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Old 26th July 2013, 05:19 PM   #2981
RNMarsh is offline RNMarsh  United States
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On a side note--- The older HP THD+N analyzers - like the HP331-4 series have a variable freq notch filter which could be modified/upgraded for use as a flexable pre-notch filter in combination with an ADC card, such as the QA400 for FFT measurements.

THx-RNMarsh
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Old 27th July 2013, 12:16 AM   #2982
davada is online now davada  Canada
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Quote:
Originally Posted by RNMarsh View Post
On a side note--- The older HP THD+N analyzers - like the HP331-4 series have a variable freq notch filter which could be modified/upgraded for use as a flexable pre-notch filter in combination with an ADC card, such as the QA400 for FFT measurements.

THx-RNMarsh
You should read Dick's trial and errors with this analyzer on his web page.
High noise and distortion.

Cheers,
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Old 27th July 2013, 01:51 AM   #2983
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Quote:
Originally Posted by RNMarsh View Post
On a side note--- The older HP THD+N analyzers - like the HP331-4 series have a variable freq notch filter which could be modified/upgraded for use as a flexable pre-notch filter in combination with an ADC card, such as the QA400 for FFT measurements.

THx-RNMarsh
Its for these kinds of applications that I use my Distortion Magnifier described in vol. 0 of Linear Audio. It works for DUTs with a flat frequency response, like amplifiers. It just subtracts a properly-scaled and phased version of the source signal (e.g., oscillator). It is basically as good as the op amps it is built with.

Phase adjustment is just achieved with a frequency-variable single-pole approximation to the DUT's HF rolloff. As a result, it provides the fundamental rejection function over a fairly wide bandwidth with only minimal adjustment for phase and amplitude. A null of 60dB is not difficult to achieve. Then some source signal at -20dB or -40dB is put back in, so that the distortion magnification is 20dB or 40dB. This keeps enough fundamental in to allow a standard THD analyzer to lock on, and also provides a fundamental reference level. Of course, one can operate without any source signal added back in and get the deep 60dB or so notch.

The distortion magnifier also works well for making two-tone measurements, like 19+20 kHz CCIF, since its cancellation of the source signal is reasonably broadband.

Compared to a variable-frequency notch, it requires adjustment of two things, namely amplitude and phase, instead of a single critical frequency adjustment. There is usually a little bit of iteration between the two as well.


Cheers,
Bob
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Old 27th July 2013, 06:32 AM   #2984
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by davada View Post
You should read Dick's trial and errors with this analyzer on his web page.
High noise and distortion.

Cheers,
I will do that. I expect to replace the active circuits with equivelent opamps. But I havent seen the schematic yet. I have the passive B&K notch filter and this would be an active equal to it. Well, I'll see if it can be made up to-date and useful. because its cheap and dont need the analyzer portion.

Thx-RNMarsh

Last edited by RNMarsh; 27th July 2013 at 06:38 AM.
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Old 27th July 2013, 06:35 AM   #2985
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by Bob Cordell View Post
Its for these kinds of applications that I use my Distortion Magnifier described in vol. 0 of Linear Audio. It works for DUTs with a flat frequency response, like amplifiers. It just subtracts a properly-scaled and phased version of the source signal (e.g., oscillator). It is basically as good as the op amps it is built with.

Phase adjustment is just achieved with a frequency-variable single-pole approximation to the DUT's HF rolloff. As a result, it provides the fundamental rejection function over a fairly wide bandwidth with only minimal adjustment for phase and amplitude. A null of 60dB is not difficult to achieve. Then some source signal at -20dB or -40dB is put back in, so that the distortion magnification is 20dB or 40dB. This keeps enough fundamental in to allow a standard THD analyzer to lock on, and also provides a fundamental reference level. Of course, one can operate without any source signal added back in and get the deep 60dB or so notch.

The distortion magnifier also works well for making two-tone measurements, like 19+20 kHz CCIF, since its cancellation of the source signal is reasonably broadband.

Compared to a variable-frequency notch, it requires adjustment of two things, namely amplitude and phase, instead of a single critical frequency adjustment. There is usually a little bit of iteration between the two as well.


Cheers,
Bob
Hi Bob.... guess what. I have your Distortion Magnifier. Great circuit. And, a very useful product. Highly recommended.

Thx-RNMarsh
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Old 27th July 2013, 01:29 PM   #2986
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Originally Posted by RNMarsh View Post
Hi Bob.... guess what. I have your Distortion Magnifier. Great circuit. And, a very useful product. Highly recommended.

Thx-RNMarsh
Hi Richard,

Thanks! I'm glad it has worked well for you.

Back in the early '80s when I did my MOSFET power amplifier with HEC, I had to use an early version of that circuit to measure the amplifier - that was the only way I could get that low with the equipment I had at the time.

Cheers,
Bob
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Old 27th July 2013, 04:19 PM   #2987
RNMarsh is offline RNMarsh  United States
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Is there a simple way to modify the HP 334A notch filter to be high Q but only have a depth of 40dB-60dB.... ?

THx-RNMarsh
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Old 27th July 2013, 04:32 PM   #2988
davada is online now davada  Canada
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Originally Posted by RNMarsh View Post
Is there a simple way to modify the HP 334A notch filter to be high Q but only have a depth of 40dB-60dB.... ?

THx-RNMarsh

IIRC it is a Wien bridge so any info on Wiens apply. Can the RC ratios be altered for higher Q like with the Bridged T?

Dick and I tried all sorts of ways of doing this with large amounts of positive FB. Although it worked and was stable the distortion was out of control.

If you can reorganize the actives like the dual inverted stage oscillators it could work. The tuning capacitor was the limit because of the way HP set it up.
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Last edited by davada; 27th July 2013 at 04:37 PM.
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Old 27th July 2013, 04:44 PM   #2989
davada is online now davada  Canada
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Hi Rick,

Something Dick and I didn't try is to use virtual grounds on the op amps and run them with single ended power supply. The grounding was the big issue. If you can get the amplifiers to float... The idea is to use an active bridge.
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Last edited by davada; 27th July 2013 at 04:46 PM.
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Old 27th July 2013, 06:15 PM   #2990
1audio is offline 1audio  United States
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Originally Posted by RNMarsh View Post
Is there a simple way to modify the HP 334A notch filter to be high Q but only have a depth of 40dB-60dB.... ?

THx-RNMarsh
Probably easier to leave the high Q and have a separate path for mixing back the fundamental. More precise and stable as well. I tried tweaking the notch filter options to control the q some time ago. Nothing worked well enough in simulation to even try in real parts.

The HP331-334 all have air caps for tuning (as I remember) meaning high Z. High Z means higher noise floor. That will be the real limit of the performance of it as a notch filter (or much of any other application) unless you jack up the voltage a lot. No good roadmap from their to here.
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