Show by Label

Saturday, October 14, 2017

Setting up an FFT Measurement System


In this post I'll describe how I use a number of components to create a system that you can use to measure harmonic distortion and noise by using FFT's. This system can be configured in several ways to verify and measure the distortion of amplifiers, sine wave generators, power supplies etc.

Here is the complete set of instruments that make up the analyzer.
From top to bottom, the modified E-MU 0202 USB Sound "Card" digitizing interface to the PC, the "Pete Millett" Sound "Card" Interface, and on the bottom the combined "pure" Victor Mickevic 1 KHz sine wave oscillator and the 1 KHz Twin-T notch filter.


First a little overview

To measure the performance (here the amount of distortion generated or added) of a black-box system (the Device Under Test - DUT), a very clean and low distortion sine wave (the fundamental) is used to stimulate the DUT, while the output is sampled. The output signal is stripped from the fundamental sine wave by means of a sharp filter, and the remaining residual, the noise and distortion is fed to an FFT system that can show the results.

Following is a picture of the setup needed to measure harmonics, distortion and noise (DHT and DHT+N) from a DUT.






This is a setup that can be used to measure sine wave generators, or to verify the Sound Card interface to the PC and the FFT software.

A high quality low distortion 1KHz sine wave is fed to the DUT, here symbolized as an amplifier. The output signal from the DUT is fed to the Soundcard Interface and is probably attenuated. This interface protects the delicate and sensitive inputs of the other intruments in the chain, like the T-filter and the USB Sound Card, and also facilitates making a few measurements. The resulting output is then fed to a Twin-T filter which removes the 1KHz fundamental. The residual (everything added by the DUT to the pure sine wave) is then fed back into the Soundcard Interface and then going to the USB Sound Card Interface, here a simple SD-AUD20040, where it is digitized and sent to the PC over a USB link. On the PC/Laptop, a software application like AudioTester, is used to show the residual (and noise) by means of an FFT.

This same setup can also be used to measure the noise performance of a power supply. In that case, the power supply output is first stripped from the DC component by using a capacitor (not shown), and then fed to the Sound Card Interface, and from there to the USB Sound Card for digitizing and on to the PC and the AudioTester FFT software.

If you want to measure the quality of sine wave generators or verify the components in the link, you can also use the following much simpler setup.



How you inter-connect the various instruments depends on what they offer. In this example, the output of the sine wave generator is fed directly to the Twin-T filter and the residual is digitized by the USB Sound Card. The Sound Card Interface is not needed, because the input levels can be set low enough, althoug you need to be carefull or use attenuation adapters.


The instruments that are used in the chain

In an earlier post on this Blog, I already described how I put the the "vicnic" very high quality sine wave generator, and an active Twin-T filter in an enclosure.
On the left is the 1KHz sine wave oscillator with a dual output connector. One the right is the Twin-T notch filter, optimized for a 1KHz sine wave.



This instrument is described here:  simple but precise 1khz distortion system

The sound card interface

I also used a DIY Sound Card Interface, to attenuate (high) signals coming from the DUT, which would otherwise destroy the input of the USB Sound Card, or the other measurement components. The output from the Interface then goes to the Twin-T filter and then to an actual USB Sound Card (to digitize the analog signal) connected to a PC. The digitized output of the Sound Card is used by the AudioTester software running on the PC to show the results, typically by an FFT diagram.





The Sound Card Interface above is another DIY project based on a design from Pete Millett and described here

The USB sound card interface

A USB sound card is used to digitize an analog signal (typically music) so it can be send to a PC for prossesing. It's still called a sound "card" interface because in earlier years this was actually an add-on board that plugged-in to a PC slot. With Laptops, that option is no longer possible so a separate intrument with typically a USB interface had to be used instead. 

The same instrument can be used in our chain to digitize the resulting signals for further processing on the PC/Laptop.

