DIY Build of the Tek SG505 Oscillator

 

The rather famous SG505 oscillator

I'm going out on a limb here. Although I already built a copy of the SG502, described here, the SG505 is a very different beast all together. At the time I did the SG502, I was not ready to attempt the SG505, a masterpiece of analog engineering by wizard designer Bruce Hofer. After a number of years mulling about it, I was recently looking at the instrument again after seeing it used in a YouTube series, and this time decided to give it a try.

As with many, if not most Tektronix designs, they incorporate special parts, or the parts are obsolete by now. The added difficulty is that with these older designs (this one was created in 1980) that pre-date the internet, you can't find any datasheets for these parts. Most manufacturers went defunct or merged and do not show these old parts on their websites.

So when looking at the SG505, I first wanted to try to build the sine wave oscillator. I used the information from the Option 2, that has the balanced outputs, because that has improved schematics and parts.

Here is where you can find a lot of information about the SG505 and it's designer Bruce Hofer.

So what is so special about the SG505

First of all, among specialists, it has the status of a real piece of analog art, designed by one of the great masters himself, Bruce Hofer. While at Tektronix, Bruce was responsible for many products and designs in the 7000 oscilloscopes and the TM500/5000 series. He also designed the AA501, the first automatic audio analyzer that worked in tandem with the SG505. As a matter of fact, I understand they designed the AA501 to verify the SG505 after production. In the manual for the SG505, you can see the rather elaborate method to calculate the total harmonic distortion, and the equipment needed. 

Just the specification of 0.0003% THD from 20Hz to 20KHz will give you an idea of the design quality. 

The SG505 has at least one patent that I know of, describing a "State variable oscillator having improved rejection of leveler-induced distortion". Have a look here : patent.

When Tektronix management was not interested in taking the AA501 and later the AA5001 distortion analyzer concept further, Bruce co-founded Audio Precision and that is the current and undisputed leader of this field.

You may ask why I'm so interested in rebuilding the SG505? First of all, it's a great design made by one of the few analog wizards. It is very well documented and also has been manufactured for several years so the selection of components went through this test. This "fitness for manufacturing" should not be underestimated, because the parts (source, value, tolerance, tempco, etc) in every possible permutation will have to meet the specification of the final product over the years. In other words, the bugs are out. This cannot be said about many other DIY designs.

The other reason for selecting the SG505 is that you can freely select any frequency by just selecting the desired range, and using the dial and vernier adjustment to select the frequency you want. There are several other pure oscillator designs that are more modern, and even have better specifications. In almost all of their applications, they are used in a THD measurement, and in those cases, you "only" need a few fixed frequencies. That makes it easier to get better specifications because you can tune both sine and cosine oscillators with fixed parts. None of them use an analog variable frequency selection.

Some Small print:
As I normally do on my Blogs, I describe the way I get to the final design, with trials and errors, ups and downs, warts and all, but hopefully in a way that you learn something, and avoid my mistakes. With my projects, it's not going to be an IKEA hand-holding type design. Some effort is required. And most importantly, there are no guarantees it will work for you too, I'll do my utmost but that's about it.

I will add information to this Blog and the Github as I go, and with this pretty extensive project, it may take a long time. As a matter of fact, I may not be able to finish it with the sought after quality, so take that into account before you heat-up your soldering iron or start to order parts. I'm not a really experienced engineer, and I'm not hampered in any way with specific know-how or experience, nor do I have the proper equipment to verify and test the design. I'll do my best, and will make you part of the process, but that is as far as I can promise.

Interesting articles and information:

There are many articles about sine wave oscillators, because building a simple one is easy, but a really good one is a real challenge. Over the years I have collected a few interesting ones:

Rod Elliot has a lot of interesting information on his site, but there is one particular posting where he explains the various architectures with their pro's and con's.
https://sound-au.com/articles/sinewave.htm

A few other articles can be found on my Github site here:
https://github.com/paulvee/DIY-Rebuild-of-the-SG505.

Then there are several Forum posts that deal with the modernization and rebuild of the Bob Cordell THD Analyzer that contains a very good oscillator, just like the SG505 (that original article is on my Github). The latest and most impressive rebuild of that instrument that is 30 years old, but now with a processor to drive the measurements is on this Github from Emile666, a very impressive and complete design. Also have a look at the diyAudio site, there are designs that deal with specific 1KHz and 10KHz pure oscillators of outstanding specifications (search for "LT AN67 1KHz or 10KHz oscillator", from Frex) The AN67 application note is listed on my Github. The author of that impressive design is the rather flamboyant Dale Eagar, who worked for my buddy in crime Bud at LT. Small world.

