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FMCW Radar beat frequency problem by using USRP-2953R 40MHz

HI

I'm using USRP-2953R 40MHz to set up a FMCW radar system. But I got a problem which is the beat frequency doesn't change at all when i try to measure the different distances between USRP and objects. Here is my FMCW spec and program below.

 

Sample rate: 15MS/s

Carrier frequency: 2.4GHz

Chirp modulated waveform: Triangle

Chirp period: 1ms

Chirp bandwidth: 7MHz

Samples number: 7000

 

Using formula : beat frequency = (4*bandwidth*distance) / (light speed*Chirp period)

I expect that when i measure the 20 meter away object from USRP, the beat frequency will increase about 2K Hz, but it doesn't change anything. Did i do anything wrong in my FMCW spec or in my program? Please give me some suggestions.

 

Regards

 

 

 

 

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Dear J_RF,

 

        I think you should simplify your approach, since there are a lot of factors which could prevent your setup from working. First of all, I am not an adept of numerical mixing of the transmited chirp with the received chirp, the way you did in your vi. Numerical mixing of the TX with the RX signal may not dirrectly yield the beat signal, as in the analog multiplication case (matter of sampling frequencies, alias etc.).

          You should perform cross-correlation instead, since your radar is closer to a chirped impulse radar rather than a FMCW radar.

        Cross-correlation (very easy to do in MATLAB) will show you the delay between the transmitted and the received chirp. It will be easy to scale the x time axis into distance (d=time_of_flight/speed_of_light).

        Please test your setup using a long coaxial cable as a delay line (then doubling/halfing the length). Do not test it in the field on the first hand, since the result will not be that relevant.

 

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