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Old 10th August 2013, 01:27 AM   #161
RNMarsh is offline RNMarsh  United States
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Default Fundemental supression-

Same as above except input is 1.5KHz (meter out of circuit). Suppressed fund. is seen... that is about right I think.

[How good the null is depends on how good I nulled it and drift before capture the screen. To get best and most stable null.... the 334A needs to be on for hours. The inside temp is warm and stable by that time and a best null an be obtained.]

1.5K null.JPG


THX-RNMarsh

Last edited by RNMarsh; 10th August 2013 at 01:30 AM.
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Old 10th August 2013, 05:29 AM   #162
RNMarsh is offline RNMarsh  United States
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I have been seeing changes in level of the harmonics at low distorton levels with low cost ADC based test equipment; The 2H and 3H change with input level when the input source isnt changing it harmonics.

there is another uncorrected 2H level change and it depends on the notch filter Q/depth. The notch affecting the level of the 2H. In the case of the 334A, a change in the depth of the notch by 10dB caused a change in the 2H of 4dB.

The auto-null affects the notch depth and thus the 2H as well. That is, the manual null vs auto null produces different notch performance with the 334A (and maybe others) and the result is the detected level of 2H is affected.

These sort of things need to be nailed down.... rather than assume what you see is what the DUT does.

Thx-RNMarsh

Last edited by RNMarsh; 10th August 2013 at 05:33 AM.
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Old 10th August 2013, 11:07 PM   #163
RNMarsh is offline RNMarsh  United States
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Re HP-334A: I reassigned the Norm-RF Det switch located just above the Input port so it will only disconnect the meter and reconnect the meter. I can now tune for null in Normal position and then disconnect the meter for lower distortion. It is fast and easy to get to.

Thx-RNMarsh

Last edited by RNMarsh; 10th August 2013 at 11:13 PM.
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Old 11th August 2013, 05:04 PM   #164
RNMarsh is offline RNMarsh  United States
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It looks to be possible to use the HP-334A as a tunable notch filter for use with ADC/FFT hardware IF the various descrete amplifier circuits can be replaced with modern opamps of very low distortion. Unfortunately, the original design used single-end, cap coupled circuits which produce a lot of 2H and beyond - making the circuits pretty much not significantly upgradable. Just need to be replaced. That is more than I need to do at this time.

I am still interested in a notch filter which is variable and has an auto-null to lock it as a more practicle next step for this subject.

Right now, the passive B&K 1607 notch filter is distortion free and with a mod to make it active, gets the job done. But, it is not auto-null. With high Q circuits, you need to be kept within .1% of the signal freq to have accurate harmonic data results. Keeping it there is where the auto-null help comes in.

Thx-RNMarsh

Last edited by RNMarsh; 11th August 2013 at 05:08 PM. Reason: Auto-Null notch filter
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Old 11th August 2013, 10:00 PM   #165
richiem is offline richiem  United States
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Just to clarify for those who haven't used these things but are interested -- if you are looking at the output of the notch filter with a spectrum analyzer, either hardware like a Tek 7L5 or with a sound card and software, then the fundamental signal does not need to be completely notched out -- a 40dB notch is easy to obtain and reduces the problem of self-distortion from the sound card's ADC to nearly negligible amounts, leaving other ADC factors as limits, like noise.

However, if reading THD on a meter, then the fundamental null of the notch filter has to be a lot lower in level than the distortion products, and for this, auto-null is absolutely needed, and it needs to be real good if you expect to see results under 10ppm/-110dB, which is about the best resolution the HP 339 can do. The 339's fundamental null can run to -115dB, or even much more as several here have determined through parts changes. But fundamental null and noise are two of the largest limiting factors for a THD meter reading.
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Old 12th August 2013, 03:29 AM   #166
RNMarsh is offline RNMarsh  United States
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I got interested in the notch during the 339A oscillator distortion reduction journey. I tried the B&K1607 on the 2H and the 3h to deduce the remaining residual into the noise floor. it worked well for that purpose also.

The 339A does not have enough adjustment flexibility to get null at each selected freq so the distortion is a compromise over the wide range of freqs. The 339A can be tuned into the noise but only for 1 or 2 freqs at a time. So the variable notch takes care of that at the source. If I tune the 339A for min 3H, I notch out the remaining 2H... or visa-versa.....all the other harmonics are too low to cae about. The Krone-Hite 4402B is a better source however. Easier to get ultra low distortion at all freqs but without the other built-in features of the 339A.

Then I found Vicktors individual oscillators on eBay came into being and they are individually optimized for one freq. So, several of those is another option for low cost ultra low distortion source. Now its onto the measuring of the distortion at ultra low source distortion levels. Where a notch is used with ADC/FFT. Davada turned us onto the QA400. But for the area below -100dB, sometimes a more flexable notch (variable) filter with auto-tune would be a nice additional feature.

-RNMarsh

Last edited by RNMarsh; 12th August 2013 at 03:43 AM.
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Old 12th August 2013, 04:08 AM   #167
1audio is online now 1audio  United States
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I followed up on a discussion here Sound Quality Vs. Measurements on cancelling harmonics caused by agc circuits. I feel I'm crossposting almost but its relevant here and in the oscillator thread.
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Old 12th August 2013, 04:48 AM   #168
davada is offline davada  Canada
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Well that's kind of a very long thread. Which part?
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Old 12th August 2013, 04:51 AM   #169
richiem is offline richiem  United States
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Here you go:
Patent US4560958 - State variable oscillator having improved rejection of leveler-induced ... - Google Patents
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Old 12th August 2013, 02:52 PM   #170
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Quote:
Originally Posted by RNMarsh View Post
It looks to be possible to use the HP-334A as a tunable notch filter for use with ADC/FFT hardware IF the various descrete amplifier circuits can be replaced with modern opamps of very low distortion. Unfortunately, the original design used single-end, cap coupled circuits which produce a lot of 2H and beyond - making the circuits pretty much not significantly upgradable. Just need to be replaced. That is more than I need to do at this time.

I am still interested in a notch filter which is variable and has an auto-null to lock it as a more practicle next step for this subject.

Right now, the passive B&K 1607 notch filter is distortion free and with a mod to make it active, gets the job done. But, it is not auto-null. With high Q circuits, you need to be kept within .1% of the signal freq to have accurate harmonic data results. Keeping it there is where the auto-null help comes in.

Thx-RNMarsh
What is the value of the coupling cap that is producing the 2rd H ?
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