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Old 17th December 2012, 08:38 AM   #1191
RNMarsh is offline RNMarsh  United States
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Do I need agc? If it causes that much trouble.... I can manually adjust the level before each measurment. -RNMarsh
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Old 17th December 2012, 02:16 PM   #1192
davada is offline davada  Canada
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Quote:
Originally Posted by RNMarsh View Post
Do I need agc? If it causes that much trouble.... I can manually adjust the level before each measurment. -RNMarsh
Unfortunately yes you do need some form agc. A lamp at least.
Without it all you'd be doing is setting the level. You wouldn't have time to take your measurement.
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Old 17th December 2012, 06:04 PM   #1193
RNMarsh is offline RNMarsh  United States
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Are there other jFET choices available today for lower H2?

Control fet --> push-pull compl jFEt's ?? (170/74) for cancelling the H2?

I can live with longer agc time constant/settling time if it helps.

Last edited by RNMarsh; 17th December 2012 at 06:22 PM.
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Old 17th December 2012, 06:40 PM   #1194
davada is offline davada  Canada
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Originally Posted by RNMarsh View Post
Are there other jFET choices available today for lower H2?

Control fet --> push-pull compl jFEt's ?? (170/74) for cancelling the H2?

I can live with longer agc time constant/settling time if it helps.
The problem lays in the linearization of the channel length modulation.
As we have seen the Jfet works best operated near it lowest R. The AC voltage across the fet is minimal and length modulation is less significant. The fet need to operate bipolar so the ususal techniques of cascading with other transistors is limited to another fet which has the same problem of the channel modulation. The trick is to make the fets involvement minimal.

Another way of controlling the loop gain is with negative resistance but then you still need a control element to vary the gain. That control element could be a variable capacitor.
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Old 17th December 2012, 06:46 PM   #1195
richiem is offline richiem  United States
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Richard, were you askling about the auto-set-level and its LDR, and not about the oscillator AGC? Yes, manual set-level would work fine, and that may be an important change to make -- I wonder if the relative adjust control would serve for that?
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Old 17th December 2012, 07:40 PM   #1196
davada is offline davada  Canada
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Richard, were you askling about the auto-set-level and its LDR, and not about the oscillator AGC? Yes, manual set-level would work fine, and that may be an important change to make -- I wonder if the relative adjust control would serve for that?
Hi Dick,

The relative adjust control is on the dc side of the meter circuit.
It works in level mode only. But I see no reason why the pot itself couldn't be used for this purpose. But of course we would loose this function from the level mode.
You would still need to make up for the 10dB of gain it provides at input voltages below 1Vrms.This is all the auto set level does. The other question is will it really make that much difference. Bypass it and put 1Vrms into the analyzer and see if there is a significant difference. I don't think there is. Past the notch filter output is the 80dB of gain so what matters is the noise at that point. If you look back in the thread I calculated this for Dimitri and the answer is the residual of the 339a exactly. So it is the input circuitry and the notch filter that are responsible for the initial noise which from there is just amplified.

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Old 17th December 2012, 09:03 PM   #1197
richiem is offline richiem  United States
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Yes, I meant re-purpose the pot. But I thought I remembered in one of your posts, David, that the auto-set level amp caused noise and distortion problems at the low end of the range when it's making up lots of gain -- was that just due to overall noise increase, and not an increase in distortion? I'm clearly confused here.
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Old 17th December 2012, 09:48 PM   #1198
1audio is offline 1audio  United States
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Originally Posted by RNMarsh View Post
Are there other jFET choices available today for lower H2?

Control fet --> push-pull compl jFEt's ?? (170/74) for cancelling the H2?

I can live with longer agc time constant/settling time if it helps.
H2 is a symmretic nonlinearity so adding two won't undo it. Think of it as a big voltage coefficient on the resistance. The feedback (two resistors providing a reduced signal to the gate) reduces the nonlinearity to a first order.

If you are willing to trade settling time and absolute voltage stability you can make the resistor in series (200 Ohms I think) larger and use a lower RDS on fet. J110 or even a J108 comes to mind. It would require some careful balancing of the level adjust control and you would tweak it to operate near the on resistance across the band.

The Amber 3501 oscillator has a second harmonic cancellation trick you can try. I think I posted it earlier but I could again. It works with state variable oscillators by taking a phae shifted signal and adding some back in the right place.
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Old 18th December 2012, 01:01 AM   #1199
davada is offline davada  Canada
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Yes, I meant re-purpose the pot. But I thought I remembered in one of your posts, David, that the auto-set level amp caused noise and distortion problems at the low end of the range when it's making up lots of gain -- was that just due to overall noise increase, and not an increase in distortion? I'm clearly confused here.
Hi Dick,

No doubt the auto set level contributes noise. I made a mistake and as it turns out the noise dropped because the gain was about -5dB low. Once I balanced things up there was no savings. I thought I mentioned that in a later post.

If you take the noise measurement off the notch filter output with the QA400 or off the EMU 0204 and calculate the 80dB gain and work out the percentages it's the residual HP quotes for the analyzer.

The op amps are all low noise and I don't think they are contribute much. I think it's the amplified passives noise. The input amplifier and buffer before the notch filter are running t unity with these measurements. If I step down 20dB on the oscillator and up 20 on the analyzer input range the noise rises by that mount. The input amplifier is adding 20dB of gain at a range setting of -20dB. There is noise from the error detection circuits but if you put your scope on the output of the integrators the outputs are surprisingly quiet.

I tried bypassing the buffer stage but I'm at the noise floor of the QA400 so I can't say for sure if the noise dropped. The distortion when up so I re connected the buffer.

I'm not sure I have a great deal of trust with the QA400 as far as distortion is concerned.
I measured Victor's oscillator and got a distortion about 12 dB higher than what was measured on an AP and it doesn't compare to what others have reported on this thread. I'm going to try again using battery power because the disto may have been high from interaction of my power supply, the QA400 and the 339a. I should also try it directly with a Twin T. I haven't got to measuring the noise with the 7a22 but I will get to this. I've been focused more on getting the distortion down.

I guess what I getting at is, take care of the input noise and noise in the notch filter then the rest doesn't matter. If I disconnect the output of the notch filter from the rest of the analyzer the meter goes to zero.

Do we really need an input impedance of 100K? The input attenuator does not keep a constant load on the input amplifier's input. The overload protection diodes are suspicious. The dual gate fet is a big question mark. If we add gain at the input amplifier we improve the SNR at the cost of 3 times the distortion. But again I don't trust the QA400 at higher signal levels.

I'm still wondering about the huge amount of second harmonic on the TP for the dual gate fet in the input protection. It's coupled through some 390pF or thee about.


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Last edited by davada; 18th December 2012 at 01:05 AM.
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Old 18th December 2012, 04:42 AM   #1200
RNMarsh is offline RNMarsh  United States
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The problem lays in the linearization of the channel length modulation.
What property of the fet is needed --- short/long channel length to minimize the effect/distortion/channel length modulation?
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