[sdiy] High Frequency ripple in PSU - no clue..

[Mike Beauchamp]

Hey all,
I'm trying to debug a build of a VCO. As I sweep the frequency,
there's noticeable interference the higher the frequency gets. As the
oscillator gets closer to past my hearing range, the output starts to
sound sort of like an AM radio tuning in dead-air.

Looking at the waveform on my scope, the output has jagged edges that
have a period of about 6 uS (166KHz).

Looking at the power supply (+/-15V LM317/337), the positive output
shows a rising sawtooth wave with the same period @ .1V p-p. However,
the negative rail shows this high frequency sawtooth at nearly 1V p-p!

If I disconnect the VCO from the powersupply and measure the negative
rail, the same 1V p-p sawtooth is there, only the frequency has
changed to have a period of about 28 uS (35KHz)

I'm pretty much out of ideas to be honest. At first I thought it was
the oscillator itself, but seeing that 35KHz sawtooth on the negative
rail of the power supply with nothing attached has led me back to the
power supply. Different dummy loads on the power supply will affect
the frequency of this.

The power supply is basically an exact duplicate of the LM317/337's
datasheet, so basically this
(http://www.musicfromouterspace.com/analogsynth_new/POWERSUPPLY2009/POWERSUPPLY2009.php)
but set up for +/-15V. I can post a screenshot of the actual schematic
and I have some scope shots as well if that will help.




-- 
[mike]