Re: Question about the HBF in the DDC


I have another question about the bandwidth of the HBF. For a wide band
incoming signal, let’s say 20MHz, can we still use this HBF to filter
for this incoming signal, as the pass band of the HBF is only 0.6MHz.

On Tue, 1/4/11, Tom R. [email protected] wrote:

From: Tom R. [email protected]
Subject: Re: [Discuss-gnuradio] Question about the HBF in the DDC
To: “peng senl” [email protected]
Cc: [email protected]
Date: Tuesday, January 4, 2011, 8:31 PM

On Tue, Jan 4, 2011 at 1:13 PM, peng senl [email protected] wrote:

I get a question about the half band
filter in the DDC. I found that the LO (local oscillator) in my
daughter board is set at 0.2MHz less than the carrier center
frequency, which means that the IF of the output signal is center at
0.2 MHz.

I am assuming the NCO in the FPGA is
generating a sinusoid at 0.2MHz to mix the IF signal to base band.
Then we would get a ase band signal and a high order frequency part
at 0.4MHz. But the half band filter after the CIC filter has a pass
band of 0.6MHz from the reference materials what I have read. This
means that the high order part won’t be filtered by the HBF.

It’s generating a complex sinusoid. It’s not modulating the signal
like an analog LO but performing complex multiplication.


The passband of the HBF scales with your data rate. The bandwidth is
about 70% of the data rate.