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

[Mike Beauchamp]

Hi Oren,
I'm not at the workshop right now, but I remember measuring about
18VDC before the regulators. FWIW, I'm using a 24VCT transformer and
1N4002's for rectifying.

I'll be sure to get an accurate measurement tomorrow, I'm guessing
that -18VDC going into the LM337 set to output -15V is cutting it a
bit close, although the datasheet I have doesn't even give a minimum
differential!

Mike


On Tue, Sep 13, 2011 at 7:56 PM, Oren Leavitt <obl64 at ix.netcom.com> wrote:
>
>
> 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
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>



-- 
[mike]