During my first baby steps in putting this system together and collecting some experience, I used this inexpensive (around 25 Euros) USB Sound Card Interface:


It worked OK as you can see in the posts I mentioned earlier, but I was not very impressed with the results. It needed further tweaking, adjusting and modifying. After getting it all working and playing with the chain of intruments and making some measurements, I moved on to other projects. So for a few years, I really didn't need this setup so didn't spend any more time on it.

New technology and upgrades broke the chain

When I recently wanted to profile my DIY Tek SG502 rebuild, I put the system together again and quickly made some changes to the inter-connect cabling that makes the connections between the units easier.

However, problems showed-up right away that I had not seen before. It was probably caused partially because in the meantime, I switched to a newer and different Laptop and I also upgraded to W10. I also could have done something wrong during some of my experiments, because I suspect that something in the USB Sound Card box could be damaged because it now shows a lot more harmonic distortion then I remember having seen before.

At first I was mystified to the cause, and could not put my finger on it. I now attribute it to a combination of my W10 Laptop, the W10 sound drivers, the W10 drivers for the USB Sound Card, plus something wrong with the drivers of the AudioTester software, because it is now crashing all the time.

Fault finding...

There were way too many variables in play so I started to address them one by one.

When I used my FY6600 DDS Function Generator, it showed THD+noise performance that looked pretty good and only a little worse than the specifications. 
My unprofiled and just finished Tek SG502 was also just outside the distortion specifications, but that was to be expected. So far so good. 
However, the ultra pure 1KHz sine wave generator designed by Victor Vickenich (vicnic) showed results that were only a little bit better than the other two, so initially I started to suspect the 1 KHz reference. 

To address my first suspect, I got in contact with Victor because I suspected a problem that I must have created with his reference. With his patient and excellent help, we found out by making some measurements on the oscillator itself, that the problem was not due to his sine wave generator. Of course it wouldn't.

I then focussed my attention to the PC software side. I have a license for the AudioTester software, but I was not happy with the overall driver situation after I switched to W10 and I also had problems with the calibration. To eliminate that aspect, I tried another software package, ARTA. I selected this, because there are many references on the web and examples of measurements made using Victor's oscillator in combination with the ARTA software, so I could start to compare. However, it still did not improve on the root cause of the problem I was having, the excessive harmonic distortion on Victor's oscillator. Have a look here...



A better USB Sound Card

It was now time to take the next step, and invest in a better USB Sound Card. After quite a bit of research, I purchased a (brand new) ASUS Xonar U7 from Amazon. To my utter dismay, I found out that the CD that came with the unit did not have W10 drivers. Also online was nothing to be found. C'mon ASUS, we're in the middle of 2017, and you have not updated the CD yet? Obviously, that did not give me a lot of confidence. Making a few measurements did not improve the situation much, so I returned the unit the next day.

Time for a reset. I realized that I lacked the knowledge to get to the bottom of this issue, so I had to learn a lot more first of all. I literally spend a few weeks going through all kinds of Forums and Blogs to see what other people were using, and to learn more about the overall system and the components in the chain in much more detail.

Eventually, after going through many blogs and forum's, I found that there were a number of USB Sound Cards that stood out. The majority were from the same company, the E-MU 0202 and the E-MU 0404 and also a few others. The good news was that they were available on eBay now and then for reasonable prices.
I decided to try to score one of these and after some miss-hits, I scored a used E-MU 0202.


Modifying the E-MU 0202

After I scored the bid for an E-MU 0202, we were out of the country for 7 weeks, so the unit would arrive but I would not get my hands on it. This gave me the time to do some more investigations, so I started to study more about the 0202. 

The ultra-pure 1KHz sine wave generator I have, was designed by Victor Vickenich (vicnic), and I found that he also published a couple of modifications to his own E-MU 0202 Sound Card, with lots of pictures together with the FFT results using the same 1KHz source. Victor was able to get incredibly good results. 

That was to be my reference now.

During my investigations, I also learned that the schematics for the 0202 are very similar to the 0404, although in a different layout and with several value changes and part numbering changes. Schematics for the 0404 are available, but I did not find good ones for the 0202. The various sources of 0202 schematics I found on the Web had errors, or were incomplete.

