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Re: [Discuss-gnuradio] Carrier leakage on transmit

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] Carrier leakage on transmit
Date: Tue, 01 Jun 2010 09:57:28 -0700
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20100430 Fedora/3.0.4-2.fc12 Thunderbird/3.0.4

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.


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.


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