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Old 25th November 2012, 07:33 PM   #881
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Originally Posted by scott wurcer View Post
Time of year to find plenty of C7 bulbs.
Referring to this article: Injection-lock a Wien-bridge oscillator

EDN Glen Brisebois - October 31, 2012

Injection-lock a Wien-bridge oscillator | EDN
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Old 25th November 2012, 07:56 PM   #882
RNMarsh is offline RNMarsh  United States
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I think I might use that AM detector input as the notch output port. I will never use the AM det. so i can just unsolder that wire and replace it with the notch output and have a nice convenient front panel port.
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Old 25th November 2012, 08:21 PM   #883
davada is offline davada  Canada
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I think you are saying that the auto set level is done with an LDR pair at the input to the notch filter that maintains the level there at 3.16V RMS. The LDR's will be a limiter for the distortion. You could upgrade them to the Silonex equivalent which are supposed to be better or use several in a series/parallel network to reduce the voltage drop across them which will reduce the distortion. The distortion is voltage (or maybe current, I'm not sure) dependent so a smaller signal is lower distortion.

A constant level in the notch is pretty important for the auto-notch circuits to work. If you tap before the level meter attenuator for an external FFT you will have a pretty predictable level/known distortion level on the monitor out. If your FFT has low enough noise and the notch circuit is low noise then you should have all you need.

The FET opamp probably has better common mode rejection than the original which will reduce the internal distortion.

Hi Demian,

This first screen shot is with the 339a in level mode and the second is the same input level in distortion mode all from the monitor output.
Attached Images
File Type: png Level mode.PNG (59.8 KB, 193 views)
File Type: png Distortion mode.PNG (65.2 KB, 188 views)
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Old 25th November 2012, 08:55 PM   #884
davada is offline davada  Canada
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Originally Posted by RNMarsh View Post
I think I might use that AM detector input as the notch output port. I will never use the AM det. so i can just unsolder that wire and replace it with the notch output and have a nice convenient front panel port.

Good idea Rick.

We have to test the theory first. I haven't tried this.

If we use the monitor output switched to the notch output then the Am input can be used to monitor the RMS detector output or what ever your preference. This will give an exact rms input level to the notch filter and scaling is simple from here.

If padding is going to be used it is best to place it right at the output of the notch filter amp. This offers isolation of the op amp to the external.

The wide band amplifiers used in the 339a are prone to RF parasitic oscillation so we have to be careful about what we do.
I will touch on this in another post.
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Last edited by davada; 25th November 2012 at 09:05 PM.
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Old 25th November 2012, 09:37 PM   #885
1audio is offline 1audio  United States
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I now follow. Its similar to the auto set level in the ST1700 series. The input is manually ranged (detail I had forgotton) and the output has a small AGC range to track the input across a 5:1 range or so. In the St17XX it can be defeated with a manual knob. You are indicating that the 339 doesn't have a manual adjustment? Still an AGC across that range should have very little distortion. The second harmonic you show is typical for LDR's. (The Lightspeed attenuator guys don't seem to see this?)

Looking at the circuit finally I see how it works. I think the easy change would be to get two or four matched Silonex LDR's and convert. Also returning the ground on the Distortion analyzer LDR to a more optimum location should reduce the crosstalk and distortion coming in via that common connection. Remember the specs for the HP339 were -80 dB, not -100 dB+. Good engineering generally stops pursuing a infinite goal when you meet the specs with reliability and good yield. Improving in this case may just be attention to details like this.

Does anyone have specs on the "photomodule"? or measurements of resistance vs current? What is needed is a cold, off DCR. Preferably after it has been completely off for 4-5 hours. Then 100 uA, 1mA and 10 mA will give a good clue as to its R vs I curve and matches can be found. That stage should have less than .1 % distortion. Actually, since the fundamental has been removed its not clear where the distortion is getting, in except from the notch filter itself. Noise will be a function of resistor values and the noise from the source. Look for really low noise FET opamps for the notch?

