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Old 9th January 2013, 08:21 AM   #41
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by klewis View Post
The last 50 pages of low distortion audio range oscillator have dug pretty deeply into capabilities and significant modifications-improvements of the 339. It's in the equip & tool sub-catagory
It is quit easy to get well below .001% with the 339A. -RNMarsh
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Old 9th March 2014, 12:00 AM   #42
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Has anyone tried modifying their 8903A for THD improvement?

I tried improving the THD on my 8903A by swapping out the 7x NE5534 with LM49710 in the generator section from the oscillator to the output buffer. I get a slight improvement in THD (measured up to the 5th harmonic) from 0.0004% (mainly 2nd harmonic) to 0.00029% (mainly 5th harmonic). It doesn't look like it's worth the effort
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Old 12th March 2014, 01:20 AM   #43
1audio is offline 1audio  United States
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Did you measure using the unmodified analyzer as a filter? Those numbers are really quite good. Getting better is difficult. Most of the products of this generation are limited by the internal impedances and opamp performance. Just swapping opamps is not enough to see if they work. In certain locations you need specific opamps to get improvements. For example at the difference amp on the input the AD797 gives much better results than the LME49710. However in other locations its not better, and way expensive.

I can look at the schematic and make suggestions if interested. I have an 8903A in storage but no time to fiddle.
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Old 12th March 2014, 05:24 AM   #44
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I use a 96kHz M Audio 5.1 Revolution and an FFT software to measure the THD. No I didn't use the 8903A's input filter for the measurement. My test setup is not ideal as I get a lot of 60Hz harmonics and spurs between 1kHz and 2kHz when measuring 1kHz THD. The number I quoted are strictly from the harmonics. If you factor in noise and IMD from the 8903A and the soundcard, THD+N is 0.007% unweighted. Hope this clears things up.

The AD797 is pricey as the 8903A generator section has 7 op amps directly in the signal path, $2 x 7 vs $8 x 7. The 8903A's analyzer has way too many things along it's path that you are limited by them.

To be honest, the THD figures of my 8903A's generator is adequate for my needs, which is high end FM tuner alignment. I'm just exploring the possibilities of adding more margin to my test setup

Quote:
Originally Posted by 1audio View Post
Did you measure using the unmodified analyzer as a filter? Those numbers are really quite good. Getting better is difficult. Most of the products of this generation are limited

<snip>

I can look at the schematic and make suggestions if interested. I have an 8903A in storage but no time to fiddle.
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Old 12th March 2014, 07:07 AM   #45
1audio is offline 1audio  United States
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You are probably better than the best modulator already. The distortion you measured may be close to the limits of the M-audio. Getting the hum and other noises out of those measurements is very difficult.

The AD797 would only be used in specific places. The circuit is pretty complex but the critical parts are the opamps in the oscillator loop and the AGC multiplier circuit. The LME49710 are fine for most locations. Since its not socketed swapping opamps is not a simple task. Tuning the analog multiplier may be the next step.
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Old 12th March 2014, 09:20 AM   #46
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Be careful with the AD797--it has subtle stability issues if the inverting input sees a too low impedance at HF (which it will if used as integrator).

A brief look at the schematics of the generator a few years back suggested that the main distortion/noise contribution is from the leveling loop, so opamp upgrades might not help much if at all.

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Old 12th March 2014, 02:14 PM   #47
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My FM generators measure THD <0.0015% w/ 75kHz deviation at 1kHz rate using an HP 8901A as the demod. Using the HP 8903A as the external modulation source increase the THD number to <0.002%. Again using the same soundcard + FFT setup for THD measurement.

Hum and noise worsens the THD number and I get THD+N <0.01%.

I agree, I'm hitting the limits of my soundcard.

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You are probably better than the best modulator already. The distortion you measured may be close to the limits of the M-audio. Getting the hum and other noises out of those measurements is very difficult.
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Old 12th March 2014, 02:23 PM   #48
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The leveling loop has 2 S/H, integrator, voltage reference and the multiplier but these are more on the DC domain. I wonder how much these stages contribute. A quieter/more stable reference as opposed to the temp stabilized zener, teflon caps vs polycarbonate. newer multiplier or lower DC offset op amps might help.

The state variable oscillators have identical stages, maybe having the values matched including the JFET switches might improve things

Swapping op amps was the easiest route. I have them socketed right now

Quote:
Originally Posted by Samuel Groner View Post
A brief look at the schematics of the generator a few years back suggested that the main distortion/noise contribution is from the leveling loop, so opamp upgrades might not help much if at all.
Samuel
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