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Old 15th December 2012, 09:06 PM   #1161
davada is online now davada  Canada
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Default 339a Oscillator

I've done more testing of the oscillator and found that adjusting the level trim R30 to yield best distortion at 1KHz is not the best for 10KHz and 20KHz. Adjusting this trim forces the operating voltage level of the Jfet gate to change. Therefore it has become necessary to have one trim pot for each range so the trim can be set appropriately for that range. The difference between 1KHz and 10KHz / 20KHz is 20dB so this is worth going after.

The best way to to do this is to add a wafer to the range selector switch.
Does anyone know where I can find a single or double pole 4 position wafer that will fit the shaft and align with the existing wafers? It need not be exactly the same it just has to fit mechanically.

No amount of fudging with capacitors in the positive feedback loop has any positive effect.
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Last edited by davada; 15th December 2012 at 09:28 PM.
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Old 15th December 2012, 09:32 PM   #1162
RNMarsh is online now RNMarsh  United States
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That looks pretty darn good. Nice update and switching idea,too. The switch wafer size looks somewhat standard so maybe finding a wafer wont be too hard. Thx -RNM
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Old 15th December 2012, 10:15 PM   #1163
davada is online now davada  Canada
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Quote:
Originally Posted by RNMarsh View Post
That looks pretty darn good. Nice update and switching idea,too. The switch wafer size looks somewhat standard so maybe finding a wafer wont be too hard. Thx -RNM

Hi Rick,

I just checked the lower two ranges and the oscillator won't run at all for what's best for 1KHz. It does run for the 10K range though.

Short of finding a wafer that will work.

Another solution is to tap the capacitors at the range switch for the AGC signal input. The ripple here is 45mVpp riding on 9VDC. Add a quad op amp to buffer and drive relays to switch in the trim pots. A simple and elegant way to do this.

Still a switch wafer would work, it's about the same work and cost. The circuit can be constructed on a perf board or such or make up a PCB.

Edit: A quad comparator, LM339 with open collector, would be better than an op amp for driving the relays.

Cheers,
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Last edited by davada; 15th December 2012 at 10:20 PM.
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Old 16th December 2012, 12:57 AM   #1164
RNMarsh is online now RNMarsh  United States
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Default HP339A THD+N

Quote:
Originally Posted by davada View Post
I've done more testing of the oscillator and found that adjusting the level trim R30 to yield best distortion at 1KHz is not the best for 10KHz and 20KHz. Adjusting this trim forces the operating voltage level of the Jfet gate to change. Therefore it has become necessary to have one trim pot for each range so the trim can be set appropriately for that range. The difference between 1KHz and 10KHz / 20KHz is 20dB so this is worth going after.

The best way to to do this is to add a wafer to the range selector switch.
Does anyone know where I can find a single or double pole 4 position wafer that will fit the shaft and align with the existing wafers? It need not be exactly the same it just has to fit mechanically.

No amount of fudging with capacitors in the positive feedback loop has any positive effect.
Maybe trimming each R set inside rather than a seperate wafer? Reed relays? Using just the mods: for 1KHz (1 X 1000 dials) and the THD+N at 1.7vac output from the 339A is about .00028% (NO filters are in !)
But, when I change freq dials -the THD goes up to .001% (mostly H2). So I need to trimm for several freqs of popular interest, first.
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File Type: jpg 339A THD+N.jpg (1,000.7 KB, 140 views)

Last edited by RNMarsh; 16th December 2012 at 01:07 AM.
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Old 16th December 2012, 01:21 AM   #1165
davada is online now davada  Canada
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Default A look at Victor's oscillator on the 339a

This is Victor's oscillator attenuated to 1Vrms.

Every now and then it breaks into parasitic oscillation.
I have two of these and the parasitic in the other is too much to use the oscillator.
The noise floor is high by comparison and the only harmonic speaks for itself.
This reads 0.00325% on the 339a meter.

Cheers,
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Last edited by davada; 16th December 2012 at 01:24 AM.
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Old 16th December 2012, 01:23 AM   #1166
davada is online now davada  Canada
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The plot
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Old 16th December 2012, 01:41 AM   #1167
davada is online now davada  Canada
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This is the 339a oscillator under the same load.
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File Type: png 339a osc.PNG (443.5 KB, 120 views)
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Old 16th December 2012, 02:26 AM   #1168
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Quote:
Originally Posted by davada View Post
I've done more testing of the oscillator and found that adjusting the level trim R30 to yield best distortion at 1KHz is not the best for 10KHz and 20KHz. Adjusting this trim forces the operating voltage level of the Jfet gate to change. Therefore it has become necessary to have one trim pot for each range so the trim can be set appropriately for that range. The difference between 1KHz and 10KHz / 20KHz is 20dB so this is worth going after.

The best way to to do this is to add a wafer to the range selector switch.
Does anyone know where I can find a single or double pole 4 position wafer that will fit the shaft and align with the existing wafers? It need not be exactly the same it just has to fit mechanically.

No amount of fudging with capacitors in the positive feedback loop has any positive effect.
Try separating the AC feedback from the DC. I think one of the schematics I saw had coupling caps on some aspect of the FET. The AGC will change a lot as a function of the component mismatches in the notch networks. The AC linearization of the FET should be independent of the gain of the fet to some degree.
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Old 16th December 2012, 02:33 AM   #1169
1audio is offline 1audio  United States
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Quote:
Originally Posted by RNMarsh View Post
Maybe trimming each R set inside rather than a seperate wafer? Reed relays? Using just the mods: for 1KHz (1 X 1000 dials) and the THD+N at 1.7vac output from the 339A is about .00028% (NO filters are in !)
But, when I change freq dials -the THD goes up to .001% (mostly H2). So I need to trimm for several freqs of popular interest, first.
RM- Take the analysis output from the 725 into an FFT. The fundamental is 425 Hz (I think) regardless of the incoming frequency. The harmonics will line up at multiples of that with the noise essentially gone. If you are looking at harmonics its fantastic and easy to see the relative levels.A little fiddling and you can get the absolute levels of the harmonics. I figure anything below -130dB is not too important.

Its a shame the box is so big. I just don't have room on my bench so mine is in storage a lot.
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Old 16th December 2012, 03:27 AM   #1170
davada is online now davada  Canada
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Oops. I keep forgetting to turn the 400Hz filter off when switching to the lower ranges.
The oscillator is running just couldn't see it on the meter.

Demian how would you go about separating the AC feedback from the DC feedback?

There is a particular setting for the feedback where all ranges are stable but this is not optimal for best distortion in each range. The answer is to have separate setting for each range. I can't fix this with simple equalization on the AC side of things so switching in four different trim pots for each range seems reasonable. The oscillator works fine for the residual of the 339a analyzer. But we are greedy and we want more.

Cheers,
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