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Old 18th October 2012, 06:19 PM   #731
richiem is offline richiem  United States
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@Scott Wurcer -- thanks for the clear explanation about the voltage coefficient of resistance. I wasn't aware of this effect and had only considered the thermal response.

Like Sam, I was thinking of it as a coiled up coil of wire whose inductance and capacitance were too small to care about, leaving thermal response as the only and dominant issue.
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Old 18th October 2012, 06:20 PM   #732
richiem is offline richiem  United States
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@KSTR -- thanks for the plots -- really excellent information.
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Old 18th October 2012, 06:58 PM   #733
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Quote:
Originally Posted by Samuel Groner View Post
Interesting--do you knwo an explanation why the lamp has significant voltage coefficient? So far I've suspected that this is mostly a well behaved metal wire.

Samuel
I don't know, I did all this work on the oscillator Jeff Smith and I did (you know the Bateman one ) I can't tell you how much time was wasted. I measured the lamp Jim Williams used and the high frequency distortion checked with the results of the whole circuit. Try the sim, with a simple third order compressive nonlinearity, you get a Q and amplitude stabilization just like with the AGC loop.

I tried the two op-amp Wein bridge with two 797's stabilized by a Tellabs precision wire wound PTAT (-3000ppm/C) resistor which has unmeasurable voltage coefficient at these levels. The output drove a little heater on the resistor. I could not tame the slow wander but when it locked in it was something else. Years later I asked Jim about the distortionless oscillator with a distortion trim, that's when he admitted that the distortion is trimmed below the noise floor citing the Oliver paper.
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Last edited by scott wurcer; 18th October 2012 at 07:01 PM.
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Old 18th October 2012, 07:16 PM   #734
davada is offline davada  Canada
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Quote:
Originally Posted by scott wurcer View Post
The lamp has two non-linearities, the thermal one and a voltage coefficient of resistance. Measure the lamp's distortion at 10kHz by comparing it to a resistor. The voltage coefficient of non-linearity is what actually stabalizes the oscillation with the thermal one setting the amplitude.

Try a sim with a perfect amplifier, Wein network, and a very small third order non-linearity in the amplifier. Kick it with a current pulse and the oscillation builds up to an amplitude determined by the non-linearity alone. The oscillator has a complex pair of poles that "walk" back and forth on the axis according to the instantaneous gain (gain of exactly 3 is on the axis) . The instantaneous gain is perturbed by the small nonlinearity and the amplitude is stable when the integrated gain over one cycle is exactly 3. Now keep decreasing the non-linearity and the amplitude keeps rising. A low frequency amplitude control is also needed.

I have tried to make a Wein bridge oscillator with no nonlinearity (<-140dB) in the circuit, indeed it never stabalizes except for a few seconds at a time.
You may be interested in what I'm doing right now. I'd like to share it with you but it's a novel idea and I'd rather not post it here until all my work is complete.

I don't mind email exchange.
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Old 18th October 2012, 07:27 PM   #735
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I had a chance to hook Viktors' 1kHz Oscillator on the AP and compare it against AP's own generator (direkt GENMON routing) and the Tek SG505. Test level was 2.7Vrms (Full scale on Victors')

No shielding of the Osc, and fed by lab supply.

The results speak for themselves, or in one word : INCREDIBLE!

Only with massive synchronuous averaging (4096 runs, checked for no/little loss of harmonics magnitudes) it was possible to see the distortion of this oscillator and it fares better than AP's by 10dB...15dB. The SG505 (with only 1024 blocks averaged) is far behind and plaged by hum which also seemed to disturb the autotracking of the notch filter of the AP.
Wait a minute--what analyzer settings did you use to get these plots? Can you post a screenshot or e-mail me the setup file?

A SYS-2722 has no way to directly measure distortion below -130 dB with any reasonable uncertainity.

Samuel
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Old 18th October 2012, 08:04 PM   #736
KSTR is offline KSTR  Germany
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Quote:
Originally Posted by Samuel Groner View Post
Wait a minute--what analyzer settings did you use to get these plots? Can you post a screenshot or e-mail me the setup file?

A SYS-2722 has no way to directly measure distortion below -130 dB with any reasonable uncertainity.

Samuel
I used massive time domain averaging (the full max of 4096 blocks, took several minutes to record) to discard some 20dB of noise/hum. I'll email you the setup tomorrow when I'm back at work, at least the genmon trace should be replicable (though we have outboarded the xformer to get the noise floor as low as possible). Any double-checking is appreciated.
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Old 19th October 2012, 01:50 AM   #737
1audio is offline 1audio  United States
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Quote:
Originally Posted by KSTR View Post
I used massive time domain averaging (the full max of 4096 blocks, took several minutes to record) to discard some 20dB of noise/hum. I'll email you the setup tomorrow when I'm back at work, at least the genmon trace should be replicable (though we have outboarded the xformer to get the noise floor as low as possible). Any double-checking is appreciated.
The plot seems to show 155 db notch depth. Is that real, or software?

I have never been able to duplicate the averaging noise reduction they get with other systems.
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Old 19th October 2012, 02:57 AM   #738
davada is offline davada  Canada
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Quote:
Originally Posted by scott wurcer View Post
The lamp has two non-linearities, the thermal one and a voltage coefficient of resistance. Measure the lamp's distortion at 10kHz by comparing it to a resistor. The voltage coefficient of non-linearity is what actually stabalizes the oscillation with the thermal one setting the amplitude.

Try a sim with a perfect amplifier, Wein network, and a very small third order non-linearity in the amplifier. Kick it with a current pulse and the oscillation builds up to an amplitude determined by the non-linearity alone. The oscillator has a complex pair of poles that "walk" back and forth on the axis according to the instantaneous gain (gain of exactly 3 is on the axis) . The instantaneous gain is perturbed by the small nonlinearity and the amplitude is stable when the integrated gain over one cycle is exactly 3. Now keep decreasing the non-linearity and the amplitude keeps rising. A low frequency amplitude control is also needed.

I have tried to make a Wein bridge oscillator with no nonlinearity (<-140dB) in the circuit, indeed it never stabalizes except for a few seconds at a time.
Thank you Scott.
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Old 19th October 2012, 11:32 AM   #739
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I quickly measured distortion of a small (10r cold resistance) lamp I had around (NOS replacement for Tek 7k scopes) and I've not been able to find any voltage coefficient effects. With 0 dBu across the lamp, distortion (pure 3rd) was -100 dB at 5 kHz. This falls at exactly 20 dB/decade, e.g. at 500 Hz I measured -80 dB. No flattening out at high frequencies was detectable--above the audio band distortion drops below the SYS-2722 residual.

So far it looks to me that all I've seen is pure thermal stuff. It's surprising at what high frequencies one still sees them. Perhaps there are lamps with different behaviour. I'll check if I find another one to measure.

Samuel

Last edited by Samuel Groner; 19th October 2012 at 11:49 AM.
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Old 19th October 2012, 02:28 PM   #740
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I wonder if a vacuum tube heater would be a good application here.
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