My (limited) experience is that the data aquisition systems don't anti-alias filter, and many folks using them aren't aware of aliasing issues.
Always sampling at a high rate then digitally filtering and decimating would be cool. I wouldn't want to have the decimation ratio be anything but an integer one, however.
-- ------------------------------------------- Tim Wescott Wescott Design Services http://www.wescottdesign.com
How do data acquisition systems handle anti-aliasing with variable sample rates? I ask this because the design of the filter is based on the sampling rate and the sampling resolution*, so one filter would not seem to work.
Some systems with variable sampling don't have any filtering at all, but doing this strikes me as a problem waiting to happen.
Alternatives I can think of are anti-alias detection, or one sample rate and discard unnecessary data, which gives the appearance of variable sampling rates.
-- * The objective being to have the amplitude of the analog input > 1/2 LSB at Fsample/2. The filter fall-off determines the F-3dB point. Phase and step input response depend on the system needs. Thanks, Dave
Hello Dave,
They use variable filters. In the LF range that could be as simple as switched capacitor filter chips where the clock rate is varied proportionately to the sample rate.
At higher rates there are two main tricks that I have used:
a. Variable LC filters: Either varicaps or "other means". I prefer the latter, often using PIN diodes or FET arrays to pull reactive components in and out of the game.
b. Heterodyning: Mix the input signal to where filters are easily built and easily tunable, then mix back to baseband. That can be really elegant. Blows peoples minds, too. I have heard comments like "How on earth does this thing work?". Add an hour and a 2nd thermos of coffee to the design review meeting when you go that route.
If they can be sure there is not much there in the spectrum above Nyquist it may be ok. Otherwise, not so good.
Group delay flatness and stuff is where things often become very tough...
Regards, Joerg
There are companies that make digitally controlled filters. Basically they have DACs inside to set the frequency.
If the number of frequencies is small, you can just design a filter for each and switch them in.
A common trick is to do the digitizing at a constant rate and digitally filter the data before resampling.
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In article , Tim Wescott wrote: [....]
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If the decimation factor is large enough and rational, it isn't all that bad of a problem. The cut off of the anti-alias is so much below the Nyquist of the basic sample rate that you can consider the function a straight line between samples.
Meaning like this:
Y(0) = X(0) Y(1) = (X(1000)+X(1001))/2 Y(2) = X(2001) Y(3) = (X(3001)+X(3002))/2 Y(4) = X(4002)
-- -- kensmith@rahul.net forging knowledge
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