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THD digital FM demod

Started by Unknown June 2, 2007
Hi,

I have designed a digital FM demodulator...

I have a 70 MHz FI signal that I downconvert to baseband

This demodulator works fine with Analog Bessel 24 order filter on I
and Q signals before demodulating.
When I change this filter with a digital FIR filter (phase lineartity)
I have a -50 dB THD

My message signal is a pure 10 MHz filter sine

My -50 dB THD is due to a 20 Hz sine (2 order harmonic) with the
analog bessel filter I have -80 dB THD

I don't have the 20 MHz sine not on I and Q, not on IdQ-QdI but this
second harmonic is on I=B2+Q=B2 !!!

So on the (IdQ-QdI)/(I=B2+Q=B2) signal I have this 20 MHz at -50 dB !!!

What kind of digital filter must I put on I branch and Q branch ?

thanks.

On Jun 2, 12:36 pm, fpga.vhdl.desig...@gmail.com wrote:
> Hi, > > I have designed a digital FM demodulator... > > I have a 70 MHz FI signal that I downconvert to baseband > > This demodulator works fine with Analog Bessel 24 order filter on I > and Q signals before demodulating. > When I change this filter with a digital FIR filter (phase lineartity) > I have a -50 dB THD > > My message signal is a pure 10 MHz filter sine > > My -50 dB THD is due to a 20 Hz sine (2 order harmonic) with the > analog bessel filter I have -80 dB THD > > I don't have the 20 MHz sine not on I and Q, not on IdQ-QdI but this > second harmonic is on I=B2+Q=B2 !!! > > So on the (IdQ-QdI)/(I=B2+Q=B2) signal I have this 20 MHz at -50 dB !!! > > What kind of digital filter must I put on I branch and Q branch ? > > thanks.
Hello - Some things I'm not clear on with your design. Is one case where you down convert, analog filter, sample and then demodulate, and the other case where you down convert, sample, digital filter and demodulate? If this is the case, you just might have a problem with aliasing. Also if you are inputting 10MHZ, then each of I^2 and Q^2 will have 20MHz components. But they should be 180deg out of phase so I^2 + Q^2 is constant. Make sure your internal sample rates will allow for the double frequency components. If you can, give more details about the signals, where the sampling and filtering occurs and in what order. Also are you sure that your I and Q signals are out of phase by 90 degrees and have equal gains. Feed them to a O'scope in x-y mode, and you should just plot a circle when you feed in just an unmodulated carrier. Set the carrier frequency so that after downconverting, it is not 0 hertz, but maybe something like 1000Hz. Are you really using a 24th order analog Bessel filter in your design? Wow! Clay
Hi,

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yes i sampling first the signal then downconvert to baseband then
filter (in one case with analog 24th order bessel and second case FIR
filter)
It's only in matlab simulation I do that

I look at the good digital filter that have the same THD performance
than the bessel analog filter

I don't know what kind of filter to put in theory this filter is just
here to remove the double NCO frequency that do the downconversion but
it have importance on the THD at the output of the demodulator

do you know what kind of digital filter is to have a very good THD
thanks