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Created 6-14-2012

Updated 8-11-2012

Re-greening the Heathkit IG-18: IG-18 #4
The IG-339B -- an HP 339A oscillator using the IG-18's tuning network

What can you expect from this build? --
I have three spectra for you to view -- one at 1kHz fundamental, one at 10kHz, and one at 100Hz. The output signal from the IG-339B was 6.7VRMS for each frequency. This output was then adjusted to 1VRMS (0dBV) with an attenuator pot and fed into the analysis test system. The three plots were taken with the feedback and distortion cancelling pots unchanged from plot to plot, so I consider their settings optimized.

These spectra were plotted using my Active Twin-T notch filter to reduce the fundamental to -60dBV, while leaving the harmonics unchanged. The signal was then fed into an E-MU 0204 USB sound module connected to my PC. The ARTA Audio Analysis software then plotted the resulting spectra. The test system linearity is excellent over a 130dB dynamic range, and very good to -140dB. The sigma-delta analog-to-digital converter gives a rising noise slope above about 40kHz in the 10kHz spectrum, but the important harmonics show up well.

The system can capture harmonics up to around 88kHz, so it works fine for all important products in a 10kHz fundamental, and is OK at 20kHz, but not great, since the 5th H. is missing.

Here's the 1kHz plot:

IG-339B distortion at 1kHz

The calculated THD is 0.12%. Divided by 1000 to account for the -60dB fundamental, this means the distortion is 0.00012% referred to the 0dBV signal level, and that's really very good. The only significant harmonic is the 2nd -- the canceller adjustment has done its best, but it's not good enough -- if this peak could be reduced 6dB, then this version would be the full equal of the A version and the HP 339A.

Here's the plot for 10kHz:
IG-339B distortion at 10kHz

The calculated THD is 0.46%, or 0.00046% referred to the 0dBV fundamental. Again, very good. And again, it's the 2nd H. that is the largest. The best canceller pot setting for 10kHz is also the best setting for lower frequencies.

And here's the plot for 100Hz, where line frequency products get in the way a bit:

IG-339B distortion at 100Hz

The calculated THD is 0.16% or 0.00016% referred to the 0dBV fundamental. This number is inaccurate because the signal's 3rd H. at 300Hz falls under the line-frequency multiple of 300Hz. A slight retuning of the oscillator to say 110Hz would reveal that the distortion is about the same as at 1kHz or a bit lower even.

The goal --
This project was intended to explore the relation of the Bridged-T tuning capacitor ratio to overall performance. Much of the work on the IG-339A is germane to this "B" version of the HP 339A oscillator. Please see that page for discussion about the HP 339A oscillator and the build of an IG-18 that uses the circuit and tuning cap ratio of the HP 339A. Since the PC board I used for the two IG-339s is based on Steve Lafferty's IG-18SL mod project page, please read his excellent report on his mod of the IG-18 to the HP 339A circuit -- I use his part numbers, which are also mostly those used by HP, in discussing the variants.

Importantly, the HP design uses a very large 100:1 ratio of the tuning capacitors in it's Bridged-T frequency selective network to achieve a relatively high-Q peak in the oscillator's amplifier. This has some advantages in controlling the bandwidth of the oscillator to help reduce distortion. But it also requires much higher open-loop gain and unity-gain bandwidth from that amplifier, because the amp's gain needs to be around 34dB. This means that the feedback margin is lower at higher frequencies. With a maximum frequency of 110kHz, does this matter for this oscillator?

As noted on my other IG-18 mod pages, the Heath IG-18 uses a 10:1 tuning cap ratio for it's Bridged-T network, which only requires an amp gain of about 16dB, increasing the feedback margin. Using the Heath 10:1 cap ratio provides some other benefits -- the Heath 2% tolerance caps can be used, as well as the Range switch with its clever switching that uses 5 caps to do the work of 8. However, the Range switch needs two more poles in order to do the AGC filter switching needed for the HP 339 circuit. But I will have suggestions to make about this presently.

A trial run --
The IG-339B 10:1 cap ratio system ran nicely from 300Hz to 100kHz during the build of IG-339A. Some resistor values on the PC board needed to be different from those in the 339 circuit used by Steve Lafferty in his IG-18SL mod because of the big change in gain. I added some socket-pin receptacles so that I could easily plug in different resistors and different JFETs, and all my opamps are socketed. I'll have more to say about the IG-339B parts values later.

Do the lower tuning cap ratio and the higher signal level on the AGC JFET really make a big difference? --
Having the 10:1 cap ratio implicitly means that the gain of the oscillator amp drops from 34dB to 16dB, from 51 to 6, which means a different, higher resistance range is needed for the JFET in the AGC system -- I've chosen to keep the feedback leg resistance pretty close to Steve's and HP's value of roughly 4k ohms in order to minimize loading on the oscillator amp and reduce distortion.

