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sampling freq for combined filter

Started by egan_nc August 5, 2003
Hi,
Can anybody please tell me how to set up the sampling freq for a
combined filters.
The sampling freq for the input signal to the first filter h1 is
1.5Mhz, the output of the first filter is then downsampled to 96khz
and feed into the second filter h2. h12 is the convolution of
h1*h2. To plot the freq response of h12 with freqz, what is the
sampling freq to use?
freqz(h12,1,2048,'whole','fs?????')
Thank you,
EngGan



You cannot convolve the two filters because their input rates are not
equal. You can:
1. Multiply the real-frequency responses together, H12(f) = H1(f)*H2(f)
= H1(w*1.5e6/2/pi)*H2(w*96e3/2/pi); or
2. Upsample h2 (the time-domain impulse response) from 96kHz to 1.5MHz,
then convolve h12 = conv(h1,h2), then use freqz with 1.5MHz as the
sample rate.

For #1, f is the actual hertz, and w is the "digital angular frequency",
i.e., w at Nyquist = pi.

Sincerely,
Glen Ragan

-----Original Message-----
From: egan_nc [mailto:]
Sent: Tuesday, August 05, 2003 3:34 PM
To:
Subject: [matlab] sampling freq for combined filter Hi,
Can anybody please tell me how to set up the sampling freq for a
combined filters.
The sampling freq for the input signal to the first filter h1 is
1.5Mhz, the output of the first filter is then downsampled to 96khz
and feed into the second filter h2. h12 is the convolution of
h1*h2. To plot the freq response of h12 with freqz, what is the
sampling freq to use?
freqz(h12,1,2048,'whole','fs?????')
Thank you,
EngGan
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