Hi, I modulate a 10.7 MHz center frequency with 20 kHz deviation. When I send a 1 kHz sinus signal to modulate the 10.7MHz, I demodulate this FM signal with IdQ-QdI. So this result is a goos sinusoide signal at 1 kHz But when I calculate I=B2+Q=B2, there's a 20 kHz sinus on this signal and so when I do (IdQ-QdI)/(I=B2+Q=B2) I have this 20 kHz signal modulated on my 1 kHz sinus wanted.. Is for calculate I=B2+Q=B2 I must add a delay on the I branch to have I and Q in phase before compute I=B2+Q=B2. Or may I filter the I=B2+Q=B2 filter before doing division ? But If I filter it, the division process won't remove a AM signal added to my FM signal. (AM is unwanted). Thanks.
FM demodulation
Started by ●October 22, 2006
Reply by ●October 22, 20062006-10-22
Patrick, You have been on this problem for a while. The I and Q must be in phase. The dI, dQ must be in phase to match each other and the I and Q. If you have a purely FM signal, smoothly modulated as you describe, wouldn't you expect the AM signal to be constant? If it is not too long, post your MATLAB script. Dirk Bell DSP Consultant patrick.melet@dmradiocom.fr wrote:> Hi, > > I modulate a 10.7 MHz center frequency with 20 kHz deviation. > When I send a 1 kHz sinus signal to modulate the 10.7MHz, I demodulate > this FM signal with IdQ-QdI. > So this result is a goos sinusoide signal at 1 kHz > > But when I calculate I=B2+Q=B2, there's a 20 kHz sinus on this signal and > so when I do (IdQ-QdI)/(I=B2+Q=B2) I have this 20 kHz signal modulated on > my 1 kHz sinus wanted.. > > Is for calculate I=B2+Q=B2 I must add a delay on the I branch to have I > and Q in phase before compute I=B2+Q=B2. > > Or may I filter the I=B2+Q=B2 filter before doing division ? But If I > filter it, the division process won't remove a AM signal added to my FM > signal. (AM is unwanted). >=20 > Thanks.
Reply by ●October 22, 20062006-10-22
patrick.melet@dmradiocom.fr wrote:> Hi, >=20 > I modulate a 10.7 MHz center frequency with 20 kHz deviation. > When I send a 1 kHz sinus signal to modulate the 10.7MHz, I demodulate > this FM signal with IdQ-QdI. > So this result is a goos sinusoide signal at 1 kHz >=20 > But when I calculate I=B2+Q=B2, there's a 20 kHz sinus on this signal a=nd> so when I do (IdQ-QdI)/(I=B2+Q=B2) I have this 20 kHz signal modulated =on> my 1 kHz sinus wanted.. >=20 > Is for calculate I=B2+Q=B2 I must add a delay on the I branch to have I=> and Q in phase before compute I=B2+Q=B2. >=20 > Or may I filter the I=B2+Q=B2 filter before doing division ? But If I > filter it, the division process won't remove a AM signal added to my FM=> signal. (AM is unwanted). >=20All members of the (I dQ/dt - Q dI/dt)/(I^2 + Q^2) should correspond to=20 the same instant of time, and your sample rate should be considerably=20 higher then the deviation. You may have to compute the derivatives with=20 FIR differentiators. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
Reply by ●October 23, 20062006-10-23
In fact, I've simulated the FM demodulation under Simulink. But know I'm on the real world, I've described the FM demodulation with VHDL langage and implented the recevier under Altera Stratix chip. So I see my new problems in the real world, not in VHDL simulations... So I have compute I and Q with a downconverter. I first subsample my 10.7 MHz FM at 5.136 MS/s and so I've got a 428 kHz FI, then I downconvert to banseband by multiplying this 428 kHz FI with cosine to have I and sinus to have Q, these cosine and sinus have a frequency of 428 kHz. And so when I compute I=B2+Q=B2, I and Q are not "really" in phase because they are produced with cosine and sinus. I is in-phase and Q is in-quadrature of phase. It seems that the oscillation that I have on I=B2+Q=B2 is due to a delay of 13 samples beetween I and Q... I don't try ever to delay I by 13 samples and to see if I've already have the oscillation on I=B2+Q=B2... because I don't undersatnd why I have to put this delay !! I don't use a FIR to compute dI and dQ, but I think IdQ-QdI is right because I have a pure sinus at 1 kHz at this step, and I sent a 1 kHz into my FM modulation... It's just when I do (IdQ-QdI)/(I=B2+Q=B2) that my sinus is distorted by the oscillation that I can see on (I=B2+Q=B2).. I put IdQ-QdI in phase with I=B2+Q=B2 before doing the division But I think my problem is only on the I=B2+Q=B2 calculation...=20 Thanks...
Reply by ●October 23, 20062006-10-23