Since I had nothing else to do at the moment, I took the time to capture all information I could find in my schematic capture program (DipTrace), so I would have a record of the original status, and could put a description together for the modifications Victor did.

Based on the various sources and photographs of the PCB, here is the resulting schematic I was able to put together for the E-MU 0202 "B" channel input to the ADC, the well known AK5385AVF. Here is the data sheet from DigiKey.

E-MU 0202 Front-End Schematic Diagram Channel B 



And here is the slightly more complex "A" channel:

E-MU 0202 Front-End Schematic Diagram Channel A




For the time being, I left out all power related parts. I may add them when needed, because at the time, I was considering adding an external separate power input, instead of using the (typically very noisy) USB 5V coming from the PC. That's a potential project for later.

My plan was that as soon as I got home and could start to work on my unit, I would start with the modifications that Victor made to his 0202. He eliminated much of the front-end of the unit and only used the "B" channel. He also uses an attenuation of his own, so there was no need for the input section. 

I will be using my Sound Card Interface only for this application, so this is no limitation for me either. Cutting that input section out of the loop saves a number of dB's in noise, and turns the Sound Card (more accurately a digitizer) more into a tailored measurement instrument.

Modifying my E-MU 0202

The EMU0202 I scored on fleabay was supposed to be working, but the output amplifier did not. I could not find the error, but I really didn't care. I wanted the digitizing front-end, so I applied the same modifications Victor published and did on his 0202.

Here are the two links to Victor's modifications and measurement results, they start with post #171 on page 9:
Victor's modifications and results

Scroll to page 10 and post #184 to get to his contribution with the photographs and the modifications. 
Note that a little below this post is another one from him with a correction to the value of R46, which needs to be 6K8. This is in post #187.

Below is the schematic information I put together for his modifications to the "B" channel, and highlighted the parts to be removed to isolate the front-end components to ADC input.




The volume control potmeter is removed to make place for an RCA or BNC connector on the front panel. I used a BNC connector myself.
The two removed series resistors, R54 and R32, both 1K4, will isolate the front-end input from the drivers for the ADC. The two resistors that are used to create the dynamic zero balance, R35 and R41 need to be removed too.

Following are the value changes and the new additions to form the new input circuit to the ADC.


The RCA or BNC connector can be mounted on the front panel in the hole of the R-Hi-Z/Line potmeter. I had a BNC connecter and this fitted perfectly. After that, the input series resistor, the capacitor and the input Z resistor can be mounted Manhattan style. Note that Victor mentioned that this resistor can be tweaked in value to remove artifacts. I kept mine to 220K.

Three feed-back resistors change in value.
R28 : replace the 1K value to 6K8.
R46 : replace the 1K value to 1K5
R34 goes from 1M to 1K. (Victor used the original removed 1K resistor from R28 and simply soldered that on top of R34 with the 1M value.)

Finally, the connection from the output of U6-B to the input of U6-A can be created by soldering the second removed 1K4 resistor, (the originals are too small, I used a new 1K5 0603) "Tomb Stone" and with a small wire to the input of U6-A. His detailed photographs show the way.

The results are stunning

Connecting his reference oscillator to my now modified EMU0202 shows rather stunning results I think:


So now my modified EMU0202 digitizing front-end together with Victor's high quality oscillator, is a perfect reference combination for my distortion measurement applications.

As you can see from the picture below, I used a BNC as the input connector.





Here is a screenshot of the modified EMU0202 with my analog DIY Tektronix SG502:


The measured THD of 0.026% of my DIY re-build is actually better than the Tektronix specification of 0.035%. Not bad at all!

And here is the result from my digital FeelTech FY6600-30 Dual Channel Function/Arbitrary Waveform Generator:



So with the described changes to my setup I finally have solved the issues I had in the chain. 

Next step will be to add in the Pete Miller Soundcard Interface and do some more measurements.