Component Issues & Challenges

For this particular project, let's discuss the road-blocks and challenges for a rebuild, based on the components that are used in the instrument.

First off, there are several special parts and some of them are no longer available on the market.

The potentiometer that sets the frequency is a high quality single turn dual 10K wirewound version without a stop. It was originally made by Spectrol, but is now part of Vishay. The part used is a Model 100 with the following part number: 100B2-103-103-XXXX

According to the datasheet, it has +/- 3% tolerance and the independent linearity between the two sections is only +/- 0.5%. Unfortunately, the datasheet does not specify the THD. This part is still active and manufactured it seems, but nobody has it in stock. 

The next item is a mechanical part that is used in combination with the potentiometer, to create a finer adjustment by reducing the number of rotations 6:1. The Tek part number is 401-0161-00 and the manufacturer is Jackson Bros (London). Their part number is listed as 4511/DAF in the SG505 manual, and believe it or not, you can still buy this part for a reasonable price, here. This reduction unit also gives a very smooth and controlled action on the potmeter. Highly recommended!

I am the lucky owner of having the potmeter and the reduction unit, it is used in my SG502, but for others, I'm going to try to find a replacement part or method for it. It's the most critical part of the design so it would be critical to find an alternative. I have some ideas using other potmeters, even digital ones but I have no experience with them whatsoever so I need to give that a try when I have a functioning oscillator.

The next item in the list is the special drum switch (or cam shaft) that Tek manufactured themselves that select the output attenuation. This in addition to the special pushbuttons that are used to select the frequency multipliers. In my design for other Tek instruments, like my 5CT1 (below) where I added readout capabilities and also for the SG502, I used reed switches or a small relay in combination with a simple rotary switch and diode matrixes to re-create the sometimes rather complex switching arrangements. 

Luckily, the SG505 switching used is quite simple. At first I was contemplating using analog CMOS switches or MOSFET's, but I understand that will add too much resistance or add to the THD distortion budget, so I will most likely use reed switches again.

Next-up is the J-FET used in the AGC circuit. The Tek part number is 151-1021-00 and is used in many Tek products. In the manual, they show the FN815 from Siliconix. I have not been able to find any information about that part at all, and I know I'm in good company because of the frequent use in other products. Many makers are looking for replacements. There is another J-FET 151-1025-00, an SFB8129 made by TI that is also used in many designs. This J-FET is used in the power supply as a current source, but that one is a lot less critical. Lastly, that same J-FET is also used in the output buffer amplifier. 

One of the most critical components, in terms of the distortion budget, is the special hybrid substrate that is used to select the dBm output settings. I happen to know that the construction used in the SG502 is the source of some extra distortion, so Bruce Hofer created an unofficial modification to change that for the SG502, significantly reducing the total harmonics from an original 0.035% THD to well under 0.002%. My assumption is that using discrete components is going to be a challenge, but on the other hand, we now have very good SMT devices Bruce didn't have at the time.

Lastly in this list is the set of matched timing capacitors used in the oscillator phase-shift amplifiers. Luckily, for a DIY project, the absolute values that are matched in the SG505, are less critical. In a manufacturing process, you don't want to tune the product, it has to work right of the production line with as few adjustments as possible. The critical factor for the original design was to create a situation by which the frequency dial is set to say 1, and by selecting the X10, X100, X1K and X10K, you would would get the exact reading of 10Hz, 100Hz, 1KHz, 10KHz and 100KHz +/- a few percent, but we don't really care do we? Besides, we don't have the frequency dial anyway. We will need to work around that.

I already have a little and simple side-project working, using an Arduino Nano and a small OLED display to measure and show the frequency.

Simplifications for the DIY design

From the outset, I will be concentrating on the sine wave oscillator itself. I'm not planning to add the differential output that the Option 2 provides. What I will eventually use as the sync output is still open. All I want is a way to trigger my DSO on the signal, and an edge of a square wave is actually more stable to trigger on than the slope of a sine wave. 

I will also not implement the Intermodulation Module because I don't see the need for it in my projects.

