Reply by Fred Marshall June 24, 20082008-06-24
It seems to me that Doppler shift is *not* like AM modulation.  It's more 
like time compression / expansion which for a sinusoid appears like a 
frequency shift.

Anyway, this is CW radar which normally transmits a fixed-amplitude, 
sawtooth in frequency waveform.  Then, in the receiver, you see leakage from 
the transmitter and you see the received signal.  At any instant there are 
two frequencies which are separated by the time delay to the target and back 
and the rate of change of frequency in the sawtooth / frequency sweep.

If things are just right, re amplitudes then you may be able to see a beat 
frequency in the received signal amplitude.  The beat frequency would be the 
difference frequency which is what you want to determine range / distance /
delay.

So now the task would be to design the processing that will determine the 
(relatively stable) frequency difference between transmit and receive 
signals.

Fred 


Reply by night June 24, 20082008-06-24
>On 24 Jun, 13:09, "night" <sandracorr...@gmail.com> wrote: >> I've aprox. 13secs of samples at 20KHz so I have a great number of
sample=
>s >> but I took only a few because I wasn't able to see anything performing
th=
>e >> FFT over a lot of samples... >> >> So, I should generate a reference baseband signal =A0and correlate it
wit=
>h >> the received signal (I know how to do it but...at what frequency u
think =
>I >> should modulate it?I only know that the trasmitter uses one carrier and
i=
>t >> doesn't use frequency modulation signal transmission) > >You need to read up on Doppler. A Doppler shift is >conceptually the same as an AM-modulation. This >modulation happens because of the interaction between >target and pulse, not because of anything happening in >the radar. > >You need knowledge about the baseband signal in order >to estimate Doppler shift, since you need to compare >the recieved signal with AM-modulated versions of >the reference signal. > >Rune
Thank u...I think I've explained me in a wrong way talking about the frequency..anyway u've told me what I needed.. I should look for getting my baseband reference signal first of all...:)
>
Reply by Rune Allnor June 24, 20082008-06-24
On 24 Jun, 13:09, "night" <sandracorr...@gmail.com> wrote:
> I've aprox. 13secs of samples at 20KHz so I have a great number of samples > but I took only a few because I wasn't able to see anything performing the > FFT over a lot of samples... > > So, I should generate a reference baseband signal &#2013266080;and correlate it with > the received signal (I know how to do it but...at what frequency u think I > should modulate it?I only know that the trasmitter uses one carrier and it > doesn't use frequency modulation signal transmission)
You need to read up on Doppler. A Doppler shift is conceptually the same as an AM-modulation. This modulation happens because of the interaction between target and pulse, not because of anything happening in the radar. You need knowledge about the baseband signal in order to estimate Doppler shift, since you need to compare the recieved signal with AM-modulated versions of the reference signal. Rune
Reply by night June 24, 20082008-06-24
I've aprox. 13secs of samples at 20KHz so I have a great number of samples
but I took only a few because I wasn't able to see anything performing the
FFT over a lot of samples...


So, I should generate a reference baseband signal  and correlate it with
the received signal (I know how to do it but...at what frequency u think I
should modulate it?I only know that the trasmitter uses one carrier and it
doesn't use frequency modulation signal transmission)

I'm not finding documentation about continuos wave radar (without
frequency modulation) so thanks for your help!!!!



Reply by Rune Allnor June 24, 20082008-06-24
On 24 Jun, 12:43, "night" <sandracorr...@gmail.com> wrote:

> &#2013266080;I'm new on radar comunications but not on signal proccesing and I'm > having problems...I'm trying to perform the FFT to get the doppler > frequency offset but I'm not sure about what I'm doing because I haven't > found it. > > &#2013266080; &#2013266080; To obtaining such frequencial resolution I've taken 200 samples of the > received signal and I've perform the 2048-FFT.
How many samples do you have? As a concept check, use all available data and compare the spectrum with the spectrum of the baseband reference data. Apart from that, I think the usual way to estimate Doppler shift is to modulate the baseband reference up or down in frequency, and use a correlator to do the comparision in time domain. Use several correlators, one for a Doppler interval, and search for the correlator with highest output magnitude to find the Doppler shift. Rune
Reply by night June 24, 20082008-06-24
Hi,

  I'm supposed to analize a received signal on a Doppler continuous wave
radar for an university practice. 

  I only have a file with samples taken at 20KHz. The only information I
know about the radar is its center frequency (24GHz) and trasmitted signal
power. The recieved signal values that I have are the baseband outputs of
the front-end that have a DC offset of 2.6Volts and seem to be a sinusoidal
wave. 

 I'm new on radar comunications but not on signal proccesing and I'm
having problems...I'm trying to perform the FFT to get the doppler
frequency offset but I'm not sure about what I'm doing because I haven't
found it.


    To obtaining such frequencial resolution I've taken 200 samples of the
received signal and I've perform the 2048-FFT.


    Could someone help me?? I know I'm making a "very simple" mistake but
I'm not being able to find it....

     Thanks!