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

Richie Burnett rburnett at richieburnett.co.uk
Fri Jan 27 19:49:46 CET 2017


> You say that this kills most of the noise for you, so I'm a little suspicious that I'm not seeing the same thing. Did you need to do anything else?

I just had the DAC outputs AC coupled straight into two channels of a mixer for my tests. So I guess all of the noise was coming from the DAC because there wasn't anything else following it.

I stopped the DAC by switching off its clock source. Switching from on-board oscillator to auxiliary ACLK or something, I don't have th e details in front of me. 

>  So, if I take it as given that the DAC is spitting out a good portion of this noise, I've got a slightly different question:
> 
> What can I do to the differential amp and/or filters to improve their filtering performance at high frequencies? Looking on the scope still suggests that a good amount of this noise is at RF frequencies, so I'd be interested to see if that helps.

Why do you want to filter out the RF Tom?

Your hearing stops at 20kHz or so, does it not? So if you are hearing noise it must be below 20k. Audible hiss and conducted RF emissions are two seperate things.

The filtering that follows your DAC outputs shouldn't do anything to any frequencies that you can hear. It's designed to attenuate higher frequencies in order to:

1. Stop ultrasonic crap getting into the ADC of something that is recording the audio and cause aliasing, but any decent sound card or whatever should have its own anti-aliasing filters before the ADCs these days. 

2. Stop high frequencies getting into power amps, causing heating, burning out tweeters, etc.or getting into tape recorders beating with the bias oscillator, etc.

3. Stop RF leaving the enclosure of your product so that it doesn't become an unintentional radio transmitter. Something that is important if you were wanting EMC compliance for example.

I wouldn't expect these anti-imaging reconstruction filters to reduce audible noise, unless they have a gain of less than unity in the audio passband.

What are you listening to the output of your dsp system with? I.e. What is the signal chain following your dsp system?

Also, if you take one of your scope probes, clip the ground lead to the tip of the probe, and then touch the grounded tip to the ground plane of your running board, do you still see excessive RF noise?

Do you have your dsp connected to a computer via a debugger /programmer when your doing your measurements? If so, flash the chip and run.it stand alone. Usb in particular incredibly noisy. 

Hope this helps, 

-Richie, 




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