How to measure frequency of received chirp wave?

[firstking]

I am able to transmit a chirp signal. 

Now I want to measure the transmitted signal from a receiver. In order to calculate frequency different thus the range of the source I need to get an array of frequency values over time. 

How can I get frequency - time plot from received chirp wave.

Thanks,

[AgentAstronaut]

What hardware are you using?

Without any context my first thought is for you to use MT Demodulate FM to get the RF signal you are looking for. The alternative is to take an fft and do a peak search.
 
Cheers,
Michael Bilyk

[firstking]

Hi thanks for your reply. 

I am using usrp n210 with sbx40 daugtherboard.

Generating chirp signal with linear frequency change. Than, ı want to obtain that frequency change over time in receiver side. Actually want to get the delay in frequency to get beat frequency to calculate range. 

Regards, 

[AgentAstronaut]

Interesting. Well I think that the you may still be able to do the method I described though it may require some testing to see if it works the way you expect.. It is worth a shot since it is pretty easy. Just connect the MT Demodulate FM VI to your IQ data. The carrier offset output should give you the baseband offset of the signal.

Cheers,

Michael

[firstking]

Getting the baseband rf signal is not the problem that I ask. 

My question is how to get the frequency change of that signal over time. Demodulation just returns me the message signal. Then I will need to read the frequency change of the signal.

Let's say, I have signal whose frequency changes from f1 to f2, then I must have a ramp like plot whose slope is constant and increases from f1 to f2 on vertical axis where time is on the horizontal axis.

[AgentAstronaut]

Hi firstking,

The demodulation VI also has an output called carrier offset in addition to the demodulated signal which should give the frequency offset of the signal from the actual carrier frequency. For instance, if you had a carrier frequency at 1.5GHz and a CW at 1.501GHz, the offset would be 1MHz. In this case you would be able to determine that you signal is at 1.501GHz because you know the Carrier and you know the offset. In the case of a chirp, if the chirp is relatively slow, you should be able to read portions of the chirp into the demodulation VI and get the frequency offset, adding that to whatever you set the carrier frequency to be should give you the information you want. We are essentially taking advantage of the built in frequency detection of the FM Demodulation vi. Have you given this method a try? It should only take a moment to get a feel for how it would work.

I hope this helps. Another thing I could suggest is taking an FFT and doing a peak search, but that will be less accurate. You may have to experiment to determine what works best for you.

Cheers,

Michael

[Mihai_Liviu]

Hello firstking,

    I assume you want to know the delay of the received chirp to be able to use your system for radar purposes. You intend on obtaining an instantaneous frequency plot versus time, in which you can estimate the received signal delay easily. I do not think that is a good approach. The characteristics of the FM demodulator may influence the delay you are looking for

    It will be a lot easier if you save both the transmitted and the received chirp and use a mathematical approach to compute the delay. Cross-correlation works very well. You can use the predefined crosscorelation function from any signal proceesing software (like Matlab).

BR,

Mihai

[Stewie2K]

 Hi,

I suppose I have landed up in the same place as firstking.

I am successful in transmitting a Chirp Signal(a signal whose frequency increases linearly with time) through a USRP and receiving it back.

Now I don't have a Target Emulator by which I mean I have simulate the delay in the received signal and then compare it with the Transmitted IQ, so that I could get the beat frequency.

I have padded zero's in the Transmitted IQ at the beginning, this would essentially be the Simulated delayed RX chirp.

Now have a A-T graph and a A-F graph, I wanted to have a F-T graph so that I could find the difference Frequency and in-turn find the range.

Any leads in going ahead will be really appreciated.

Thanks in advance!!

[Mihai_Liviu]

Hello Stewie2K,

          Use spectrogram in order to plot the F-T graph of both transmitted and received chirp. But that is not such a precise method in order to determine the beat frequency, especially in a real scenario, with multiple targets. Multiply the baseband versions of the TX and RX signals, low pass filter them, then apply FFT to get the beat frequency. A simpler approach will be to use crosscorrelation and find the range profile.

BR,

Mihai