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Old 16th December 2012, 04:19 AM   #1171
1audio is online now 1audio  United States
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David:
First here is some good background on how the linearization works: http://www.vishay.com/docs/70598/70598.pdf Looking at HP's schematic the linearization is ac coupled (60 pF cap) which means it changes with frequency. Possibly both a trim for DC and a cap coupled (at the wiper) trim for ac in parallel. The sketch shows what I am thinking- R4+R2+C1 are the ac (hf) distortion trim and R3 + R1 are the DC (LF) trim.

Attached is the circuit from the KH4400. It really works but you need something like the 725 to really use it. The trimmer really nulls the 2nd harmonic right out. It takes feedback from the voltage across the fet in positive phase along with a small negative phase (trim resistor) which seem to find the right balance. There are some caps for HF compensation.
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File Type: jpg KH4400 AGC.JPG (30.3 KB, 137 views)
File Type: jpg AC+DC feedback FET AGC.JPG (25.2 KB, 136 views)
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Old 16th December 2012, 05:40 AM   #1172
RNMarsh is offline RNMarsh  United States
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Hi Demian.... there is an R and C in series across the FET (gate to Drain) as well as in series with the gate. The C value is large --60 Mfd.

I had put a trimmer in place of each R and adjusted. It helped but not a lot and couldnt null out the H2. Changed the 60Mfd to bipolar as well. Maybe a more elaborate network as you show with J1.?

Right now, if i use the X1K range (optimised for that range) and adjust the freq dial from 1 to 10.... It'll cover most freq of interest; 1KHz to 10Khz. The ShibaSoku monitored the 339A generator shows good ageement with the 339A analyzer.. even though the levels are all small and in the lower 10% of the meter range. Good meter.
I get same reading value on both instruments meter displays. Nothing measurable above H2, btw: HP339A at 1KHz is .00023% thd+n and 10KHz it is .0008% thd+n. This is almost usable. I need .0001 or better. I havent replaced some opamps which could further reduce the THD. [And all filters IN on 339A analyzer.]

I can now try the FFT on the 725 analyzer using the .00023%, THD, 1Khz freq. 725 already told me its all second harmonic, though. The 725 will display the level of 2,3,4 and 5th harmonic as well as the total. Will measure Victors osc as well. Thx-RNmarsh

Last edited by RNMarsh; 16th December 2012 at 05:43 AM.
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Old 16th December 2012, 06:02 AM   #1173
davada is offline davada  Canada
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[QUOTE=1audio;3286832]David:
First here is some good background on how the linearization works: http://www.vishay.com/docs/70598/70598.pdf Looking at HP's schematic the linearization is ac coupled (60 pF cap) which means it changes with frequency.

Yes this is exactly the problem. You can see it monitoring the control voltage on the gate.
It changes with range and frequency.

I can use the summing amplifier which drive the jfet gate in the AGC to sum the signals in.
I'll give it a try.

Cheers,
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Old 16th December 2012, 06:12 AM   #1174
davada is offline davada  Canada
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Quote:
Originally Posted by RNMarsh View Post
Hi Demian.... there is an R and C in series across the FET (gate to Drain) as well as in series with the gate. The C value is large --60 Mfd.

I had put a trimmer in place of each R and adjusted. It helped but not a lot and couldnt null out the H2. Changed the 60Mfd to bipolar as well. Maybe a more elaborate network as you show with J1.?

Right now, if i use the X1K range (optimised for that range) and adjust the freq dial from 1 to 10.... It'll cover most freq of interest; 1KHz to 10Khz. The ShibaSoku monitored the 339A generator shows good ageement with the 339A analyzer.. even though the levels are all small and in the lower 10% of the meter range. Good meter.
I get same reading value on both instruments meter displays. Nothing measurable above H2, btw: HP339A at 1KHz is .00023% thd+n and 10KHz it is .0008% thd+n. This is almost usable. I need .0001 or better. I havent replaced some opamps which could further reduce the THD. [And all filters IN on 339A analyzer.]

I can now try the FFT on the 725 analyzer using the .00023%, THD, 1Khz freq. 725 already told me its all second harmonic, though. The 725 will display the level of 2,3,4 and 5th harmonic as well as the total. Will measure Victors osc as well. Thx-RNmarsh
Hi Rick,

Try working the 'Amplitude Adjust' R30 along with the distortion trim.
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Old 16th December 2012, 05:21 PM   #1175
RNMarsh is offline RNMarsh  United States
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Default 4 zero's and a 2 --

I have a variable twin-t filter and I applied it to the 1KHz signal from the 339A. I tuned the H2 out and now the distortion is -- .000025% (4 zero's) ! [maybe mostly noise.. have to check with FFT]

Though it will be great to have it all done in one box. I'll follow on further lowering; espec that H2. Thx-RNMarsh
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Last edited by RNMarsh; 16th December 2012 at 05:25 PM.
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Old 16th December 2012, 05:49 PM   #1176
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The B&K is also lowering the 3rd and up harmonics (and the fundamental) so its not clear what you are getting. The 725 will go down to .00001% with the ag15. Too many zeros and its easy to get confused.

To tweak the distortion of the HP just select H2 on the 725 and tweak for the lowest.

Attached is a plot of the distortion from the AG16 measured with an AK5394A demo board. All of the harmonics are -124dB or below. This is probably the limitation of the AK5394A since the 725 indicates much less distortion.

I think -120 dB and below for harmonics becomes a point of no real return for effort. No analog recording system ever came close and this is possibly the best digital today.
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Old 16th December 2012, 05:58 PM   #1177
RNMarsh is offline RNMarsh  United States
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I factored that all in. The input level is adjusted to be at 100%. Some 3rd plus may be attenuated... dont care as it and the other higher H are Much lower than the H2. Besides, thats all for the better.

Now, I'm back to the same situation as before.....but now with the 725; On the lowest range and at 10% of the low end of the meter. Wonder if I can get another -20dB somewhere. One more zero. -Thx RNMarsh
[just kidding]

My interest is in being able to accurately measure my own amplifier designs which are very low analog. Now I can tune them for lowest Harmonic distortion. I want to be at least 10-20dB better than my amp circuits.... which are already pushing the -110 (or more) mark in practice (not sim).

Last edited by RNMarsh; 16th December 2012 at 06:09 PM. Reason: 4 zeros
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Old 16th December 2012, 07:06 PM   #1178
RNMarsh is offline RNMarsh  United States
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How is that residual output port on the 725 work? Something seen at near 500hz -- whats that? Thx-RNM
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Old 16th December 2012, 07:32 PM   #1179
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The fundamental is converted to something around 415 Hz I think. Its harmonics are in the output jack with the fundamental removed. The noise is also removed. The FFT will show you.

I have been looking at the simple FFT displays on e-bay to see if one would work for this. No need for exotics.

Attached is a snapshot I took of the distortion output measuring Victors oscillator. The tall peak is the 2nd Harmonic. 2-10 are shown in a bandpass to 5 KHz. This does not change with input frequency. You can also sync to the trigger output and see phase relationships.
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Old 16th December 2012, 07:47 PM   #1180
davada is offline davada  Canada
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Default Unbelievable. Another small mod

Just when I thought it was at it best.

You can just barely see that 2nd sticking it head out of the noise.
I just can't seem to shake that 4th H though.

Cheers,
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