Possible to change RFX400 LO tuning step size?

GNU Radio
1

Is it possible to change the tuning step size on the RFX400 to something
different than the 2 MHz step size it’s on now? I’ve been looking
through db_flexrf.cc and have had no luck.

Tim

2

Newman, Timothy wrote:

Is it possible to change the tuning step size on the RFX400 to
something different than the 2 MHz step size it’s on now? I’ve been
looking through db_flexrf.cc and have had no luck.

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.

Matt

3

Matt E. 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