It may take a while, I have a few other projects I'm working on, but stay tuned for more...

Enjoy!

If you like what you see, please support me by buying me a coffee: https://www.buymeacoffee.com/M9ouLVXBdw




Friday, September 15, 2017

DIY build of a Tektronix SG502 Sinewave Generator

Because I sold all of my Tektronix gear, I was a bit sad to loose three particular instruments.
One was the DIY 5CT Curve Tracer with my readout modifications that was already covered in another post, the 5A22N Differential Amplifier, and the SG502 Sinewave Generator.


When I was at Tek, I build several instruments from parts, the SG502 being one of them.



I really liked the SG502, for its simplicity using analog(!) discrete parts only, and the overall specifications that made it perfect for most if not all of my applications. This instrument covers the frequency range from <5Hz to >500KH and has a pretty good distortion performance with only 0.0035% THD between 20Hz and 50Khz. The output is 5V RMS open circuit or 2.5V RMS into 50 Ohm. It also has a good step attenuation ranging from 0-70dB. Finally, it has a 5Vp-p square wave output that can also function as a trigger out.

The reason I seldom used it over the last years was that it comes as a TM500 plugin, and that uses a lot of real estate. The TM500 series are very deep, and they are heavy. I don't have room for them on my desk or counter anymore, so eventually I cut the umbilical cord and sold everything I had from Tek. Well, not really everything (sorry Raymond). I kept a few goodies.

I also have the so called Victor Mickevich Ultra Low Distortion 1kHz sinewave generator with a reported 0.00001% THD for special measurements (also described in another post: simple-but-precisice-1khz-distortion-tool), and I recently purchased the FeelTech FY6600S-30 14 bit DDS. That instrument is a Dual Channel Function/Arbitrary Waveform Generator, very versatile, but, it's digital...

If you consider building one, the SG502 has a few critical or rather special (unobtainium) parts. One is the dual FET (Tek p/n 151-1021-00), used for the input differential amplification, a J-FET (Tek pn/ 151-1054-00) used in the AGC, and the precision dual 10K tracking pot and satellite adjustment contraption, used to set the frequency. Less critical is the matched capacitor set (10uF, 1uF, 0.1uF 0.01uF and 0.001uF). They are used in the bridged T notch filter in a rather clever dual purpose way. The last rather special item is the SG3501D (156-0208-00) IC that is the center of the dual tracking +/- 20V supply. I recently found that this IC died in my SG, but I was able to order replacements on flea-bay.

I happened to have all these items as "spare" parts, and kept them since the early 70's, waiting to be used again. I did not have the special matched set of timing capacitors, but individual ones and testing revealed that they were so close and precise that they didn't need any further matching or adjusting.

Just to kinda take care of my guilt in letting my trusted and self build Tek gear go, I decided to rebuild the SG502, but in a much smaller enclosure. I use the TEKO KL22 enclosure in black/aluminum a lot for my projects. They cost less than 15 Euros, and have just the right size for most of my projects. I used them already for my three power supplies, my DC load, and now my SG502.

I first dabbled with the idea to upgrade the design with modern OpAmps and see if I could improve on the specifications. After thinking about this for a while, I decided not to. I could have build the SG505, Tek's own upgrade to the SG502 which uses OpAmps, but in my opinion, this classic should stay the way it was designed in the early 70's. Period! 

BTW, the PG505 has been described as a real masterpiece of analog design wizzardry. The instrument was designed by Bruce Hofer who now is at Audio Precision, and is a true analog design genius. So, my statement "I could have built the SG505", must be taken with a handful grains of salt. It will not be easy to replicate that instrument.

In contrast, rebuilding the SG502 instrument turned out to be rather simple. If you follow some common sense design and layout rules, anybody with a little above average skills can do it. If you can't get your hands on the critical parts, you could try to find an SG502 unit on flea bay or on one of those surplus markets where they sell old electronics. Sometimes these SG's can be bought for less than 20 Euro's. Because all components are THT, it's real easy to harvest and use the most critical parts.

