Re: [Discuss-gnuradio] Carrier leakage on transmit

[Matt Ettus]

On 05/25/2010 03:17 AM, Charles Brain wrote:
> Hi,
>
> Is there any way of calibrating out or mitigating the carrier at the WBX
> tx frequency? I am sending
> a wideband signal and the carrier (which I assume is due to dc coupling
> in the WBX) is at a significant level compared to my signal.


What Johnathan describes is a way of hiding the DC offset by moving it 
outside your passband.  The proper thing to do is to actually correct 
it.  In order to do that, you'll need to write a little bit of code, but 
it isn't too complex.  The DC offset will be a function of many things, 
including LO frequency and temperature.  These are the steps you need to 
follow:

1 - Tune your TX to the frequency you want and turn it on.  You don't 
need to transmit a strong signal, but you do need to transmit.  You can 
transmit at a very high interpolation as well, since there is really no 
signal there.

2 - Tune your RX to a different but nearby frequency.  +1 MHz away is 
reasonable

3 - Measure the amplitude of the TX DC offset as received by the RX (in 
this case at -1 MHz).  Iteratively adjust your I TX DC offset number 
until you get to the lowest power you can see.  Then do the same for Q. 
 If it still isn't low enough, do I again.


Matt