Next up is a more simple instrument setup. The original SG505 has to function in a TM500 or TM5000 mainframe, together with a host of other instruments right next to it. Some of them can be very unruly and noisy neighbors. The SG505 is completely floating from real earth ground inside this environment. My plan is to keep it that way, but build it in an enclosure that has no earth ground connection anyway, so it will be floating from the outset.

The power supply is a critical element in the chain and will deserve a lot of attention, but I will address that later. I will use my lab dual power supply in the beginning.

My Plan

My plan is to build a prototype PCB that will give me a functioning oscillator that will allow me to test and try some different strategies and components, and build a reference platform for my measurements using my mostly home-brew set of inexpensive tools. Some if which I already have, some I may need to build to verify and test the specifications. I envision something similar as the design of the VBA Curve Tracer, where we went a step at a time over the course of about two years.

After going through the documentation many times, and watching YouTube videos and strolling the internet for information, I started by creating a schematic in KiCad. I then spent many long hours pouring over the Replaceable Electrical Parts list selecting and finding the parts for the prototype. I used Google, Mouser, DigiKey and LCSC for days, and eventually decided on a BOM to order from LCSC because of the total price including S&H. It turned out that they did not have one particular resistor value from all the parts I need, so I ordered a combination of two to get the missing part.

I did not save money on the parts but selected the best quality. All resistors are 1% metal film, better than the original THT resistor specifications. I also matched or exceeded the higher precision resistors (like 0.1% at 25ppm). However, I did not use Mica capacitors, but good quality NPO with the same 1 or 2%  tolerance. The electrolytes are all Tantalum except for the two used in the supply rails.

Almost in parallel, and now knowing the parts and footprints, I started to design a PCB and ordered it as well. Both the parts and the PCB's will be on their way soon. Talking about on their way, another "way", PCBWay has agreed again to sponsor me with this project, and I'm grateful to get boards from them for free and with a very high quality. I may need to go through a number of re-designs while progressing so this support is very helpful.

The schematic for the prototype

This is after I fixed the discovered errors.

The component names and values are the same as in the original schematic in the Option 2 manual. Where they do not, or I had to use other parts, I indicated that.

However, I deviated quite a bit from simply redrawing the original schematic in the manual, to show the many intricate and not-so-obvious feed-back and feed-forward connections, as well as the many compensation circuits, especially around the inverting Opamp U1510. This is where most of the magic happens.  I suggest you read the Theory of Operation in the manual and also the patent application for the feed-forward invention, that allowed Bruce to significantly reduce the harmonics. In essence, he applied his wizardry to remove the fundamental frequency AC signal from the AGC circuit DC-level in several places. I know that the picture above is not of great quality, but a Github project is available here and I will upload files as the project progresses.

The three hierarchical boxes in the schematic are "hiding" the frequency and dampening switches and parts but are inconsequential at this moment. I just followed the original design, but used jumpers instead of switches.

The 2-layer PCB prototype

The layout of the PCB largely follows the schematic, so there is a full circle with the circuits following the State Variable design as is in the schematic as well.

I made the decision, right or wrong, to use ground fills on both sides of the PCB. We'll see if that will show up as an issue, in which case I need to go to a "star" grounding method with all ground going to the output BNC connector.

For this prototype, I will be using jumpers to select, add and try components and also activate the capacitors for the frequency selection and the AGC damping. For the frequency selection, I will use a normal dual (stereo) cermet 20K potmeter, just to have a functioning oscillator. 

J-Fet Selection

I added two kind of foot prints for the J-FET so I can use SMD and TFT to experiment. The TFT version has a socket. I actually have the original but now elusive unobtanium J-FET, and I also have a Curve Tracer, described here, so I can verify or measure the specifications if needed. I already ordered or had a small selection of J-FET's to try.

The elusive, unspecified & unobtanium 151-1021-00 J-FET

While waiting for my parts, that were returned to sender because a very bright DHL delivery person could not find the apartment I lived in for 17 years, I started to look for alternatives for the 151-1021-00 JFET. I already had a number of candidates, and more are in the shipment so I could get started. As a spoiler alert, I think I have been successful, but I now need to test the alternative 2N4391 in the real circuit, and compare the results with the original 151-1021-00 part.

Because I did not do a write-up for testing JFET's with our Curve Tracer yet, I added that episode to the following document. Scroll down to the end in that posting to see the JFET result.