Checking the system calibration with a second oscillator mixed in at -40 or -60 dB will help with reality verification.
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Old 25th November 2012, 09:45 PM   #886
1audio is offline 1audio  United States
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Also tieing another resistor to the gate of Q3 on the auto set level board and to an appropriate supply will allow the AGC to be shut off and the stage becomes unity gain. Minimal mod fix.
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Old 25th November 2012, 10:02 PM   #887
davada is offline davada  Canada
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Originally Posted by 1audio View Post
I now follow. Its similar to the auto set level in the ST1700 series. The input is manually ranged (detail I had forgotton) and the output has a small AGC range to track the input across a 5:1 range or so. In the St17XX it can be defeated with a manual knob. You are indicating that the 339 doesn't have a manual adjustment? Still an AGC across that range should have very little distortion. The second harmonic you show is typical for LDR's. (The Lightspeed attenuator guys don't seem to see this?)

Looking at the circuit finally I see how it works. I think the easy change would be to get two or four matched Silonex LDR's and convert. Also returning the ground on the Distortion analyzer LDR to a more optimum location should reduce the crosstalk and distortion coming in via that common connection. Remember the specs for the HP339 were -80 dB, not -100 dB+. Good engineering generally stops pursuing a infinite goal when you meet the specs with reliability and good yield. Improving in this case may just be attention to details like this.

Does anyone have specs on the "photomodule"? or measurements of resistance vs current? What is needed is a cold, off DCR. Preferably after it has been completely off for 4-5 hours. Then 100 uA, 1mA and 10 mA will give a good clue as to its R vs I curve and matches can be found. That stage should have less than .1 % distortion. Actually, since the fundamental has been removed its not clear where the distortion is getting, in except from the notch filter itself. Noise will be a function of resistor values and the noise from the source. Look for really low noise FET opamps for the notch?

Checking the system calibration with a second oscillator mixed in at -40 or -60 dB will help with reality verification.
Hi Demian,

I don't think the distortion from the auto set is very significant at the tiny levels coming from the notch filter. However this is an lm318 following the notch filter. Not the best by today's standards. What your suggesting will help the 339a analyzer, but I still think shortening the signal change is the answer. There is still the question of the LDR in the notch filter and I think we can still do better with a different op amp here as well.

It's not my goal to hot rod the 339a more than I have because I don't think this form of analysis is the best. Notch then FFT is what works for me. We can make good use of what we have with the front end of the 339a which is what's missing from our sound card FFT arrangement. So any improvements are focused here.

Additionally, the phase detector can use some attention as well. At bit more gain at the input might bring the notch down more and getting the noise down some may help because the at this point the notch is near the noise floor and there's nothing left to work with.
Also the band width of the input op amp to the phase comparator is a bit narrow which effects the higher frequency null. Actually with sound card and FFT the null doesn't need to be that deep but noise is an issue.

But all this needs to looked at in more detail.
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Old 25th November 2012, 10:07 PM   #888
davada is offline davada  Canada
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Originally Posted by 1audio View Post
Also tieing another resistor to the gate of Q3 on the auto set level board and to an appropriate supply will allow the AGC to be shut off and the stage becomes unity gain. Minimal mod fix.

These all great suggestions which don't require hacking the 339a at all.
Minimal is good.
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Old 25th November 2012, 10:14 PM   #889
davada is offline davada  Canada
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Also tieing another resistor to the gate of Q3 on the auto set level board and to an appropriate supply will allow the AGC to be shut off and the stage becomes unity gain. Minimal mod fix.

There is a test point sitting right beside U10 + input so we don't even have to do any soldering here
and test point 1 is on the output of the RMS detector.
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Old 25th November 2012, 11:38 PM   #890
davada is offline davada  Canada
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Default Test point 8 of 339a

With all else the same this is a screen shot of test point 8.
The output of the notch filter. As you can see the noise floor is much lower but
still has the x3 scaling from the input amplifier. With this scaled down 3:1 we have a nice spectrum of the input.

The second shot is with the input attenuator stepped up to the next 3Vrms range.
The harmonics change proportionally but the notch didn't change as much.
Definitely needs to be attenuated on the output side of the notch filter.
It looks like it needs to be scaled down about 45dB to get the true value which seems kind of odd. I'll need to see what the actual level is going in to the notch.

This is the residual noise floor of the input, buffer and notch filter. The notch won't go any lower than this. This is pretty much the bottom line for this analyzer as it is.

Overall this is much better.
Attached Images
File Type: png TP8.PNG (64.6 KB, 173 views)
File Type: png TP8 3Vrms.PNG (63.1 KB, 174 views)
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Last edited by davada; 25th November 2012 at 11:43 PM.
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