I'm not going to cover the design, you can find the Tektronix Instruction Manual online, it has everything you ever wanted to know about this instrument. One thing you should note is that Tek made some important changes to the original design (especially the ACG, the voltage supply and the output attenuation circuit), and I used the latest available Change Reference (M34075 from 1-19-79) in my redesign.

One of the challenges is to get or replace the S50 push button switch set for the frequency selection and also the S160 push buttons used for the output attenuation. I used the same technique again that I already used for my DIY 5CT, and that is by using (reed) relays to do the switching. This will allow you to use inexpensive single deck rotary switches, in combination with diode matrices if required.

Here is the schematic of the range switching for the frequency selection:


Here is the schematic for the AGC damping (top) and the output attenuation:


After completing the unit, and testing it, I noticed a "design flaw" in my output attenuation switching design. When you switch between especially the higher attenuation settings, there is a short moment in-between the "clicks" that the output goes back to full scale. I need to add a delay to the relay fall-off times, to create the equivalent of a make-before-break action, change the rotary switch to make-before-break, or add a master output relays contact that prevents glitches to the DUT in-between setting changes.

I also redesigned the power supply somewhat. Normally, the SG502 uses the big transformer, diode bridge, electrolyte smoothing capacitors and the power transistors from the TM50X mainframe. I measured that the original SG502 uses about 70 mA on each 20V supply rail, so I could get away with a much simpler design.

First of all, I am a big fan of not putting mains transformers into the measurement enclosures. It keeps the hum out, and you don't have to deal with the bulky transformers, the main switch, filter, bridge etc. It allows me to use smaller enclosures, and put the transformer and the needed other stuff in a separate box that I can put someplace out of sight or away from my precious desk or bench space. Another benefit is that I can get multiple usages out of these transformers/supplies boxes.

Here is a photo of the 24-0-24V AC 160mA transformer box :



I didn't produce a schematic for the supply, so let me describe what I did. In the enclosure above, I put a 24-0-24VAC 160mA PCB transformer. I put a dual pole switch, a fuse and a neon indicator lamp on the primary side. On the secondary side, I connected the 24-0-24 AC outputs to 4mm binding posts.

Because the current demands are so small, I put the rectifier (1N4002) diodes directly on the 4 mm binding posts in the SG enclosure and also mounted the two smoothing caps (1000uF/50V) Manhatten style on them. On a little circuit board, I mounted the power section directly from the SG502 manual, and used two smallish power transistors that I had for the series transistors. I selected the voltage setting resistor (R348) to get as close as possible to the +/- 20V DC. Tek also hand selects this resistor, and I ended up with a value of 14K7, probably due to the fact that I used different power transistors.

To drive the two sets of (reed) relays, I wanted to balance the transformer and rectification loads a little, so I used an LM317/LM337 pair with 270 and 820 Ohm resistors to get + and - 5V rails. The +5V section is used to drive all the reed relays for the frequency selection and AGC dampening, and the -5V drives the 3 output attenuation relays. The grounds of both the 5V supplies are not connected to the analog ground on the analog circuit boards.

Here is a photo of the power board:


The left section on the board deals with the +/-5V and the right side with the +/-20V. I just happened to have the SG3501D chip, otherwise I could have used another set of LM317/337 to obtain the +/- 20V supplies. With these  more modern components, I really don't believe the supplies need to be tracking, because the LM317/337 are stable  and good enough.


The oscillator section is mounted on the main board, and looks like this:

Top left is the AGC damping section with 5 reed relays. On the right half is the frequency selection with the two sets of 5 reed relays. The large 10uF precision capacitor is mounted on top of the 1uF and 0.1uF capacitors to save some space. On this picture, the two output transistors(Q82 and Q83) are still the (isolating plastic!) 2N3904 and the 2N3906, they have been replaced by the 2N2222 and the 2N2907 metal can transistors after I was happy with the performance and took the picture. (Watch out for the different pin-out between these transistor types, as I forgot myself (;-o) )


I'm showing the backside with the rats nest, because it's a testament to the quality of the original design that I could stay well within the specifications without using a properly laid out PCB.

