 12-bit, 60 MS/s digitizer/oscilloscope with up to 60 MHz analog bandwidth
    • NI PCI-4461: dynamic signal analyzer, basically a $10k sound card, still produced today – according to NI, it can be bought until 2024-12-31!
    • Agilent/Acqiris AP200: 1 channel, 500 MHz, 2 GS/s high-speed digitizer/averager card (used in radar tech or mass spectrometers)
  • Yokogawa/HP 4274A Multi-Frequency LCR Meter. I have a bunch of HP fixtures for components, unfortunately the LCR meter needs a repair. Should be hopefully an easy fix since the errors appear after warm-up. Fun fact: The Japanese government was very restrictive and protected their markets in the early days. In order to enter the Japanese market, companies from foreign countries such as United States were obliged to “team up” with a local Japanese manufacturer so they could sell their products in Japan. I guess HP teamed up with Yokogawa and Tektronix had a partnership with SONY. That’s the reason why there is occasionally a double company logo on some pieces of test equipment.
  • Tektronix TM5006 chassis equipped with
    • Tektronix FG5010, 20 MHz frequency generator (GPIB-controllable)
    • Tektronix AA 5001, audio analyzer capable of measuring Total Harmonic Distortion (THD), also GPIB-controllable
    • Tektronix SG505, audio frequency generator with exceptional ultra-low distortion sine-waves, perfect for audio amplifier testing
    • Tektronix DC 503, digital counter (doesn’t work, needs a repair)
  • HP 3562A Dynamic Signal Analyzer. A very capable FFT analyzer e. g. for audio and vibration testing and frequency response analysis in the µHz … 100 kHz regime
  • Fluke 5100B/5101B Calibrators. They can provide AC and DC voltages and currents and also resistances for calibration of up to 5.5 digit DMMs. Unfortunately, both need repair (power supply issues). Currently they are not in use – I’m hoping to repair them either later this year or maybe in early 2025

Surrounded by the rack mount are my book shelves with many books about physics, chemistry, operating systems and other stuff. A HPM 7177 digitizer can be seen at mid-left. There are only three cables left which are connected externally to the chassis: 230 V power, GPIB and Ethernet. I could replace GPIB with an NI GPIB-USB and Ethernet with WiFi. This would of course simplify the cable management but I’m very happy how everything turned out. I’ll need to find some lifting feet for the rack mount since it’s a bit overloaded with ~200 kg of equipment and it could cause damage the floor and the wheels over time.

Most of the test equipment seen here has been acquired during the past 3-4 years. Some pieces were really cheap (< 100 EUR), others rather expensive (> 300 EUR). It will be used for my laser interferometer and accelerometer calibration project which I plan to build in the future.

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Tektronix TDS 3034B bandwidth upgrade

Tektronix / /

Tektronix TDS 3034B four channel color digital phosphor oscilloscope.

A “new” oscilloscope arrived in the lab just recently thanks to my friend Matt, who relayed the offer to me. It’s a wonderful instrument from the early 2000’s era: a Tektronix TDS 3034B Four Channel Color Digital Phosphor Oscilloscope. The oscilloscope’s analog bandwidth is specified at 300 MHz with a sample rate of 2.5 GS/s per each channel. It has a ton of options (e. g. FFT, 3LIM, 3TRG) and a communication module with VGA monitor output and GPIB/Ethernet/RS-232 for communication and control. There is also a battery compartment, however, the battery was not included. The “Waveform Intensity” knob simulates the intensity control of an analog oscilloscope with a cathode ray tube – therefore a “Digital Phosphor Oscilloscope” or DPO. The size of the oscilloscope is very convenient and perfect for a desk use – doesn’t take much space and it’s fast and responsive compared to my older Tektronix TDS 754D oscilloscope.

Tektronix TDS 3034B back side with battery/probe compartment.

I was able to set up the communication to my PC via GPIB (IEEE 488). Since I already had the Tektronix WaveStar Software for Oscilloscopes installed on my Windows 10 PC, I was able to quickly transfer screenshots/hardcopies, measurement data and sample data from the instrument to my PC. No diskettes or USB drives were needed. It is possible to control the oscilloscope remotely which is absolutely fantastic for measurement documentation and measurement automation purposes! It’s a great addition to the lab and it will serve to me as my daily workhorse among other oscilloscopes, of course.

Tektronix WaveStar for Oscilloscopes. Easy way to transfer screenshots and data from Tek TDS 3034B to a Windows PC.

Bandwidth Upgrade

It seems to be very common (even nowadays in 2024) that the oscilloscope manufacturers design a generic oscilloscope parts which can be used for different models. The oscilloscopes are then assembled with similar to identical hardware components (presumably because it’s cheaper manufacturing-wise) but the measurement capabilities are determined either by hardware (jumper settings) or by software (locked/unlocked options).  For example, Rigol DHO 800/900 Series Oscilloscopes can be upgraded from 70 MHz to 100+ MHz via software. Thanks to Matt, he gave me a hint that the guys at EEVblog have figured out how to perform an upgrade of the Tektronix TDS Series oscilloscopes to different models. So already having an excellent 300 MHz & 2.5 GS/s oscilloscope at my fingertips, I was curious if I could upgrade it to the 3064B model which has 600 MHz analog bandwidth and 5 GS/s sample rate. I tried out the EEVblog’s upgrade procedure and it seemed to work without bricking the oscilloscope. I’ll just quickly summarize the procedure here.

