[sdiy] Mixed-signal problems (dsPIC digital delay project)

rtarcan rtarcan at superonline.com
Fri Jan 27 16:52:54 CET 2017


 

May be the Vref voltage is not filtered well with electrolitic
Condansator C22 . Ceramic Cap. is neccessary parralel to C22. There is
also positive feedback possibility via Leds in overload condition.


Regards 

2017-01-26 18:34, Tom Wiltshire yazmış: 

> Hi All,
> 
> I'd
like some advice. I recently did a PCB for my DigiDelay project:
> 
>
http://electricdruid.net/diy-digital-delay/ [1]
> 
> This is a
commercial project in as much as I'm selling chips and PCBs. If my
asking for help for that bothers you, please stop reading here and
accept my apologies.
> 
> The pedal is good, but it has more hiss than
I'd like. A certain amount is going to be inevitable because the circuit
uses the dsPIC's on-chip 12-bit ADC for audio, and the delay processing
is only 16-bit. But that's not the problem I'm seeing.
> 
> The problem
I've got is high frequency noise on the 9V power supply rails. This
looks like spikes at the 64KHz sample rate (the audio is sampled every
other sample, the interleaved samples are used for the control pots).
The spikes are causing an oscillation at around 2.7MHz. This comes out
as basic white noise on the audio.
> 
> There's an image of the top of
the PCB here:
> 
>
http://electricdruid.net/wp-content/uploads/2016/12/DigiDelayComponentValues.jpg
[2]
> 
> I thought I'd done as much as I could to reduce interference
between the digital and the analog sides of the circuit: The PCB divides
the two areas completely (analog on the left, digital on the right),
both have separate ground planes. The grounds only meet at a single
point back at the power supply input. The dsPIC's analog and digital
3.3V supplies are generated by separate regulators. This is all supposed
to help, right?!
> 
> It seems to me there must be some simple thing I
can do to remove such a high frequency. The audio from the dsPIC is
already heavily filtered once it's back in the analog world. This helps
(there's less noise after the filters than before them), but it's not
eliminating it, I guess since the noise is getting in through the
op-amps' power pins.
> 
>
http://electricdruid.net/wp-content/uploads/2016/10/DigiDelay-Schematic-Pg1.jpg
[3]
>
http://electricdruid.net/wp-content/uploads/2016/10/DigiDelay-Schematic-Pg2.jpg
[4]
>
http://electricdruid.net/wp-content/uploads/2016/10/DigiDelay-Schematic-Pg3.jpg
[5]
> 
> Any pointers or things I could try would be appreciated.
> 
>
I'd like to get this properly sorted since I'd like to do a rack version
with delay time modulation like the old Ibanex DM2000.
> 
> Thanks,
>
Tom
> 
> _______________________________________________
> Synth-diy
mailing list
> Synth-diy at synth-diy.org
>
http://synth-diy.org/mailman/listinfo/synth-diy [6]




Links:
------
[1] http://electricdruid.net/diy-digital-delay/
[2]
http://electricdruid.net/wp-content/uploads/2016/12/DigiDelayComponentValues.jpg
[3]
http://electricdruid.net/wp-content/uploads/2016/10/DigiDelay-Schematic-Pg1.jpg
[4]
http://electricdruid.net/wp-content/uploads/2016/10/DigiDelay-Schematic-Pg2.jpg
[5]
http://electricdruid.net/wp-content/uploads/2016/10/DigiDelay-Schematic-Pg3.jpg
[6]
http://synth-diy.org/mailman/listinfo/synth-diy
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