Re: anti-aliasing


"John Larkin" <jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:85ckk3lgasdmpe5mf12dpu2fus4i25lka0@xxxxxxxxxx


Suppose one (actually, me) were firing an adc at some fixed rate,
ballpark 12 KHz in this case, and the input to the adc was a sine wave
of fixed but unknown frequency. The adc will take a bunch of samples,
ballpark 1000 maybe, and I want to compute the mean (ie, dc value) and
the mean of the abs value of the samples (ie, the ac value). That
works, but sometimes the input frequency aliases against the sample
rate and messes up the data, like gives a big average dc value when
there's really none there.

(This is not a Nyquist issue; the sample rate may be above or below
the sinewave frequency.)

So maybe I can fuzz up the sample rate so that it can't alias against
any constant sinewave frequency.

I could add a pseudo-random delay after every adc sample shot; but the
mean sample rate wouldn't change a lot. Or I could add successive
delays, essentially sweeping the sample rate down.

Any ideas?

John


If the granularity of your "pseudo-random delay" is small enough, and you
take enough samples, then the answer will converge on the correct one (i.e.
no effects of aliasing will be seen).

How many samples to take as a function of delay granularity to give an
acceptable amount of error? That's the hard part.

You should probably repost on comp.dsp, or go to comp.arch.fpga and ask for
Ray Andraka's help. He's smarter than everyone here -- combined!

Bob


.

Relevant Pages

  • Re: anti-aliasing
    ... Suppose one were firing an adc at some fixed rate, ... ballpark 12 KHz in this case, and the input to the adc was a sine wave ... any constant sinewave frequency. ...
    (sci.electronics.design)
  • Re: anti-aliasing
    ... Suppose one were firing an adc at some fixed rate, ... ballpark 12 KHz in this case, and the input to the adc was a sine wave ... I could add a pseudo-random delay after every adc sample shot; ...
    (sci.electronics.design)
  • Re: anti-aliasing
    ... John Larkin writes: ... Suppose one were firing an adc at some fixed rate, ... ballpark 12 KHz in this case, and the input to the adc was a sine wave ...
    (sci.electronics.design)
  • Re: anti-aliasing
    ... Suppose one were firing an adc at some fixed rate, ... ballpark 12 KHz in this case, and the input to the adc was a sine wave ... I could add a pseudo-random delay after every adc sample shot; ...
    (sci.electronics.design)
  • Re: anti-aliasing
    ... Suppose one were firing an adc at some fixed rate, ... ballpark 12 KHz in this case, and the input to the adc was a sine wave ... so I can't measure crossings to any useful accuracy. ...
    (sci.electronics.design)