  • Boot up the device and set up the communication via GPIB (e. g. GPIB address and talker/listener mode). Connect the device to your GPIB controller
  • I used the National Instruments GPIB-USB-HS controller device along with current NI’s GPIB drivers (driver version doesn’t matter much, you could also use the VISA drivers from Keysight, Tektronix or Keithley). The TDS 3034B Firmware Version was v3.39
  • The communication between the PC and the oscilloscope was established with NI Measurement & Automation eXplorer’s (NI MAX) own GPIB Instrument Communication. I guess you could use any GPIB communication software, e. g. pyvisa for Python
  • Check whether the oscilloscope responds to the *IDN? query. If yes, proceed, if not – try to find the problem and fix it (obviously)
  • Send the following commands to the oscilloscope
    PASSWORD PITBULL
    MCONFIG TDS3064B
    Just send (write) the commands in the order as shown above! Do not use a “GPIB query” since there will be no response action from the oscilloscope. Upper/lower case letters don’t matter.
  • Now power-cycle the oscilloscope (turn it off, wait few seconds and turn it on again). It should boot up with a new screen and new model. If it doesn’t show the new model, something went wrong with command transmission via GPIB. As far as I could tell, some models accept only different MCONFIG-commands, such as “MCONFIG TDS3054” without the letter “B” at the end. Anyways, after sending the commands via GPIB and power-cycling the oscilloscope, it should show up with the new model
  • Changing the TDS model will result in an uncompensated waveform. I observed a DC offset and noise on my CH1 through CH4 waveforms post-update. The solution to this problem is quite easy: wait some time (at least 10-15 minutes) for a warm-up and perform the Signal Path Compensation (SPC), which can be found in the Utility → System Config → Cal
    menu. This will perform an internal calibration where the noise and offset errors are compensated. The oscilloscope should be ready for performing measurements afterwards

That’s how it worked out for me. It took me just few minutes to convert a Tektronix TDS 3034B into a TDS 3064B. I’d recommend to check out the linked EEVblog thread for further information on the TDS 1000/2000/3000 Series upgrades. Of course I can’t be held responsible if you try it out and damage your instrument (e. g. calibration data or warranty is lost or the device is bricked) since I’m sharing this information and tried it out on my own oscilloscope. This is solely done at one’s own risk! Please be careful when trying out this procedure and please read the EEVblog thread prior to changing the instrument. Always make backups of your oscilloscope firmware prior to changes. Also hacking the device in order to use unlicensed software options is… well… “illegal”… I guess… Keysight’s Agents will hunt your PC down! 😉

Excerpt taken from the Keysight N1500A EULA. This is not a joke (well, it’s a well-known meme), see it for yourself: https://helpfiles.keysight.com/csg/N1500A/License_Agreement.htm

Checking the Oscilloscope Analog Channel Bandwidth

I did some initial bandwidth testing with my Leo Bodnar Fast Risetime Pulse Generator. The pulser generates repetitive 10 MHz square waves (~1 Vpp) with a rise time of around (30 ± 2) ps. It can be used to easily test the oscilloscope’s analog channel input bandwidth. The analog bandwidth of an oscilloscope can be calculated as follows:

\( \mathrm{BW [GHz]} \approx \cfrac{0.35}{t_r ~\mathrm{[ns]} } \)

So a measured 10%-90% rise time of \(t_r = 1.000 ~\mathrm{ns}\) results in an analog bandwidth of \(0.35/(1.000 \cdot 10^{-9} ~\mathrm{s}) \approx 0.350 \cdot 10^9 ~\mathrm{Hz}\) or 0.35 GHz which is basically 350 MHz. So prior to the bandwidth upgrade we were expecting an oscilloscope bandwidth of ~300 MHz at a 2.5 GS/s sample rate, post-upgrade it should be around ~600 MHz and a 5 GS/s sample rate. I’ve measured the rise time of all four channels in order to see if there are any significant differences between them. The oscilloscope settings were as follows: Coupling: DC, Termination: 50 Ω, Trigger: External, Acquisition Mode: 64× average per acquisition at 10k points and 5.00 GS/s. The trigger was delayed by approx. -10 ns.

The measurement results are summarized in the table below.

Analog bandwidth calculations from measured rise times for each oscilloscope channel. Comparison between Tektronix TDS 3034B before model upgrade and after model upgrade to TDS 3064B.

As we can see, the full bandwidth of 600 MHz for a Tektronix TDS 3064B has not been achieved. However, there is a measurable improvement. Channels 3 and 4 seem to have a slightly higher bandwidth than channels 1 and 2. The rise time measurements deviate on each acquisition so I would estimate an uncertainty in the rise time measurements of approx. ±10 ps. I also wonder if the overshoot/undershoot calculations are correct. I’ll have to look into this manually, since the data samples can be exported in a CSV file.

Conclusion

Nevertheless, the bandwidth upgrade was successful! 500 MHz analog bandwidth at 5 GS/s is more than enough for my needs before stepping into the GHz time domain regime. And no need for a battery if there is a food compartment in your oscilloscope! Tested successfully with carrots! 😉

Tektronix TDS 3034B food compartment.

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