On 12 Sep 2006 13:34:58 -0700, "MadEngineer"
wrote:


Henry Kiefer wrote:

So the phase shifter is NOT for flatten the group delay variance (source is
the Tayloe mixer low-pass)?
If I understand your question correctly, that is true. The IQ output
from the Tayloe mixer (or any IQ mixer) is not upper and lower
sideband. To get those you must do further signal processing, which
usually involves shifting the I and Q channels ninety
degress with respect to each other then summing or subtracting the
channels depending on which sideband you want.

Is multiplying I with Q enougth to demodulate FM as in a quadrature
demodulator?
I cannot find a suitable theory page to look for.

I can't either. The IQ signals multiplied do not make an FM quadrature
detector as the phase does not really shift much over the NBFM
bandwidth. There may be a clever way to extract FM more direcly from
the IQ channels without first detecting a sideband, but I don't know
it.

FM the problem is not responding to amplitude changes but frequency
changes. So any system that can count and measure frequency and
render a pattern based on changing frequency.

Synthetic PLL or simple a software PLL. would do it.
The synthethetic PLL approach means a software oscillator locked
to a varying frequency external signal. The error word generated is
the demodulated signal (apply to D/A or use raw).

How much dynamic range do I need? I thought about 100dB?
If I recall theory I loss 2dB if limiting the signal to remove AM sensitivy.techniques,

Dynamic range is one of those the more the better but, many things
like noise eat away at it. These days a radio with 80db is good
and 90DB excellent, 100 Db is attainable.

You need good dynamic range up to the limiter.

You need good dynamic range up to the first selectivity that can
remove offending close in signals.

Then you can limit hard. If you limit before or without adaquate
selectivity you will have intermodulation problems.

This can be done with
analog circuits as discussed. After the limiter an A/D converter is
not even needed in theory--a fast-running timer hooked to a digital
port could do the trick. I don't remember any loss by removing AM in
the limiter (since all the the information is contained in the
frequency of the signal), but my theory is in the distant past, back
when FM meant 'funny math'.

If the signal is limited then zero crossings are enough. That could
be expressed as 1bit. Your now working in the time/frequency
domain.


Allison