Basic question about filter

GNU Radio
1

Hello everyone:
You see in theory there should be a low-pass filter after ADC for
eliminating the image. Also after DDC there should also be a
filter. But in USRP there are not such things. So do the cic filter and
half band filter implement that function besides decimation?

Best regards

2

On 18/06/12 09:46 PM, signalswdm wrote:

Discuss-gnuradio mailing list
[email protected]
Discuss-gnuradio Info Page

Actually, in typical architectures using I/Q, there are low-pass filters
in front of the ADC to eliminate aliasing prior
to sampling. So the CIC decimator “sees” a signal that has already been
band-limited appropriate to the sample
rate of the ADCs.

The CIC decimation process is filtering, so out-of-band products
appearing at the output are usually suppressed
by at least 80dB, and usually more.

3

On Jun 18, 2012, at 6:54 PM, Marcus D. Leech wrote:

Indeed, to re-enforce Marcus’s point with precise English terminology:
Downsampling is a process that reduces the sample rate of a digital
signal by simply discarding samples.
Decimation is the compound operation that incorporates downsampling and
an appropriate low-pass filter.

4

Thank you
To make it clear, I use USRP1. Does that mean in common the
signals that come out of ADC chips (in USRP case it is the signals come
out of AD9862 before processed in FPGA) is already filtered and
aliasing prior is partly eliminated ? In the datasheet I do not see
anything about that. If I am wrong please correct me.

Best regards

At 2012-06-19 09:54:11,“Marcus D. Leech” [email protected] wrote:
On 18/06/12 09:46 PM, signalswdm wrote:
Hello everyone:
You see in theory there should be a low-pass filter after ADC for
eliminating the image. Also after DDC there should also be a
filter. But in USRP there are not such things. So do the cic filter and
half band filter implement that function besides decimation?

Best regards

Discuss-gnuradio mailing list
[email protected]://lists.gnu.org/mailman/listinfo/discuss-gnuradio
Actually, in typical architectures using I/Q, there are low-pass filters
in front of the ADC to eliminate aliasing prior
to sampling. So the CIC decimator “sees” a signal that has already
been band-limited appropriate to the sample
rate of the ADCs.

The CIC decimation process is filtering, so out-of-band products
appearing at the output are usually suppressed
by at least 80dB, and usually more.

5

In general that is correct, analog filtering has been applied to the
signal chain prior to the ADC to prevent significant spectral folding.
Specifically, the individual radio daughter cards provide filters of
appropriate bandwidth for each design, there is no analog filtering
circuit in the actual USRP1 motherboard. The BasicRX and TX boards are
the sole exception to this, they provide no on board filtering.