Making measurements with the VBA Curve Tracer

Capacitor Selection

For the frequency setting capacitors, I used good quality Polypropylene film capacitors, but now that modern NPO and COG ceramic capacitors are available in good quality and with very precise tolerances, I may want to experiment with them as well. I will most likely trim the Polypropylene values to match by using the ceramic parts, and measure the effects.

What are the chances of success for the rebuild?

I'm very anxious to see what the results of this prototype is going to be. When I built the SG502, I was pleasantly surprised that my rats-nest construction (no PCB) produced a slightly better THD specification than the production version one had. A testament to the conservative specs and the quality of the original design. 

Who knows what I find with this one...

How am I going to verify the results?

As I stated earlier, I do not possess all the proper equipment that is required to really verify the SG505.

I will have to do with less and see how far I can get. If needed and possible, I can build something, like the passive notch filter (067-0938-00) that the Tek engineers designed specifically to test the SG505 performance. 

I already have an active T-notch filter for 1KHz that I may need to duplicate or extend for the other frequency ranges (10Hz, 100Hz, 10KHz and 100KHz), we'll see about that when we get there.

Luckily, I have a kind-off reference oscillator, the Viktor Mickevich 1KHz design that has a very respectable THD. He claims (verified by others), a whopping <0.00001% THD. 

With my simple DIY equipment, (look at this post and this one) I could measure the THD for his oscillator as far down as 0.00096%, so that is a very good start.

For the rest, time will tell.

Possible solutions as an alternative to the dual 10K potmeter

As I mentioned earlier above, this frequency adjustment potmeter is a very critical part that is crucial for the operation and more importantly, the specification of the oscillator. The specification needs to be tight because of the "State variable" design of the oscillator. Each section of the pot sets the frequency for the two 90 degree phase shift circuits (states). If there is a difference in the resistance between the two potmeter sections, it will negatively impact the phase shift. The timing capacitors also have an equally important role, but they can be more easily matched to each other or the value tuned by adding parallel capacitors.

The combination of the original 1-turn 360 degree 10K potmeter together with the 4511/DAF 6:1 reduction allows the sensitivity or rather resolution to be as follows. For the potmeter, 10K / 360 degrees is 27.7Ohm per degree of rotation, further reduced by 6 to 4.6 Ohm per degree of rotation. And then there is the vernier adjustment of +/- 1% of the frequency. 

Looking at it from a frequency perspective, with the frequency multiplier at 1K, the potmeter can adjust the frequency from just below 1KHz to just over 10KHz. That translates to 1,000 Ohm for every 1,000 Hz, or 1 Ohm per Hz. 1 degree of rotation is 4.6 Ohm and therefore 4.6 Hz. The vernier range is 1% of the frequency so that translates to 10Hz at 1KHz and 100Hz at 10KHz. That will be not so easy to duplicate.

So, provided you can't get your hands on the original Model 100 potmeter, what are the possible options or alternatives?

After quite some time looking at alternatives, I came up with a few potentials.

1. Use a dual (stereo) cermet potmeter
With the right 6mm shaft diameter, it can also be used in combination with the 4511/DAF 1:6 mechanical reduction. The challenge is that these potmeters do not have very tight specifications. A good quality one will still cost about 25 Euro's and at best has a 10% specification, but I could not find a specification for the linearity between the two halves, and that is the more critical specification. An overall value miss can be adjusted with resistors at the beginning or end, but not in-between. This type potmeter is what I'm going to use with the prototype. I can use a trimmer to make sure both halves have the same value at a particular setting.

2. Use a dual 10 turn precision potmeter
This will give you a much higher resolution in setting the frequency, and you don't need the 4511/DAF reduction. The vernier circuit can stay as it is. The specifications for tolerance (3%) and linearity are better than for a cermet potmeter, but there is a significant price differential. A suitable one will cost about 46 Euro's. The downside is that you need to turn the knob a lot (10x) to go from the beginning of a range to the end. Even with this quirk it could still be the most viable solution. I searched at Mouser for candidates, and by the time you have made the criteria selection, there are only 4 left, ranging in price from 46 all the way up to 157 Euro's. 

The least expensive one is the Bourns model 3549S-2AA-103/103A

It has a linearity specification of 0.2%, an overall 3% tolerance and 50ppm/C. This is pretty close to the original Model 100 that has 0.5%, 3% and 20ppm/C.

