Low-distortion Audio-range Oscillator

[KSTR]

Samuel,

Thanks for your setup advice. I will try to run this test tomorrow if I manage to properly shield the osc. I would assume hum in the control loop could make things much worse than they actually are, right?

[klewis]

Following this thread got me inspired to finish building the ultrapure oscillator from AN67. I used Frex's board, powered by an old Hp 6205B power supply (eg, nothing special for the power supply). Took a bit to get it up and running, but the results are quite good. With the output at approx 1.5 Vrms I measured the 2nd H at approx. -126 dB. Tested it with Dick's Twin-T, nulled the fundamental to about -46db. The pic from left to right are the board in the enclosure, full scale measurement, and a zoomed in measurement (on this one ignore the values in the boxes as they aren't appropriate for a zoom in like this - have to re-write the VEE and am to lazy).

Ken

[richiem]

@Ken -- I don't think that's the 2nd H, which should be at 20.8kHz....

[klewis]

Originally Posted by richiem

@Ken -- I don't think that's the 2nd H, which should be at 20.8kHz....

Hi Dick,
I probably mis-stated the fundamental, it's at 10.04kHz, so the second is at 20.08khz.
Ken

[luvdunhill]

Looks like 10.4K from the screenshot.

[klewis]

Sorry guys, I went back this morning and looked at where the 10.4k came from. It's left over in the VEE programing from when I used to use my HP8903A as a source. I changed the programing this morning to read the left meter on the HP8903A to the output fundamental box. So, ignore the fundamental output box and know that the fundamental was at 10.040kHz.

I had previously changed the VEE to read the right meter to report the Vrms, but had forgotten to change it to read the left meter as well.

Ken

So, if you believe my numbers, the AN67 oscillator is quite good yes? But, from the looks of it Victor's is the better deal...

[richiem]

@Ken -- The results from the AN-67 unit are hard to understand given the general belief that this is a truly great oscillator with THD in ppb. There are many possible sources of harmonic intrusion that are not intrinsic to the oscillator.

I couldn't see any recognizable higher harmonics in the noise, but an expanded view may be warranted. Compounding the issue is that we really don't know what the residual of the Active Twin-T is -- I have yet to find a way to get at that, and was hoping the ppb oscillator would be helpful.

[klewis]

Originally Posted by richiem

@Ken -- The results from the AN-67 unit are hard to understand given the general belief that this is a truly great oscillator with THD in ppb. There are many possible sources of harmonic intrusion that are not intrinsic to the oscillator.

I couldn't see any recognizable higher harmonics in the noise, but an expanded view may be warranted. Compounding the issue is that we really don't know what the residual of the Active Twin-T is -- I have yet to find a way to get at that, and was hoping the ppb oscillator would be helpful.

Hi Dick,

I'm not running it on batteries, that might help, but, who knows when I will get to that.

kind of a catch 22 - as you say, maybe the 2ndH we're seeing is a contribution of the active twin-t...

I was thinking this morning that I should try to "zoom - in" on each of the the harmonics, seems the hp3562 resolution gets better when the fundamental (null of course) is not in the view. Is that what you would like to see?

For me, it's certainly clean enough...

Ken

[klewis]

I forgot to mention, Frex did a really thoughtful job of layout and enhancement to the schematic. Such things as parallel resistors that will allow fine tuning of the frequency, fixed resistors around some of the trimmers to allow finer trim. Good job Frex!

Ken

p.s. If anyone does intend to build one of these, let me know and I can post a bit more elaborate adjust proceedure. It took a couple of hours to get it running consistently.

[WSJ]

Originally Posted by klewis

I forgot to mention, Frex did a really thoughtful job of layout and enhancement to the schematic. Such things as parallel resistors that will allow fine tuning of the frequency, fixed resistors around some of the trimmers to allow finer trim. Good job Frex!

Ken

p.s. If anyone does intend to build one of these, let me know and I can post a bit more elaborate adjust proceedure. It took a couple of hours to get it running consistently.

Thanks for the offer to post your adjustment proceedure. I'm ordering two boards form Frex today.