Re: Complex baseband

morpheus wrote:
Howdy,
For FM/AM demod, you require a complex baseband. I am downconverting
the IF from 1MHz to baseband by multiplying the input stream (12 bits)
from the ADC with Sin and Cosine outputs of a DDS and therefore,
generating I, Q
I am using standard 18x18 signed multipliers in Xilinx to do the
mixing. Is this right or should I use a complex multiplier?

Does Xilinx pawn something called a "complex multiplier"? For what you're doing, a pair of plain old signed multipliers is appropriate.

The reason why I am asking this question is because, I had implemented
the whole datapath (Downconversion, Rate decimation, Compensating FIR)
and verified it on the bench and then implemented CORDIC for getting
FM, which never worked.
So instead I tried to do AM demod by not using CORDIC but (Sq(I) +
Sq(Q)) using standard 18x18 signed multipliers and still no AM demod
which is supposed to be simple.
Then I took the CIC and CFIR filters out and tried to do demod
straight after the mixing, still nothing.
So I'm kinda stuck here, any help will be appreciated
Thanks
-M

You should be able to put a pure tone into your IF around 1MHz and have it show up as a complex number of constant amplitude that's rotating at the correct rate (i.e. 1MHz in should be DC out, 1.001MHz in should be +1kHz, 0.999MHz should be -1kHz).

If you're not getting that, you're doing something wrong.

p.s. I don't have access to Matlab and System generator, but I'm
buying them next week. I figured, doing digital radio design without
modeling tools is like digging your own grave!!!

You may want to check out SciLab. It's free, and while it has fewer bells and whistles it does the basics at least as well as MatLab.

--

Tim Wescott
http://www.wescottdesign.com
.

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