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Old 17th April 2013, 12:01 AM   #2321
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Quote:
Originally Posted by simon7000 View Post
Then some of us were doing this not just to keep a stable FFT bin but to allow looking at 1/F offset noise. You know critical band issues.

ES
We are looking at THD at the moment, only.
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Old 17th April 2013, 12:01 AM   #2322
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by davada View Post
In a way this is what degeneration does. If we can use a none linear element where our de-gen resistor goes it might work.
could get 2h down but noise up. What about 3H?
The two topologies have also been described as compressive and expansive. Together they are more linear than either, alone.
Or, one is predistorted for the other. Who has already done this for audio or oscillators? certainly there are such circuits to apply here?
maybe? If not what can we configure?

Thx-RNMarsh

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Old 17th April 2013, 05:46 AM   #2323
1audio is offline 1audio  United States
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Predistortion works to a degree. The ST1000 uses it to get low distortion from the varactor modulator. You can get below .05% THD but the tweaking is not easy. However you are looking for matching a nonlinearity 60-80 dB lower than that. I'm not sure how to do something like that. It seems easier in the frequency domain.
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Old 17th April 2013, 06:14 AM   #2324
RNMarsh is offline RNMarsh  United States
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If you follow my thought here - this isnt really the traditional predistortion but an amplifier design used for the osc. BUt that may be beyond the scope here. (?)



Thx-RNMarsh

Last edited by RNMarsh; 17th April 2013 at 06:16 AM.
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Old 17th April 2013, 07:42 AM   #2325
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The point is the lightbulb oscillator does have two loops a thermal loop (very slow) and the compressive non-lineartiy. His point is that if you defeat the later by making it too good the amplitude "squeegs". Jim Williams thought his work applied directly to the LTC oscillator which does have level detector, etc., the S&H and other sophisticated AGC aides might change the game (I don't know).
I don't think it applies to linear leveling loops. With an ideal peak detector/multiplier, 1st order settling could be done in 1.5 cycles with zero distortion contribution. That's IIRC what Meyer-Ebrecht wrote in "Schnelle Amplitudenregelung harmonischer Oszillatoren" on this.

Quote:
Predistortion works to a degree. The ST1000 uses it to get low distortion from the varactor modulator. You can get below .05% THD but the tweaking is not easy. However you are looking for matching a nonlinearity 60-80 dB lower than that. I'm not sure how to do something like that.
Fully agree--distortion cancellation needs a dependable and simple to model (e.g. independent from or at least directly/inversely proportional to frequency) distortion source. The after "good contemporary design practice" remaining distortion sources are anything but this--complex in nature, and sometimes with large spread from specimen to specimen.

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Old 17th April 2013, 02:05 PM   #2326
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Samuel, I guess I'll let that point rest for now. I might play with some sims sometime to convince myself.

Here's part 1 of my Wein bridge as untra-high Q amplifier experiment. I rearranged my light bulb oscillator to make it an oscillator with "tickler". I took the most horrible function generator I had (HP3311A) diode shaping and massive jitter. and tweaked the oscillator to just below oscillation and injected the HP via a 2Meg resistor. I could stablilize it at 20V p-p and take measurements. There was one very unexpected and interesting result. Apparently since the Wein network has an abrupt phase transition a lot of the jitter is converted to AM, my guess since this was visually very apparent. The spectral results were OTOH quite dramatic.

EDIT - PIM detector!?
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Old 17th April 2013, 02:41 PM   #2327
davada is offline davada  Canada
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Quote:
Originally Posted by scott wurcer View Post
Samuel, I guess I'll let that point rest for now. I might play with some sims sometime to convince myself.

Here's part 1 of my Wein bridge as untra-high Q amplifier experiment. I rearranged my light bulb oscillator to make it an oscillator with "tickler". I took the most horrible function generator I had (HP3311A) diode shaping and massive jitter. and tweaked the oscillator to just below oscillation and injected the HP via a 2Meg resistor. I could stablilize it at 20V p-p and take measurements. There was one very unexpected and interesting result. Apparently since the Wein network has an abrupt phase transition a lot of the jitter is converted to AM, my guess since this was visually very apparent. The spectral results were OTOH quite dramatic.

EDIT - PIM detector!?
"OTOH" ??? Don't know this one.
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Old 17th April 2013, 02:50 PM   #2328
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Quote:
Originally Posted by scott wurcer View Post
Samuel, I guess I'll let that point rest for now. I might play with some sims sometime to convince myself.

Here's part 1 of my Wein bridge as untra-high Q amplifier experiment. I rearranged my light bulb oscillator to make it an oscillator with "tickler". I took the most horrible function generator I had (HP3311A) diode shaping and massive jitter. and tweaked the oscillator to just below oscillation and injected the HP via a 2Meg resistor. I could stablilize it at 20V p-p and take measurements. There was one very unexpected and interesting result. Apparently since the Wein network has an abrupt phase transition a lot of the jitter is converted to AM, my guess since this was visually very apparent. The spectral results were OTOH quite dramatic.

EDIT - PIM detector!?
Why do you need the light bulb? Why one stage of wein filter? And what do you get driving it with a decent oscillator?

(Really Cool!)

Last edited by simon7000; 17th April 2013 at 02:53 PM.
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Old 17th April 2013, 03:08 PM   #2329
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Quote:
Originally Posted by simon7000 View Post
Why do you need the light bulb? Why one stage of wein filter? And what do you get driving it with a decent oscillator?

(Really Cool!)
The light bulb was removed and I made the feedback network with 1k Ohm - 50 Ohm pot - 500 Ohm. The pot acts like the tickler in a regen radio, it is tuned to just below oscillation.

Further experiments follow.

Dave this is an experiment I thought of last week after playing with the injection lock. Skip the oscillation and AGC but make an ultra-high Q amplifier. From the pics you can see huge harmonic attenuation. Oh, do you mean OTOH = on the other hand ?

I'm going to try this to detect the primary Schuman resonance (~7.5Hz) with only a whip antenna.
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Old 17th April 2013, 05:06 PM   #2330
1audio is offline 1audio  United States
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That would be a good test of a selective amplifier.
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