Re: [Discuss-gnuradio] Possible to change RFX400 LO tuning step size?

[Glenn Richardson]

Matt Ettus wrote:
> I assume you mean on the USRP1.  Change the R divider in constructor 
> of the db_flexrx_400_{tx,rx}_mimo_b in db_flexrf_mimo.cc
>
> An R divider of 16 (the current setting) make the LO step size 64 
> MHz/16 = 4 MHz, but the following stage divides it by 2 again for a 
> step size of 2 MHz.  If you want 1 MHz, use 32, if you want 500 kHz 
> use 64.  You can also use numbers which aren't a power of 2.  In-band 
> phase noise is better for bigger step sizes.
This is slightly different, but I recently brought out the control to 
change the LO offset via GRC, so that in my testing I could change 
between say, 4 and 8 MHz.  (It actually didn't dawn on me to change the 
step size too.)  The trouble I have is with amplifying this signal.   I 
have need of a 4 watt or more output, and so will run the ~100mW signal 
through an external amplifier.  However, I don't want to go splattering 
the LO all over, since it is within the bandpass the bandpass of the 
amplifier (hence the need to move the LO away)...  An external bandpass 
filter is an obvious step, but then ties me directly to a specific band 
that is smaller than the operating frequency of the rfx400.  Is there 
anything else we can do to reduce the LO?  I currently get about -30db 
of attenuation.

Thanks for your input!
Glenn