3. Use a digital potmeter.
I have no experience with using them, but I first thought this could be the solution. So after reading a lot about these devices, I found that there are actually a few stumbling blocks for this particular application. With most devices, the voltage range of the potmeter is limited to 0..5V. The regular maximum granularity is 256 steps (there is a special one with 1024 steps). This means that from a 10K value, every step will be 39 Ohm. That's 8.4 times more than the original resolution, so the vernier adjustment has to be modified to cover the difference. That can be done, but digital pots have other issues, like the linearity and tolerance, and then there are the resistance of the wiper and, a bit less important, the end and beginning resistance. When you change from one setting to the next, there could be a jump or glitch in the frequency, although there are ways (zero crossing) to avoid or suppress that. On top of that they are not that good with temperature changes. 

Based on our particular circuit, one of the best solutions could be the TI PGA2310 designed for hi-end audio stereo volume control. It has excellent specifications, even removing some of the issues with other digital pots. The chip is not cheap however, with a price of  about 28 Euro's.

Most importantly, these digital pots will need some form of digital logic to drive them. Normally, you would use a rotary encoder as the input, and use logic or a processor to drive the electronic potmeter settings. I have not discounted this solution, it provides some potential but it's not my favorite to develop at the moment.

4. Use a rotary switch.
When using a rotary switch, you need one with a double deck and one pole per deck with at least 10 positions, so you can create 10 segments of 1KOhm each by using fixed resistors. By using tight specifications for the resistors, you can really get the best linearity and matching between the two sections and probably also get the best Tempco results, surpassing the original design. The downside is that you can't smoothly turn through the whole frequency range, you have to select a section (1/10th of the range) and than use the modified vernier (modified to 10x the adjustment) for a finer resolution within the section. You can select shorting and non-shorting versions for these switches to avoid large frequency jumps when you switch. 

The suitable devices I found will set you back by about 45 Euro's. This is a rather complicated solution for a rather cumbersome user experience, so not my favorite to develop. For almost the same 45 Euro's, you could also buy the dual 10K Bourns.

Summary:

Depending on your particular application for the oscillator, there are a few options you can select from.

Frequency Display

As mentioned above, because we don't have, or will have a dial, I'm planning to use a small OLED display to show the frequency of the SG505.

Here is the prototype with an Arduino Nano and the tiny display I'm planning to use. I like the yellow color. The dimensions are 0.9" with 128x32 pixels.  I tested the FreqCount library from Paul Stoffregen with my function generator, and it easily covers the range of 10Hz to 200KHz.

The boards arrived.

I received the boards from PCBWay within a week of posting the Gerber files. They are really fast.

There is no gold plating, that is the light fixture playing tricks. The quality of the boards is great, as I came to expect from PCBWay. They also sponsor me for this project which is highly appreciated!

Update on the parts ordering saga:

Unfortunately, the parts I ordered at LCSC are still in limbo. They have been returned by DHL to the European distribution center and will then be sent out again. Hopefully this time DHL selects a more intelligent driver that can find the apartment I have lived in for 17 years. In his ultimate wisdom, he determined that I don't live here, and instructed to have the shipment returned to sender. DHL was unable to reverse this mindless decision. Great! The order was placed on March 4 and I'm told it may take another 1-2 weeks before I can get my hands on them. I just received notice (April 2nd) that the parcel has been re-discovered and this time I instructed DHL to deliver it at a pick-up point so I can pick it up tomorrow. Finally.

Well, on April 3rd, I finally have the parts in my hands and can continue with the project.

To create the best possible frequency selection, I selected the best matching capacitors for the timing ones. Because my minimum ordering quantity was 5 for the 1uF and 20 for the 100nF, 10nF and 1nF all with 5% tolerance, I could easily get matching pairs, but not precise values. That can be tweaked later if needed, but I don't think I need that. The matching between the values is much more critical for the distortion budget.

I also should mention that I'm using the Tektronix 151-1021-00 JFET in the AGC circuit. I'll investigate alternatives later.

First power-up and test

I used my new hotplate reflow solder station to solder all the SMD parts, and that went very well. I then added all the THT parts, and after thoroughly soaking and cleaning the board twice and inspection with a microscope, I tested the power rails for shorts. No issues, so I then used my Lab supply to provide +/17V at 50mA each and gave it a go. My DSO was showing a nice sinewave! Great!

