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

[Oren Leavitt]

On 9/13/2011 5:56 PM, Mike Beauchamp wrote:
> 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,

What is the unregulated input voltage at the regulators?
- Oren