Re: [Discuss-gnuradio] IQ imbalance...

[Matt Ettus]

On 04/16/2010 12:42 AM, Per Zetterberg wrote:
> I am measuring IQ imbalance. The values are generally quite good.
> However, one thing I don't understand is that the mirror frequency
> doesn't turn up on exactely "-f" but a few Hertz off. I don't get this.
> Are we sure that the down-conversion on the FPGA is completely turned off ?


No, the down converter is not.  The way the tune command works is that 
the daughterboard code tries to get as close as possible to the 
requested frequency.  It returns the actual frequency which it tuned to, 
not the requested one, and these can be as much as about 100 Hz 
different.  The next layer up tunes the CORDIC to remove that remaining 
frequency offset.  If you modify the daughterboard code to return the 
requested frequency instead of the actual frequency, that delta will be 
zero and the cordic will be effectively disabled.

Matt