While going through the range selection with the jumpers, I noticed that I goofed with the schematic and subsequent layout for the 10K multiplier selection jumper J15. A few trace cuts and jumper leads fixed that. I also noticed that turning the frequency potmeter didn't really change the frequency. That was caused by me flipping the 3-pin connector the other way around for one half of the potmeter on the PCB. Swapping the connector leads also fixed that. I now had a fully functional sine wave oscillator.

Frequency selection

Because I use a "kind of" dual 10K potmeter (see above), but with separate adjustments for either half, my plan was to see the effect of "in-equality" of the two pots, and adjust them to be exactly the same by using an ohm meter. The goal was to see what effect the in-equality (not only the linearity) would be on the distortion. I will get to that later. However, because of my goof with the connector swap, I stumbled on an optical verification of the in-equality or tracking. When you swap the connector, so one half of the pot will drive one phase change circuit with an increasing value, and the other phase change circuit with an "equal" decreasing value, the frequency should stay at the same value. But only if the two pot halves track each other 100%. Mine does not, there are slight changes in the frequency when I turn both pots together. 

Here is what I'm seeing. At fully CW or fully CCW, the frequency is almost the same at say 308Hz. During the rotation of the potmeters together, I see the frequency dipping down to a minimum frequency of 259Hz somewhere at 75% of rotation and then rather rapidly going up to 308Hz again. My calculator shows that to be an error of a whopping 19% in frequency which is quite a lot. 

To put that in perspective, the 10K potvalue creates a frequency range between 83 and 1.100Hz, or 1.017Hz. That means 9.83 Ohm/Hz, so a tracking error of 308-259=49Hz/9.86Ohm=5.96% of 10K. That's more reasonable, but it shows how important the tracking specification is. And we don't even know yet what the distortion contribution of this tracking error is.

I will do that same measurement with my original Model 100 precision potmeter later.

Does it meet spec.?

I use my dual lab supply to supply +/17.00V. The current consumption is 24mA for the positive rail and 26mA for the negative rail.

The sine wave output is 2.04VRMS (6.00Vp-p) into 1MOhm , and that is according to the specification.

The frequency ranges are within specifications.

• 10x     8Hz to 111Hz
• 100x   83Hz to 1.1KHz
• 1Kx    833Hz to 11.7KHz
• 10Kx  8.8KHz to 107KHz

When I go beyond 107 KHz, the sine wave collapses, so that's really the maximum.

The Vernier adjustment at a frequency of 10KHz goes from 9.92 to 10.12KHz, also good enough.

The AGC circuit works well, visually, but I did not test that any further.

When I set the frequency in the 10x multiplier setting, and adjust the frequency to mid-range and then select the other multiplier settings I see this:

• 10x     50Hz
• 100x   515Hz
• 1Kx    5.26KHz
• 10Kx  55.2KHz

This to me a fine, but can be further tweaked by using trimmer capacitors.

So what is the distortion?

I had some issues to find and load the drivers for my modified Creative EMU-0202 USB "Sound Card" (look at this post for more information), after I went to a new Laptop a while ago after a major W10 induced debacle. Most of the installed software had to be installed again, but I didn't need the FFT capabilities until now.

After some experimentation, here are the results with the PCB just lying on my desk. This is with the range setting at  x100 (100-1000Hz), and using jumpers instead of the frequency select potmeter, to eliminate the leads and the potmeter tracking differences. That puts the frequency at the top of the range at about 1.1KHz. I'm using a DIY 600 Ohm -10dB attenuator, and the E-MU0202 and the ARTA software is calibrated with it. Note that this is taken straight from the oscillator output, and not from a "real" output amplifier. I do not yet know what impact the 600 Ohm loading is for the circuit.

Not a bad result I think, but not near the specification of <0.0008% in the 20Hz-20KHz range.

After several hours trying, searching, testing and pulling some valuable hair, I now know why these results are too far away from the original specification. First of all the settings in the ARTA software.

Second, the 600 Ohm -10dB pi attenuator I was using to feed the E-MU with a lower input signal is indeed loading the oscillator circuit and is the cause for the higher THD distortion. Rather than showing all the previous results I made, I'm now reverting to the setup with the 100K loading of a potmeter at the output of the oscillator circuit so I can reduce the signal amplitude. 

This is the result:

The specification is <0.0008% THD so we're significantly below that figure now.

As a reference, I also made a measurement using the Victor Mickevich oscillator, and that shows this:

It seems my measurement setup is OK, apart from the larger amount of noise. Earlier measurements from a few years ago using the same modified E-MU0202 and oscillator, but with older versions of the software and drivers and a different laptop, produced seemingly better results so something is a little different.

