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Old 4th December 2012, 05:21 PM   #1021
RNMarsh is offline RNMarsh  United States
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There are several tantalum coupling caps betwen osc and thd monitor output. They are not even back to back. these will be changed to film or bi-polar type to reduce distortion by another X-dB's. Bipolar caps on order... will change and report. Thx-RNMarsh

Last edited by RNMarsh; 4th December 2012 at 05:24 PM.
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Old 4th December 2012, 05:37 PM   #1022
davada is offline davada  Canada
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Quote:
Originally Posted by dirkwright View Post
Well, maybe just the oscillator board(s)? Power supply? I dunno, just throwing ideas out there.
Sure maybe when it's said and done.
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Old 4th December 2012, 05:42 PM   #1023
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Quote:
Originally Posted by RNMarsh View Post
Scotts (?) Thx-RNMarsh
No that was Mr. Brisbois' exactly in fact except for the op-amp. My oscillator is the one Mr. Bateman re-published, it uses an SSM VGA and rms detector.

I suspect the performance of the Brisbois oscillator would be highly compromized (or take several minutes to settle) at 100Hz.
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Old 4th December 2012, 06:08 PM   #1024
davada is offline davada  Canada
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Okay so we'll continue here.

All the oscillator measurements taken have been with large averaging. I turned the averaging down to 1 and found the 2nd H rising to a little over -120dB and then dropping back down disappearing into the noise floor. Rather odd behavior. I notice HP use shielded cable for just about ever connection in the oscillator except for the leads going to the level pot, the frequency vernier and few other minor connections. I twisted the wires for the level trim pot and the effect stabilized. Even at 1KHz the radiation is strong and somehow is causing some beating. The answer to this is to use shielded cable or coax on all the controls.

I only removed components necessary to disable the circuit from the op amp U1 and I think removing any dangling components will help with radiated effects.This and a really good cleaning of the board should do it I think. The pots are old and beyond cleaning, get rid of them.

I replaced A1U3 with a LME49710. I would have used a 1468 but they don't load well into 600 ohms. It raises the distortion.

I'm going to try converting the buffer to an inverting type because I think the the distortion will be better and the circuit impedance less sensitive to loading effects. This way we can change the gain a bit for the level vernier and if that doesn't work then leave the pot as an attenuator.

Cheers,
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Old 5th December 2012, 07:41 AM   #1025
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by richiem View Post
Rick -- note that the quad amp used by HP for the filters is a real stinker noise wise, and will need to be replaced with something much quieter if you're going to get lower distortion readings on the meter.
I'll be using the OPA1654 Quad opamp for the filters- also on order. -RNM
[the original IC is a quad of 741 opamps !! Designers must have been drunk that day.]

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Old 5th December 2012, 01:14 PM   #1026
davada is offline davada  Canada
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Originally Posted by RNMarsh View Post
I'll be using the OPA1654 Quad opamp for the filters- also on order. -RNM
[the original IC is a quad of 741 opamps !! Designers must have been drunk that day.]
LOL.

You've got one in the oscillator as well and two in the auto null circuits.
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Old 5th December 2012, 02:19 PM   #1027
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[The original IC is a quad of 741 opamps! Designers must have been drunk that day.]
Well, if they just pass control signals or the residual, why not..?

Samuel
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Old 5th December 2012, 03:23 PM   #1028
RNMarsh is offline RNMarsh  United States
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Originally Posted by Samuel Groner View Post
Well, if they just pass control signals or the residual, why not..?

Samuel
Some are for cntrol, thats fine.
For .002% thd apparently it didnt matter. .0002 is another matter. The quad is used for the various filters (400, 30K, 80KHz) and contribute to increased noise in the monitored residual which limits the ability to read/measure very low level harmonics. Any additional thd may have been low compared to the residual but now it isnt low enough. Its a few DB at a time. Only newer opamps and passive parts allows for changes squeezing out more performance; the over-all fundemental design concept seems fine. So maybe they werent drunk after all? That was a bit harsh, I suppose.

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Old 5th December 2012, 05:46 PM   #1029
davada is offline davada  Canada
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Quote:
Originally Posted by RNMarsh View Post
Some are for cntrol, thats fine.
For .002% thd apparently it didnt matter. .0002 is another matter. The quad is used for the various filters (400, 30K, 80KHz) and contribute to increased noise in the monitored residual which limits the ability to read/measure very low level harmonics. Any additional thd may have been low compared to the residual but now it isnt low enough. Its a few DB at a time. Only newer opamps and passive parts allows for changes squeezing out more performance; the over-all fundemental design concept seems fine. So maybe they werent drunk after all? That was a bit harsh, I suppose.
Hi Rick,

I think the noise we are after is way out of sight for us. Possibly in the MHz. We need a wide band spectrum analyzer to see it. Much wider than what you have. Although I know someone that just got a PI for there TEK frame that might do it.

I also have access to an RF spectrum analyzer at work but it is 50 or 75 ohm input Z.
That's a hard load for what we have. I suppose I could use a buffer.
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Old 5th December 2012, 06:09 PM   #1030
richiem is offline richiem  United States
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In my experience active filters are just noisy; don't know if it has to be that way... but for me the crucial filters are 400Hz and 80kHz (for LP, I'd rather have 10kHz and 100kHz, and may make those changes). In the 339, the 80k filter is the worst for added noise -- the 400Hz HP actually doesn't seem to add much to the baseline noise, and it really is needed to control the line-related spurs.

The guy with the recently acquired 7L5 won't be looking inside his 339 until after the holidays and on into the new year a ways, maybe February. Sorry folks, just how it is. This isn't such a deal for David, who wants his 339 to be a front end for spectrum analysis, but for those who just want to see a believable meter reading, it's a different matter. Still, david could benefit from the 400Hz HP.
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