Re: [Discuss-gnuradio] Could this output be a problem with myHardware or the python codes?

[Bruhtesfa Ebrahim]

Hey Eric,

 

It is like this.

(1). when I am not transmitting anything rather just using the USRP as receiver, I see the environment signal level using USRP_fft.py in oscilloscope mode.

(2). When I start to transmit and recieve simultaneously, then the signal recived is the environment signals + the transmitted signal reflected  from the target + the signal leaking from the transmitter to reciver antenna.

 

So, I expect that the signal I receive in (2) should be much higher in amplitude than the one I receive in (1). But, what I am getting while doing this experment  is the opposite.

 Actually,I preffered to transmit and receive simultaneously using the USRP because my target is within a distance of 10meter(I am using the doppler radar for human movement detection).

 But now because of this isolation problem, I am thinking to Implement my radar as pulse radar such that I will transmit for a very short time then stop transmiting and start recieving, then transmit  etc...

So, what do you suggest me?

 

 

Thanks!

Bruhtesfa

On Fri, Nov 7, 2008 at 3:36 PM, Eric Blossom <address@hidden> wrote:

On Fri, Nov 07, 2008 at 11:19:40AM +0100, Bruhtesfa Ebrahim wrote:
>  Hey Paul,
>
> Ya! you are correct, the isolation is a problem.But what is curious is when
> i operate the transmitter and reciever simultaneously,the signal level of
> the reciver decreases too much(cancels out) . If some of the signal from the
> transmitter antenna is also recived by the reciver antenna, I think  the
> signal recived should increase in amplitude, because the frequency of the
> signals sent and recived are almost the same(except the small doppler
> shift). But, what occurs is the opposite.
>
> Bruhtesfa

You may be overdriving the input resulting in some kind of non-linear
behavior.  Have you plotted the received signal?  Does it look like
what you think you're sending?

You say that you are trying to receive a doppler shifted echo.  Given
your physical setup (antennas, their pattern and their spacing, the
distance to the object that you expect the signal to reflect from, its
velocity and radar cross section), of the power received at the Rx
antenna -- according to theory -- how much of the power will come
directly from the Tx antenna, and how much will be reflected off of
the moving object?  Assume a free space model for simplicity.

Eric