The output amplifiers for the sine wave and the output attenuation, in addition to the square wave generation and amplifier are on a separate board.


This board will be mounted through the output level potmeter to the front panel, and also on a stud to the main board.

The rest is mounted directly on the front panel:
The open hole on the left is for the output potmeter, and the hole on the right for the power LED.


To the left is the 5 position  rotary switch for the frequency multiplier then the special potmeter with the fine adjustment hardware contraption, and to the right the 8 position rotary switch with the diode matrix for the output attenuation (0 to -70dB in -10dB steps).

Together it looks a little bit cramped, but it fits easily.


The cool ribs you see on right at the outside of the back are not needed. I just stumbled on this old adhesive CPU cooler, and added it to the back initially, just in case.


And here is the front panel in detail:



I typically make a design of the front panel in PowerPoint, together with the drill map. I print the design on a color printer, using the best photo paper I can find, and in the highest resolution and best color quality.
I use double sided tape to secure the front panel and use a very sharp knife to cut the holes, I then carefully, without twisting the front layer of the paper, mount the hardware. Note that I try to use the same color scheme Tek used in the 70's. I really like it and use that for all my designs.


And here is the final unit.

While I was building the various sections, I was checking and verifying the results. When I was at Tek, I used to repair these instruments and I was amazed how well the original design worked, even with my modifications and wire nests and felt proud to have been part of this bit of T&M history. In retrospect, I'm glad I started on this project.

Using the procedure in the Instruction Manual, I verified everything as good as I could. I don't have a distortion analyzer or dedicated spectrum analyzer so I can't specify the distortion level. I used the FFT capability of my Rigol DS2302A scope, and that looked very good.

A small bit of info:
I went from using Tek equipment to a Rigol scope. Well, you probably didn't know this, but the Rigol subsidiary for the America's is located in Lake Oswego, which is only a few minutes from Beaverton, the home of Tek. You wonder why Rigol picked this location? (;-))

At a later date I will try to do a comparison with my Mickevic oscillator, my active double-T notchfilter together with an external sound card and my PC based analyzer software.

Everything else, except the rise/fall times for the square wave (>50nSec instead of <50)  is well within specification. I have not investigated this edge issue yet, for me it's good enough.

I did tweak the capacitor for the 50-500kHz range to match the other ranges. The 100Khz signal is the most critical, so I went back and force a few times to adjust the value of C55 the timing capacitor so when I switch ranges, the frequency setting is well within the specification. The original C565 value is 87pF, I ended up with 92pF.

What that means is that when I set the frequency setting to the (reference) 100.0MHz in the X100K setting, and switch to the X10K setting, the frequency is 10.17kHz, in the X1K setting 1.03kHz, in the X100 setting 101.7Hz and in the X10 setting 10.2Hz. That is excellent I think.

One caveat, and you may have already missed it. I don't have room for the frequency dial. First of all, I don't have one in the first place, but because I normally attach my scope anyway, it has a digital read out of the frequency setting, so I don't need the dial.

Even though I did not have the calculated precision resistors for the attenuation switch, I got really close by getting the closest E96 resistor value or selected a couple, and again, I was able to stay well within the specification.
If you're interested why Tek used these funny resistor value: have a look here :
matching-t-attenuator-calculator (use the 600 Ohm input/output setting and you'll see that the values match exactly to those that the Tek designers probably calculated with a slide ruler (;-))

One thing I need to do still, is to order copper sheet metal and use that on the inside of the (plastic) enclosure. Whenever I use my T12 solder iron, the high frequency pulses from the heater come right through. That's not just this unit alone though, but I want to create an extra barrier for this one.

All in all, I am mightily impressed with the design quality the Tek engineers at the time were able to pull off in the 70's. Rebuilding this unit and staying well within the original specification is again a testament to their skills. Hats off!