But this change in gain means that the ground leg resistance of the feedback network has to be much larger than in the IG-339A -- roughly 750-800 ohms instead of about 80 ohms, and it also means a higher resistance compliance range is needed for the JFET. It also means that a higher AC signal level will be on the drain of the JFET. It has been reported that lowest distortion results from the lowest possible AC signal level at the JFET's drain -- the effect of this is to minimize FET channel-resistance modulation. I can say that I find this may be true, but how much does it matter?

A partial answer --
Here's the IG-339A version at 1kHz, which closely matches the HP 339A. Note that the plot level has been boosted with 20dB of gain in the Active Twin-T, so the important number is the calculated THD, 0.000091% referred to the 0dBV fundamental:

IG-339A distortion at 1kHz

The 2nd H. is about -123dBV, and the 3rd H. is about -133dBV. These products are each about 3dB lower in the IG-339A than they are in the IG-339B plot at the top of the page. That is not a huge difference in performance.

Given the results above, why did HP use such a large tuning cap ratio? --
An interesting question. Clearly HP engineers thought it mattered, but the excellent Krohn-Hite 4400 and 4402B use a Bridged-T circuit that has a 2:1 ratio of the bridge and pillar components, and their performance is said to be in the same area as the HP 239 and 339 -- I actually have a K-H 4500, but it's circuitry is somewhat different and it only gets into the 0.001% area at 1kHz. The answer to this question can only be guessed at. On paper it clearly seems to be a good idea, and if getting the last ounce of performance out of the oscillator is important then it seems to be the right thing, at least a low frequencies.

But the HP engineers didn't have the wonderful range of opamps available to us today -- the only amp that would work well was the incredible Harris HA-2625, and HP used it to great advantage. It may be that the HA-2625 works best with the large tuning cap ratio. This is a hypothesis that I won't be testing. Steve Lafferty used the HA-2625 in his mod, but that amp requires a boatload of compensation components for stability, and the great new amps like the LT1468 and OPA1641 do not require these elaborate networks.

Sorting out the final parts values --
Please see my IG-339A page and to Steve Lafferty's page for reference to the changes here. Steve used 5% resistors in his mod and changed some values quite a bit compared to those used by HP (the HP 339A circuit diagrams and manual can be downloaded as a big PDF file from the BAMA test equipment manual website; there's also a copy on the Agilent manuals website, but that copy is missing the oscillator section!).

I have used the HP parts values pretty much uniformly, unless I just didn't have the HP values; then I subbed in different values -- this only happened with AGC filter caps. Steve's mod uses several dual opamps, and that's fine, but I used a two-single amp to dual converter socket in testing to see which amps worked best in which locations. the opamps used in this version are the same as those for the A version, except I've ended up using an OPA1642 for U4. So this IG-339B actually is the IG-339A re-configured to use the 10:1 Heath tuning caps and different resistor values for R52 and R53 -- everything else is the same.

Keeping the existing feedback-leg components meant their center value resistance was 3.9k -- I use a 3.4k fixed resistor for R31 and a 1k pot for R30 -- which means for a gain of 6 (about 16dB), the ground leg resistance would need to total around 780 ohms. Right away, it was clear that because of the relatively high resistance compliance range needed for the JFET, the choice of JFET is important. I can happily report that the PN4092 works just fine.

The resistors that limit the compliance range of the JFET, R52 and R53, obviously had to be larger. I initially thought that just raising them each by a factor of 10 would work, but after much fiddling, I settled on using 1.58k for each -- use 1.6k if you intend to use 5% resistors, but you may need one size bigger or smaller for either resistor depending on tolerances.

The distortion cancelling pot can reduce the level of 2nd H. distortion. It is placed in series with either R50 or R51, which are equal at 2.7k in Steve's build and 2.00k in HP's design, but one of them can be made smaller by 1/2 the pot's value. then the pot can find the best distortion cancellation of the JFETs channel resistance modulation, which reduces the 2nd H. distortion. You will need a spectrum analyzer to see this, but then you'll need the spectrum analyzer to actually measure the performance anyway -- whatever hardware THD analyzer you have probably won't be able to resolve it.

Building the Range switch --
As noted, the stock Heath range switch has two poles (well, not exactly -- it is a complex switch) and four positions, so the modded switch needs four poles -- two for switching the 5 tuning caps, as designed by Heath, and two for switching the two sets of AGC filter caps. I luckily had an extra Heath Range switch, so I just added its two decks to the existing IG-18 Range switch. But I had to reduce the spacing between decks. I had a bunch of aluminum spacers in my odds and ends, so I used what I had -- a trip to the local hardware store will likely yield up a great assortment of aluminum, brass, and plastic tubing and spacers that can be used.

The extra one or two decks can be scavenged from an old -- or new -- switch that has similar diameter decks with 30° rotation detents. A single deck with two poles and four positions will do nicely, IF the rotors align with the Heath frame so that the right contacts make and break, or you can use two decks, each with at least one pole and at least four positions, again making sure about contact alignment. Of course decks with more positions will make it easier to get an alignment that works with the Heath switch frame and original decks -- this is not a trivial issue, so two single pole, 12-position decks will give you the best shot at getting things to work. The alternative is to build a completely new Range switch, which is not as good since you will need two sets of four caps -- 8 in all -- instead of just 5 caps.

