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Old 1st December 2012, 03:55 PM   #951
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Quote:
Also suggests that some DC will cause 2nd harmonic and pure AC 3rd harmonic.
Those parts I've measured showed significant 2nd harmonic.

Quote:
Can some numbers be put to the DC influence? Are we talking uV, mV etc.
Same order as the AC across the element.

Samuel
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Old 1st December 2012, 04:07 PM   #952
davada is offline davada  Canada
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Quote:
Originally Posted by Samuel Groner View Post
Those parts I've measured showed significant 2nd harmonic.



Same order as the AC across the element.

Samuel
Hi Samuel,

Did you log some data and perhaps stick it on your web pages?

Cheers,
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Old 1st December 2012, 04:17 PM   #953
RNMarsh is online now RNMarsh  United States
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Well, from the thd on the LDR, the lowest is at the spec for the 339A. That leaves -- the ser/par arrangment and a distortion cancelling method to remove thd of the LDR. Is there a simple circuit to linearize the LDR... harmonic cancelling... maybe with a second LDR mixed with control LDR? Or even a multi-turn pot in place of the LDR to get thd lower than LDR can do.-- worse case. -RNM
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Old 1st December 2012, 04:36 PM   #954
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Quote:
Originally Posted by RNMarsh View Post
Well, from the thd on the LDR, the lowest is at the spec for the 339A. That leaves -- the ser/par arrangment and a distortion cancelling method to remove thd of the LDR. Is there a simple circuit to linearize the LDR... harmonic cancelling... maybe with a second LDR mixed with control LDR? Or even a multi-turn pot in place of the LDR to get thd lower than LDR can do.-- worse case. -RNM

That's assuming the thd is mostly coming from the LDRs in the 339a. I wasn't able to isolate this enough to know. I don't have the right value trims to do a manual tuning of the 339a notch. Otherwise I would have posted the results. I did the E2 LDR but not the E1.

It's not a bad assumption because I just can't see any other source for the distortion at these levels.
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Old 1st December 2012, 05:53 PM   #955
RNMarsh is online now RNMarsh  United States
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what if... adding DC offset to the opamp to bias out the 2nd from the LDR or just DC to the LDR...... cap couple the circuit output afterwards. ?? A DC trim pot/control.
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Old 1st December 2012, 06:37 PM   #956
davada is offline davada  Canada
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Originally Posted by RNMarsh View Post
what if... adding DC offset to the opamp to bias out the 2nd from the LDR or just DC to the LDR...... cap couple the circuit output afterwards. ?? A DC trim pot/control.
I think what Samuel was saying is the DC is in the same proportions to the AC for it's effect on distortion. The offset from an op amp may be in the mV or even uV so I don't think this would have a significant impact on the 2nd H or 3rd H.
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Old 1st December 2012, 06:52 PM   #957
davada is offline davada  Canada
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Default 339a mod

Here is the latest.

I have averaging in the SW now so we can see more clearly.

I soldered a 50K trim pot across R48, the resistor to the non inverting input of the notch amp. The trim was adjusted so the total R is about 10K from the stock 15k. This forces the E2 LDR to operate at a lower resistance by increasing the gain of the notch amp. The auto tune has to compensate for the gain change. I also lowered the input voltage to the notch filter from 10Vrms to 1Vrms.

The effect of increasing the notch amp gain is a lower 3rd H and the 2nd H is about the same considering the scale change.

This is a what you see is what you get view.

The auto cal is temporarily non functional. The FS is at -0.5dB of dBV
Attached Images
File Type: png 339a mod.PNG (439.1 KB, 153 views)
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Last edited by davada; 1st December 2012 at 06:56 PM.
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Old 1st December 2012, 06:56 PM   #958
RNMarsh is online now RNMarsh  United States
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Originally Posted by davada View Post
I think what Samuel was saying is the DC is in the same proportions to the AC for it's effect on distortion. The offset from an op amp may be in the mV or even uV so I don't think this would have a significant impact on the 2nd H or 3rd H.
I am suggesting put external DC bias onto LDR to increase dc offset to the same as ac level or what ever level is needed to cancel 2nd. Thx-RNM
The manual trim above is useful to me. -130 for 3rd is very good. -110 for 2nd -- only need find a few more db's there to get to the source oscillator level. :-)

Last edited by RNMarsh; 1st December 2012 at 07:05 PM.
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Old 1st December 2012, 08:20 PM   #959
davada is offline davada  Canada
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Originally Posted by RNMarsh View Post
I am suggesting put external DC bias onto LDR to increase dc offset to the same as ac level or what ever level is needed to cancel 2nd. Thx-RNM
The manual trim above is useful to me. -130 for 3rd is very good. -110 for 2nd -- only need find a few more db's there to get to the source oscillator level. :-)
Okay I see where you're going with that.

One thing that will help is to attenuate the +10 dB HP normalizes to to 0dB at the input of the notch filter. The analyzer scale will be wrong but everything else will operate normally.

I guess everything else being the level mode.
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Old 1st December 2012, 08:34 PM   #960
davada is offline davada  Canada
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I put a 50k ohm trim in series with the E1 LDR but didn't observe a drop in second H.

I'm wondering if the 339a oscillator is running a bit high in 2H right now.
The level setting pot is bad and might be getting in the way.
I could use one of Victor's oscillators right about now.
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