Re: [Discuss-gnuradio] Carrier leakage on transmit

[Matt Ettus]

On 06/01/2010 03:41 AM, Charles Brain wrote:
> > 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
>
> Hi Matt,
>
> Are the settings for the I and Q adjustments independent or do I have to
> iterate between them for the lowest error?


They are independent, but you still need to iterate.  Once the I offset 
is small enough, it will be swamped by the Q offset and you won't get 
enough precision to totally remove it until you also reduce Q.

> Also if I am sending a 0 amplitude signal can I simply AM detect the
> received signal to measure the dc error value?

Possibly, but if there is another signal coming in it will mess up your 
results.


> Finally what is the simplest way to synchronise receive, so I can make
> sure I am associating a particular DC offset error with a corrected
> transmitted block?
>
> I can see the carrier going up and down on the Analyser but my
> routine comes up with some silly values. I think it is because I have
> the routine out of sync.

If you aren't using inband signalling, you'll need to proceed slowly. 
Make a change, wait a little bit to make sure you are seeing its results 
before you make the next one.

Matt