UPDATE 22-11-2017

After a lot of issues, I was finally able to create a vastly improved setup to measure FFT's, so I can now present the THD distortion number. A measured THD of 0.026% is even better than the specification for the original, which is 0.035% for the 20Hz to 50KHz range.
Here is the screenshot:


Enjoy!

If you like what you see, please support me by buying me a coffee: https://www.buymeacoffee.com/M9ouLVXBdw


Monday, September 11, 2017

Designing a single Li-ion cell UPS for the Raspberry Pi




A friend and I set out on a quest to design a UPS for the RPi that really(!) worked for higher power applications.

Most designs, even the ones I have listed on this blog, suffer from a lack of true output power for the more power hungry RPi Models, like the Model 3, or when there are enough devices feeding off the USB port bus.

You will be hard pressed to find UPS designs, even commercial ones, that are able to supply and sustain more than 1 Amp to the Rpi. I purchased a couple, and all of them ended up in the "salvage for parts" bin. They were useless.

I designed several ones myself over the past years, and currently use the last version every day on several of my classic Model B's. I used it for my file server and web server and also while I'm doing designs and tests. It helps to protect my work and the SD card, of course.

This latest version is described here: rpi-power-supply-with-1-button-start
However, as with so many others, it is limited in the amount of power it can supply. As I said, most of them max out at about 1 Amp. I can only use it on an almost idling Model 3.

There are a few major challenges you need to overcome. One is the booster design, the amount of current and then there is the heat factor. Consider that at a minimal 3V Li-Ion cell voltage, the booster needs to get upwards of 5A from the cell to be able to supply 2.5A at a minimum of 4.8V to the RPi.

My mouse-pal friend Bud Bennet (an ex analog chip designer from Linear Technologies) and I (no qualifications to speak of), set out to design a solution. In the process, we actually designed a few solutions and stumbled several times to get it right.

Here is the blog with all the details:

single-cell-li-ion-powered-ups-for-raspberry-pi


After designing this one, and using it, Bud could not leave good enough alone and build the ultimate version:


If you like what you see, please support me by buying me a coffee: https://www.buymeacoffee.com/M9ouLVXBdw

Tuesday, August 22, 2017

DIY build of my Tektronix gear and a 5CT1 Curve Tracer

When I worked at Tektronix in the 70's, I build my own 5440 Oscilloscope and TM503 mainframe.

What that means is that I collected every part and soldered each one of them myself. Some boards came from scrapped units, so I also scavenged parts.

This in itself was not a very unusual project at Tek, but I believe I was the only one at the time to build a 5440. Several others build portable scopes, but since I repaired the 5000's, I was very familiar with them. I purchased many parts from the Tek factory in Heerenveen, and in the employee shop in Beaverton. Some very good friends at Tek helped me to get the printed circuit boards, and several special hardware items, which were otherwise not available. I remember that one time I went on a business trip to Beaverton with one suitcase, and came back with two. Imagine the explanations I would have to do at customs at Schiphol...if I was stopped. The CRT was a reject from a customer (there was a dust spectacle on one of the plates that caused a funny light reflection effect. A colleague of mine spend days ticking on it and shaking the tube to get it off the plates. Eventually he gave up, and it was decided to scrap it. So, I had my tube. When I build my scope, I never saw that effect again... go figure)

I took the transformer from a unit that had been on fire, so that was another item I got. One of the 5000 frames was bent when a customer dropped his unit. I replaced the frame, and was lucky to be able to bent the old one in shape again without breaking it. It is now my TM503 frame.

The list goes on and all in all it took me several years, but here it is:


The measurement system consisted of the following parts:


Tektronix 5440 Plugin Oscilloscope System.

D40 Single Beam Display Unit
5403 3-slot Mainframe with Display Readout Option
mainframe is typically 70MHz, could display up to 100MHz.


5A48 Dual Channel Amplifier with 50Mhz bandwidth

5B42 Delaying Timebase, to zoom in on a specific area of the trace.



