hello, I'm designing a receiver for FM demodulation and I have a little problem with the filters of the downconverter. Here's my specifications : FM carrier 10,7 MHz, 20 kHz deviation. I send to the modulator a 19.2 kHz sinus signal. At the receiver, I first band-pass filter the carrier : center to 10,7 MHz with 80 kHz bandwidth. Then I undersample at 4 MS/s : so I have got a first IF signal center at 1,3 MHz with 80 kHz bandwidth. Then I need to downconvert to 0 IF by multiplying with cos and sin to obtain the I and Q signals. So, I multiply with a sinus and a cosinus at 1,3 MHz and then I need to put out the second order frequency by a low pass filter with 40 kHz bandwidth. My problem becomes from this last filter, when I take a bandwidth of 40 kHz the ouput signal of my demodulator (after donwconverting I compute the derivative of the signal) is not my original 19,2 kHz sinus but it's a triangle and if I put a 80 kHz bandwidth my 19,2 kHz sinus is correct !! What's happen ? Any suggestions will be welcome... Best regards.
Digital FM demodulation
Started by ●October 3, 2006
Reply by ●October 3, 20062006-10-03
patrick.melet@dmradiocom.fr wrote:> hello, > > I'm designing a receiver for FM demodulation and I have a little > problem with the filters of the downconverter. >[...]> > My problem becomes from this last filter, when I take a bandwidth of 40 > kHz the ouput signal of my demodulator (after donwconverting I compute > the derivative of the signal) is not my original 19,2 kHz sinus but > it's a triangle > and if I put a 80 kHz bandwidth my 19,2 kHz sinus is correct !! > > What's happen ?Unless there is a trivial mistake somewhere, it could be the effect of the excessive band limiting and/or nonlinear phase in the filter. Those effects result in the nonlinear distortion of the demodulated FM signal. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
Reply by ●October 4, 20062006-10-04
Hi, What are your tools? software and hardware I mean. I want to design PM demodulator but my IF is 70 mhz. And I have been using C6713 DSK and Matlab/Simulink. May we help each other.
Reply by ●October 4, 20062006-10-04
hi I'm working with Matlab/Simulink to the system design and simulation of SNR at the output of the demodulator and AWGN in the channel. Then I'm going to make my models in VHDL language under Quartus and on a Stratix FPGA.
Reply by ●October 5, 20062006-10-05
patrick.melet@dmradiocom.fr wrote:> hi > > I'm working with Matlab/Simulink to the system design and simulation of > SNR at the output of the demodulator and AWGN in the channel. > > Then I'm going to make my models in VHDL language under Quartus and on > a Stratix FPGA.You may know, if you use Matlab/Simulink you can also generate DSP code automatically. Then your problem will be my problem: selection of sutiable adc card. Because Simulink has restricted adc card support for automatic code generation. pls write me private messages. murselonder@yahoo.com