Complex baseband

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?
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
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!!!

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

"morpheus" <saurster@gmail.com> writes:

> 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!!!

Check out the open-source project GNU Octave:

http://www.gnu.org/software/octave/

It's kinda like a free Matlab.
-- 
%  Randy Yates                  % "Rollin' and riding and slippin' and
%% Fuquay-Varina, NC            %  sliding, it's magic."
%%% 919-577-9882                %  
%%%% <yates@ieee.org>           % 'Living' Thing', *A New World Record*, ELO
http://home.earthlink.net/~yatescr

morpheus wrote:

> Howdy,
> For FM/AM demod, you require a complex baseband.

Not necessarily. Complex baseband is just one of the possible ways.

> 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?

????
Generating I and Q by mutiplying by sin and cos *is* a complex multiplier.

> 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

Look for bugs.

> 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!!!

Matlab is a glitter toy for students with many useless bells and 
whistles. A model of the system can be easily built in any programming 
language that you are using.


Vladimir Vassilevsky

DSP and Mixed Signal Design Consultant

http://www.abvolt.com

Vladimir Vassilevsky wrote:
> 
> 
> morpheus wrote:
> 
>> Howdy,
>> For FM/AM demod, you require a complex baseband.
> 
> Not necessarily. Complex baseband is just one of the possible ways.
> 
>> 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?
> 
> ????
> Generating I and Q by mutiplying by sin and cos *is* a complex multiplier.
> 
>> 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
> 
> Look for bugs.
> 
>> 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!!!
> 
> Matlab is a glitter toy for students with many useless bells and 
> whistles. A model of the system can be easily built in any programming 
> language that you are using.
> 
If you stick to the basic programming language of MatLab it can 
significantly speed up algorithmic development.  It's inherently 
matrix-oriented structure will let you do things at about a 1:10 ratio 
of lines-of-code compared to C or C++, and it's interpreted nature makes 
it easy to do things like hit a breakpoint then graph the intermediate 
results.

Of course, SciLab will do all of the above for free, and with a more 
solidly object-oriented programming language.  If you just know you'll 
die without the bells and whistles it isn't for you, however.

-- 

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

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