[sdiy] Digital delay - whine from serial RAM access

[Jay Schwichtenberg]

You don't say where you are seeing the noise: power supply, DAC filter,
DAC.... You should be able to see where the noise is being injected into the
system with the scope. I'd look at the power supplies and then the DAC
output from the dsPIC, DAC filter.... When measuring noise with a scope put
the scope on AC coupling (verses DC) and then turn the volts/div up. This
will get rid of the DC and you can see the noise better.

Things I'd try are:
* Cut ALL traces on the stripboard to as long as they need to be and no
more. These can act as little antennas that can transmit and receive stuff.
* Read the data sheet and ERRATA 10 times. Then make sure that all the power
supply filtering and layout is done right for the analog sections of the
chip.
* Some of the DACs and ADCs in MicroChips processors can be ran off separate
supplies. Might see if that will help if you can do that.
* Make sure that the power supply isn't stressed (i.e. loaded enough it's
not filtering correctly).
* Add some small value resistors (30-50 ohms) in series on the SPI clock and
data lines as close as possible to the signal source. This will reduce the
current some and will help on the noise.

Jay S.

-----Original Message-----
From: Synth-diy [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of
Tom Wiltshire
Sent: Sunday, May 08, 2016 11:36 AM
To: synthdiy diy
Subject: [sdiy] Digital delay - whine from serial RAM access

Hi All,

I'm currently building a digital delay. The prototype is on stripboard (this
may be a significant fact). I'm using a dsPIC 33FJ64GP802, which has a
12-bit ADC and 16-bit DAC on-chip. I had this working very nicely using the
on-chip RAM for a short delay. This weekend, I've added an external SRAM
(the 23L1024 128KB SRAM that Richie B suggested here a while back) and got
it running with that. This produces a much longer delay, which is great.

What's not so great is that it also causes a nasty whining sound in the
output. I've checked on the o'scope and the noise coincides perfectly with
the periodic block read/writes to the SRAM chip - so it's coming from the
fast signals of the SPI connection (8MBits in my circuit). I'm using a 32KHz
sample rate and read/writing blocks of 16 samples (the SRAM memory page is
32 bytes, so it's convenient) which gives me a 2KHz block rate, and a whine
at the same frequency.

I've got separate analog and digital supplies, and as far as possible I've
got the analog and digital parts of the circuit separated, electrically and
in space. The in/out op-amps and filters are all up one end of my board, and
the uP and SRAM are down at the other, with a line of track breaks dividing
the two in the middle.

Given that I can't add a ground plane to a stripboard, is there anything
else I can do that will help reduce the whining? Any other suggestions for
reducing digital noise is mixed-signal circuits like this? How does the
noise get from the SPI connection to the output? What techniques could I use
to determine that?

Thanks,
Tom

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