Re: [Discuss-gnuradio] Transmit and Received Signals are Different

[Josh Blum]

On 07/19/2012 03:30 PM, Frederick Lee wrote:
> HI,
> 
> I am currently using two USRP N210 rev4 with a RFX2400 daughter board and a
> VERT2450 antenna on each. I am having trouble transmitting a signal between
> the two USRPs even though they are right next to each other.
> 
> On the transmitting flowgraph, I have a signal source sending a square wave
> at 1kHz with an amplitude of 100m connected to a USRP sink. On the
> receiving flowgraph, I have a USRP source multiplied with a complex cosine
> to shift the signal to baseband. Then I use a lowpass filter before sending
> to a scope sink or fft sink.
> 
> Whenever I use the scope sink to look at the incoming signal, it looks
> nothing like the square wave I am sending. It sometimes looks like a sine
> wave with a frequency other than 1kHz, or it looks something like the
> picture titled received_signal.png ( I only showed one of the lines because
> it was too cluttered with both of them ). When I send a cosine or a
> constant signal, I receive a sine on the scope, but the amplitude jumps a
> little and the frequency jumps up and down one or two kHz.
> 
> I also wanted to know what happens to the signal in the RFX2400
> daughterboard and what the FPGA does to the signal. I think I read
> somewhere that the FPGA converts the signal to baseband, but I was not sure
> if that is true. I have attached the flowgraphs used for transmitting and
> receiving.
> 

Perhaps you are seeing a small frequency offset between RX and TX. I
recommend experimenting a little more in simulation:

* What happens you pass the same square wave through that low pass
filter you mentioned. Whats it look like?

* What happens when you apply a small frequency shift to the square
wave? I recommend using the channel model block for this.

-josh