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Old 18th December 2012, 04:46 AM   #1201
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by davada View Post
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. .
Would short channel lengths do better?
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Old 18th December 2012, 04:47 AM   #1202
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Originally Posted by richiem View Post
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?
I wasnt clear... both.
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Old 18th December 2012, 04:51 AM   #1203
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Originally Posted by 1audio View Post

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.

Thats worth a try.

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.
This is more along the lines of what i was looking for.... lets see if it can be applied here. yes, you did mention Amber doing so. How do they do it in schematic form? Thx-RNMarsh

Last edited by RNMarsh; 18th December 2012 at 04:58 AM.
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Old 18th December 2012, 04:57 AM   #1204
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by davada View Post
Hi Dick,



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.


Cheers,
As we are measuring outputs that can drive <600-2K Ohms or at least 10K, we dont need to use 100K (noisy) Z and other R's can be scaled down as well with the new opamps in place.

The dual-gate mosfet is for protection (CMode?). Shouldnt do much unless overloaded.
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Old 18th December 2012, 05:33 AM   #1205
davada is offline davada  Canada
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Quote:
Originally Posted by RNMarsh View Post
As we are measuring outputs that can drive <600-2K Ohms or at least 10K, we dont need to use 100K (noisy) Z and other R's can be scaled down as well with the new opamps in place.

The dual-gate mosfet is for protection (CMode?). Shouldnt do much unless overloaded.

Protection of what? It's coupled to the center of the bias string clamp through 390pF caps.
There is a lot of 2nd H on TP101.

Cheers,
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Old 18th December 2012, 05:53 AM   #1206
RNMarsh is offline RNMarsh  United States
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HF/RFI Common-Mode. Take it out. shouldnt matter much re protection. See what H2 does after that. -RNM

Last edited by RNMarsh; 18th December 2012 at 05:56 AM.
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Old 18th December 2012, 06:08 AM   #1207
davada is offline davada  Canada
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Originally Posted by RNMarsh View Post
HF/RFI Common-Mode. Take it out. shouldnt matter much re protection. See what H2 does after that. -RNM
I did take it out. It didn't do much so I put it back in. This is the first answer I've got for asking this question.

Thanks,
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Old 18th December 2012, 06:10 AM   #1208
davada is offline davada  Canada
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Rick what are getting for distortion at 1KHz 1Vrms off that oscillator of your.

Cheers,
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Old 18th December 2012, 06:17 AM   #1209
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I'll check again and let you know. -RNM
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Old 18th December 2012, 03:19 PM   #1210
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Quote:
Originally Posted by RNMarsh View Post
Would short channel lengths do better?


No, I think a better statement is that the FET wants to stay in the triode region well away from Vp. What you want is very high Vp which happens in general to go with long channels. The down side is that high Vp low Idss FET's are higher resistance and it's harder to make them be just a small portion of the feedback mechanism. The linearization tries to keep the FET looking like a resistor centered on the origin rather than off to one side, hence the trim tweaks the seconds. This trick is very old BTW well known when I started in 1973.
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