Here is a pix of the Range switch. See the chassis and PC board pix on the IG-18 #3 page for comparison. Note that the AGC filter caps are the same as in the IG-339A, and that the dual-DIP adaptor socket has been replaced by the OPA1642 opamp on a SOIC-to-DIP header:

IG-339B Range switch

Adjustments --
The adjustments are the setting of the feedback pot, the setting of the distortion cancelling pot, and setting the meter cal pot. The feedback pot should be adjusted at 1kHz for a DC voltage on the gate of the AGC JFET of about -0.6V. Then the range switch should be moved to each position and the gate voltage checked -- it should never be less than about -0.2VDC, but could be much higher, as much as -2V or more. The actual voltage depends highly on the precision of the cap tuning ratios -- the caps should be sorted to 1% value match or better.

For example, it is easiest to measure the Heath caps, find the biggest, then match the values of the others to 3 significant digits by adding smaller caps in parallel -- if your biggest is 508nF, for example, then pad the others up so their values are close to "508" too. With 508 as the value to be matched, for example, 1% matching will mean values from about 503 to 513.

I made the huge mistake in this unit of padding a couple of the Heath caps with some Chinese ceramic caps. The result was horrible 3rd H. distortion -- 20dB higher than now. I'm guessing they have a strong piezoelectric effect. So only use film caps (polypropylene is my fave, or mylar/polyester, or polystyrene) or silver-mica caps for padding the original Heath caps.

The distortion cancelling pot should only be used if you have a test setup than can see the level of the 2nd H. -- otherwise, make R50 and R51 as equal in value as you can.

IG-339B wrap-up --
After fiddling with resistors, JFETs, and the feedback pot, the overall performance of the B version has distortion under 0.0005% from under 100Hz to 10kHz, and I also think that lower frequency distortion will be well under 0.001%. This unit has 100kHz THD of 0.015% as measured by my HP 339A, while the HP 339A measures itself at 0.012%. You can decide whether keeping the caps from the IG-18 (and padding them for consistent value, if needed) and then adding two more decks to the range switch is overall easier than just building the A version as I and Steve did.

I think that the overall performance of the B version is amazingly good. But what about the questions of the importance of the large tuning cap ratio and the signal level on the JFET's drain? I think the importance of the large tuning cap ratio is a non-issue -- I just can't see that it matters at all. But I do think that the slightly higher levels of distortion in the B version compared to the A version is due to the AC signal level increase on the JFET's drain, especially the relatively higher 2nd H. where we might expect the 3rd to be the highest.

Update 8-11-2012 -- Simple shielding for the power transformer
As noted in the page for the BIG-18, IG-18 #2, using simple shielding for the power transformer greatly reduces line noise artifacts in the oscillator's output. So I decided to try the mod with this unit. The Heath power transformer mounts fairly easily in a 3-1/2" x 3-1/2" x 2-3/8" electrical junction box with a cover plate. I put 1/2" long 6-32 threaded aluminum spacers over the transformer's mounting screws, drilled mounting holes in the back of the junction box and used four 6-32 x 1/2" screws to hold the transformer in the box. I added wires for the primary connections and extended the secondary lead lengths to reach the PC board. I drilled holes through the back of the IG-18 chassis in order to use the junction box's cover screws to hold the junction box to the back panel of the IG-18. The junction box cover neatly sandwiches between the box and the IG-18's back panel. A 3/8" hole through the box cover and the back panel lets the wires through to the inside.

This is pretty crude magnetic shielding by any measure, but the results are very, very good. The spectrum below shows that the overall level of line-noise artifacts is really low -- most of the spikes have been lowered by 20dB or more, compared to those in the 100Hz distortion spectrum shown at the top of the page. Such low noise, combined with any high-pass filtering in the analysis system designed to reduce the effects of hum, will just completely remove line noise from THD calculations at 1kHz and above. The spectrum has a log frequency scale to better show the line noise:

IG-339B line noise with transformer shield

I think this is a remarkable plot (and the 1kHz distortion performance is pretty remarkable too). It might be just as well to use a "wall-wart" supply and feed pre-regulated positive and negative DC voltages to the board's DC input pads. But this was a pretty easy and inexpensive mod to make and the results justify the mechanical work needed to make it happen.

But why is the distortion lower too? Two reasons -- 1) It may be because I replaced four of the five original Heath mylar tuning caps with polypropylene and silver-mica caps -- all except for the 5uF cap for the X1 frequency range; I don't have a big polypropylene for that yet. The quality of the caps really does matter, it seems.

And 2) I inserted a 47 ohm resistor between U1's output on pin 6 and all the rest of the circuitry. I had to cut a board trace to do this but that was easy enough. This isolating resistor gives the opamp a little easier time when driving the tuning circuit's capacitive load. My friend David Barber has suggested that this resistor could be as large as 100 or 200 ohms for even better results.

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