I also tried my Arduino frequency counter, and although it works fine, just taking the input through a capacitor from the circuit output produces a lot of harmonics, so it needs a separate buffer amplifier. The oscillator prototype itself also needs a buffer amplifier, so I'm going to add that and see how the counter behaves then.

Here is a measurement using the potmeter to adjust the frequency to a precise 1KHz.

Apart from a little bit more noise (six long leads to the potmeter) there is no difference to the THD, so I'll continue with the potmeter installed from now on, and revisit the possible effect on the THD by the tracking tolerance later.

Output amplifier circuit

Because of the loading on the circuit by as little as a scope probe (added noise) and the sensitivity to loading the circuit with an output impedance (adding harmonics), I quickly put together an output amplifier circuit. In hindsight, I should have added that to the prototype, but I didn't...

I am now using the Arduino based counter to show the frequency so I don't need to use my DMM or DSO anymore, reducing the amount of loading and adding noise. With the potmeter at the output, I can now set the voltage level for the E-MU 0202 so it does not show harmonics. The input voltage to the E-MU needs to be around 1Vrms to have the least amount of distortion.

Distortion and the supply rails

Form what I can gather, there is no need to have tracking supply rails with a high accuracy. I could reduce the positive supply by 1V without  a change in the THD. However, the circuit is more critical to the negative supply. If I lower that by 1V, the is a 0.002% change in the THD+N. Lowering both rails to +/- 15V has an even larger effect. I initially thought that the increase of the rails from 15 to 17V was due to the Option2 output amplifier, but I now think that they raised the rails to get a little better THD result.

Power Supply

I've finished a simple textbook separate mains fed power supply for the prototype that has the +/- 17V rails and also the +5V for the counter, and later the relay/reed switch section.

I'm using a toroid transformer that has two separate 24VAC windings.

To get the +/- 17V rails, I'm using the LM317/377 adjustable voltage regulators, and added a trimmer to adjust them. The 5V rail is tapped from the transformer in a way to reduce the loading or digital influence. That voltage is too high for a normal LM7805, so I'm using another LM317 that can have input voltages up to 60V to get the 5V. 

The grounding for this prototype is simple. Later on I will probably split the two windings and create separate supplies for the +17V and -17V, and use a star ground at the output connector for the analog circuits. The digital circuits (the Arduino Nano and the relays/reed switches) will be kept on the 5V rail and that GND will be as separate as possible.

Running an FFT of the +17V rail with a DC blocker shows that there is a bit more work to be done:

The 50Hz mains comes through, and there is coupling back to the rails from the (1KHz) frequency of the generator.

Granted, it's all just lying on my desk without any shielding from my other equipment (10MHz master clock, and 2 x 10MHz GPSDO) or the quite noisy environment (switching power supplies, DSO, DMM, WiFi transmitter).

After some thinking about the power supply, I decided to make a detour and give the SG505 a more worthy supply. This quick-and-dirty-put-together supply was a failure, so I ripped it apart. 

I have been aware for years of the so called "Superreg" design from Walt Jung from a few decades ago. He designed it predominantly for audio projects, but hey, if it's good enough for critical hi-end audio applications, it should also be adequate for the SG505. At least that is my current thinking.

I don't want to add that project to this blog post, so I'll start a new dedicated one. Superreg

Distortion and the potmeter tracking

This effect turned out to be more difficult to measure than I anticipated due to the significantly changing THD numbers that are reported by the ARTA software. Changing the value of one of the potmeter halves is more difficult that I anticipated, due to the minute rotation change and the large change in frequency.  To get a better visual handle on this, I used my DSO in the X-Y mode and use the two input channels to look at the output of the two 90 degree phase shift Opamps. It should produce a circle, which it does, but you can't really see enough of a change relative to the changing distortion numbers. In my opinion, it is safe to say that the closer the specification for the tracking is, the better the distortion numbers will be. The overall tolerance of the 10K potmeter value is less important. If needed you could add small resistor values at the bottom and/or top of the potmeters to create equal minimum and maximum values without disturbing the tracking.

Distortion and the JFET selection

That will be next on my list.

My current test bench

Stay tuned, there will be a lot more to come now that I have a functioning prototype...

A Github repository is available here it will be updated with information during the project.

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

For those that already did, thank you!