5A22N Differential Amplifier (down to 10microVolt), with LF & HF filters and DC offset.



TM503 Mainframe
This is a TM503 with original parts, but build in a 5100 lower section scope frame.
It has no middle section, and therefore no handle. I usually have the scope on top of the TM503, so I don't miss the handle.
One the left:
DM502 Digital Multimeter, VAC, VDC, ACmA, DCmA, Ohms, Temperature(C/F), comes with 2 temperature probes and a spare tip
 
In the middle:
PS503A Dual Power Supply, (0 +20V, 0 -20V with tracking, 0-400mA and fixed 5V 1A)
This is a self-build copy of the original unit with lots of original Tek parts.


To the right:
SG502 RC Oscilator, 5Hz to 500KHz low distortion (<0.0035%) sine wave and square wave output
This is a fantastic unit for audio work, which is what I did most of the time, back then.



 

5CT1 Curve Tracer
After I left Tek, I had no time to spend on my electronic hobby, so everything was mostly stored, and followed along with the several moves we've made. Somewhere around 2000, I wanted to finish an instrument that was on my to-do list for quite some time, a Curve Tracer. Some parts were still waiting for this project, but since I left Tek, I had no access to parts anymore, at least not for reasonable prices.

I had no access to a front panel either, so I made my own, but missed the engravings for the settings. I also didn't have the customary drum rotary switch that Tek uses a lot, so I used normal single deck rotary switches. These were connected through diode matrices that drove little 5VDC DIP relays, and these mimicked the finger switches. I used the same technique to add a read-out capability for the Vertical Amperes/Div. and for the Step Amplitude. The ./. 1000 switch was added to the mix, so the display also changed from milli to micro etc.

The instrument uses a transformer to obtain voltages up to 300V. At first I used a normal 220V transformer in reverse, but I was not too happy about that. This transformer had no shielding either, so I got some extra switching noise. When I had an opportunity, I had a special transformer made. There was no information about the transformer Tek used, so I had to do some reverse engineering and the transformer company did the rest. In the end this trick worked really well and the unit is within specifications.

This is how it looks like (left compartment):
Note that since I added the readout capability at a later date, I should have renamed the 5CT1N to 5CT1. The "N" suffix in Tek parlor means "No Readout".

BTW, in case you wondered, the 7CT1N is exactly the same unit, but has a mainframe adapter board to make the circuit fit in the 7000 mainframes.






 Here I'm measuring the voltage drop of a 1N914 diode, showing 0.7V (at .5V/Div)


This is the breakdown voltage of the 1N914 at 20V/Div 


This is the voltage drop of a Schottkey diode.


Here is a trace of a 12V Zener diode (at 2V/Div)


This is the characteristic of a 2N2219 transistor.


This is the breakdown voltage of the 2N2219 at 20V/Div.
Note that the traces show in different intensity, but is caused by the sweep rate of the scope and the way I took the picture.


This is a screen shot of a tunnel diode, used quite a bit by the Tek designers in the 60's and 70's.

This is a BS170 MOSFET


Right now, I'm selling all my Tek equipment and parts. I don't have the space to keep them on my bench, and it's too cumbersome to create the needed space to use them for measurements. I have a new scope now, and I have no need for the DM501 and the PS503 anymore.

I will miss the SG502 because of the good quality sine wave forms (0.0035% THD). I may rebuild that instrument in a different enclosure at a later date, maybe with OpAmps. (one more item added to the already long Bucket List)
Update, I just finished that project, look for it on my blog.

And I will also miss the 5A22N differential amplifier capabilities and the Curve Tracer, although I'm using less and less older parts that need checking or verification. Parts are so inexpensive these days that swapping them out is a no brainer, certainly compared to the old days.

Update : I also started a project to replace the 5CT1/7CT1 Curve Tracer


If you like what you see, please support me by buying me a coffee: https://www.buymeacoffee.com/M9ouLVXBdw