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Re: [Discuss-gnuradio] IF frequency of RFX400

From: Matt Ettus
Subject: Re: [Discuss-gnuradio] IF frequency of RFX400
Date: Wed, 11 Jul 2007 13:57:06 -0700
User-agent: Thunderbird (X11/20070326)

melanio wrote:
> Hi,
> TVRX IF frequency is fixed at 44MHz, But the RFX400 IF frequency is
> 3MHz or 4MHz, so DDC is -3MHz or -4MHz.
> What is the reason does not use the fixed IF frequency?
> I think which is not fixed seems to be a bit complicated. I wonder why
> different IF frequency is necessary such as 3or 4MHz…
> Is it the constraint because of the hardware?

We normally offset the LO from the desired receive signal so that you
don't have the DC offset (and DC offset compensation) right in the
middle of your signal.  Typically we use 4 MHz for this offset, but you
can change it to whatever you like, including 0.  This is why you see
the DDC tuned to -4MHz when you are looking at a frequency which is a
multiple of 2 MHz.  The LO on the RFX400 is set to tune in 2 MHz steps,
so when you tune to an odd multiple of 1 MHz, you see the DDC at -3 MHz.

All of the above is controlled in the gr-usrp/src/db_flexrf.py file. 
You can change any of it if you wish, but usually you don't need to. 
Reasons you would want to change it include having strong interferers on
your image frequency.


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