complex baseband

Hello all,
 I am currently working on analysing a VHDL code of a FM baseband
receiver.

I have a block in VHDL which accepts 2 inputs 
 1)one from the AD converter and 
 2)other a sawtooth wave

this block produces 2 outputs
 1)complex baseband I
 2)complex baseband Q

There is a lookuptable within this module.I hope he has used CORDIC in
his design.But iam not sure what this does to the input(does it
multiply the input with sine or cos or arctan,what is the significance
of having a sawtooth wave then ?)

 when i plot the abs(fft(complex(i,q))).i find 3 spectral lines ?what
is the significance of this ?
kindly reply 
jack

"jack kilby" <jackilby@yahoo.com> wrote in message
news:18668c6c.0311180130.4b1f3d6b@posting.google.com...
> Hello all,
>  I am currently working on analysing a VHDL code of a FM baseband
> receiver.
>
> I have a block in VHDL which accepts 2 inputs
>  1)one from the AD converter and
>  2)other a sawtooth wave
>
> this block produces 2 outputs
>  1)complex baseband I
>  2)complex baseband Q
>
> There is a lookuptable within this module.I hope he has used CORDIC in
> his design.But iam not sure what this does to the input(does it
> multiply the input with sine or cos or arctan,what is the significance
> of having a sawtooth wave then ?)
>
>  when i plot the abs(fft(complex(i,q))).i find 3 spectral lines ?what
> is the significance of this ?

Jack,

The first thing that occurs to me is that the sawtooth is akin to an
oscillator/address generator - equivalent to a clock input with a counter
inside.  But, that doesn't make sense entirely either.

What is done with the sawtooth (analog) inside the VHDL (binary?)?
What is the frequency of the sawtooth relative to the center frequency of
the FM?
What do you mean by "baseband receiver"?  Do you mean the output is complex
baseband?  If so, what is your assumed input?

"more input!".

Fred

The block is simply a phase rotator.  The sawtooth is the phase angle that
the ADC input gets rotated by.  This is equivalent to doing e^jwt.  The
phase rotator could be done as a look-up table provided the phase angle
resolution is relatively limited.  The size of a lookup will grow
exponentially with the precision of the phase angle input, so it quickly
becomes impractical.  That function can also be accomplished with a CORDIC
rotator, in which case the rotator is taking a complex input and rotating
it by the phase angle specified by the phase input.  See the CORDIC paper
on my website for more details on the algorithm.  There is also an article
there I wrote for the Xilinx XCell Journal describing the CORDIC mixer,
and you can see an example in the shortwave receiver demo block diagram on
the main page.  In your case, it sounds like the input is real-only, in
which case the Q input to the rotator is just held at 0.

jack kilby wrote:

> Hello all,
>  I am currently working on analysing a VHDL code of a FM baseband
> receiver.
>
> I have a block in VHDL which accepts 2 inputs
>  1)one from the AD converter and
>  2)other a sawtooth wave
>
> this block produces 2 outputs
>  1)complex baseband I
>  2)complex baseband Q
>
> There is a lookuptable within this module.I hope he has used CORDIC in
> his design.But iam not sure what this does to the input(does it
> multiply the input with sine or cos or arctan,what is the significance
> of having a sawtooth wave then ?)
>
>  when i plot the abs(fft(complex(i,q))).i find 3 spectral lines ?what
> is the significance of this ?
> kindly reply
> jack

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