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Old 10th January 2013, 12:34 AM   #1511
davada is offline davada  Canada
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Quote:
Originally Posted by dimitri View Post
You are correct. AD797 stages before notch filter don't allow me to measure reliable <-125dB with AP2. After rejection of fundamental amplifier before an ADC and ADC itself aren't very important, at least there is always the option to use different amplifier/ADC and compare results.
Are you familiar with the concept of missing codes in ADC's?
It's a funny way of putting it. The aren't really missing they are simply not there.
It a type of inherent non linearity in ADC's. Even ADC's that say no missing codes have missing codes. If the missing codes just happen to be in the dynamic range of our measurement being as low as they are then we have distortion. This mechanism of distortion is far more significant at low levels than it is at the ADC's FS.
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Old 10th January 2013, 12:57 AM   #1512
dimitri is offline dimitri  United States
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David, look. I have 1V fundamental and -120dB 3rd harmonic, I suppress fundamental by 60db and harmonic by 6db with a notch filter, then I amplified both by 60 dB. Now I have 1V fundamental residual and -66dB 3rd harmonic on the ADC input. Should ADC be the best?
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Old 10th January 2013, 01:36 AM   #1513
davada is offline davada  Canada
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Originally Posted by dimitri View Post
David, look. I have 1V fundamental and -120dB 3rd harmonic, I suppress fundamental by 60db and harmonic by 6db with a notch filter, then I amplified both by 60 dB. Now I have 1V fundamental residual and -66dB 3rd harmonic on the ADC input. Should ADC be the best?
Well it depends on the ADC, what type of conversion it uses and more than anything how much you paid for it. If I put that into the QA400 I would have so much distortion generated
I wouldn't be able to tell what's DUT and what QA400.

My point is there are other non linearity that need to be considered which add to the uncertainty. For a sanity check we should measure our distortion by more than one means.
I find inconsistency with the method which I can't explain. For example I have a selective voltmeter which I use to confirm the measurements I get from something like a QA400 or EMU0204. But I'm still using a notch filter to get the measurement and so this aspect of the measurement still has some uncertainty. All measurement has uncertainty even the AP's. Look at the differences of what's been posted with Victor's oscillator. It's all over the board. And then there is the issue of calibration.

What you pointing out is proportionality but any error is proportional as well.
The purpose of notching and then amplifying is to make some sources of error less significant.
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Last edited by davada; 10th January 2013 at 01:45 AM.
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Old 10th January 2013, 02:09 AM   #1514
RNMarsh is online now RNMarsh  United States
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where would be the best place to put a trim to alter the phase a tiny bit - just enough to see if the notching adds or subtracts -- increases or decreases the harmonic data? -RNM
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Old 10th January 2013, 02:55 AM   #1515
davada is offline davada  Canada
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where would be the best place to put a trim to alter the phase a tiny bit - just enough to see if the notching adds or subtracts -- increases or decreases the harmonic data? -RNM
You don't need to add a trim.

If your notch filter is variable You will change the phase by tuning the frequency of the notch filter away from the oscillator frequency. Your notch will rise and you can see if the distortion rises, falls or stays the same. If you have a fixed frequency notch filter you can tune the oscillator frequency away from notch center this will change the phase. your notch will rise. Observe you distortion. The phase is at it's highest velocity at the deepest notch. The phase reverses rapidly at the deepest notch, notch center.
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Old 10th January 2013, 05:54 AM   #1516
RNMarsh is online now RNMarsh  United States
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well then... let's do it and see any affects and if so by how much a notch affects the data/harmonics. Or, just what the boundaries are. If it isnt much then we can all sleep well. -Thx RNMarsh
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Old 10th January 2013, 06:00 AM   #1517
RNMarsh is online now RNMarsh  United States
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Lamps in oscillators? what ever happened to thermistors for that role? Thx-RNMarsh
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Old 10th January 2013, 07:59 AM   #1518
1audio is offline 1audio  United States
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Originally Posted by RNMarsh View Post
Lamps in oscillators? what ever happened to thermistors for that role? Thx-RNMarsh
You seem to have missed the discussion; essentially a lamp is a very linear positive tempco thermistor. David showed a really elegant solution for making a multiplier with a lamp and some dc voltage. The remaining task is a level detector to "steer" the level with higher precision that can be had from the lamps. SG suggested using a simple rectifier/integrator, which should be fine since the response time is slow anyway.

This could be used with either a Wein bridge or a state variable oscillator. I'm not sure how practical this would be for an HP339.
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Old 10th January 2013, 09:07 AM   #1519
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I think that insertion of pi (T) attenuator between UUT and notch filter will help.
Yes, that's what I'm using to gain some measurement confidence. As you say, this again rises the question if the attenuator introduces some distortion, or the possibly changed impedance conditions (different loading of the oscillator) have some effect. Or perhaps the connectors or switches..?

Quote:
If your notch filter is variable you will change the phase by tuning the frequency of the notch filter away from the oscillator frequency. Your notch will rise and you can see if the distortion rises, falls or stays the same. If you have a fixed frequency notch filter you can tune the oscillator frequency away from notch center this will change the phase.
What is of relevance is the phase at the harmonics. Detuning the notch filter won't change much here.

Samuel
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Old 10th January 2013, 01:20 PM   #1520
davada is offline davada  Canada
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Originally Posted by Samuel Groner View Post
Yes, that's what I'm using to gain some measurement confidence. As you say, this again rises the question if the attenuator introduces some distortion, or the possibly changed impedance conditions (different loading of the oscillator) have some effect. Or perhaps the connectors or switches..?



What is of relevance is the phase at the harmonics. Detuning the notch filter won't change much here.

Samuel
"or the possibly changed impedance conditions (different loading of the oscillator) have some effect."

Or different loading of the filter.

OKay. The phase of the harmonics should be predictable at the output of a Twin T if it is the Twin T that has introduced the phase shift. We can see this in simulation.
Where is this phase shift of the harmonics coming from that cause cancellation?

There is a distinct possibility that what I'm seeing is an effect of the FFT windowing ripple.
This can be tested buy using flat top.
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Last edited by davada; 10th January 2013 at 01:22 PM.
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