Re: [Discuss-gnuradio] WBX Lo leakage to LFRX

[Marcus D. Leech]

On 12/10/2013 10:41 AM, Nemanja Savic wrote:
> hi,
> thank you Ralph.
> It looks like I should have known about not packing two receivers in 
> the same band in the sme box, but anyway, rf parts are not in the same 
> box, and I intended to keep them at the distance of arround 5 to 10 
> meters. Antenna of 434MHz in is more or less conected directly do the 
> pin, there just matching cuircuit between. The 10.7 MHz out will be 
> with 5 - 10 meters long cable connected with USRP. Power supply of 
> external frontend runs through twisted pair of a UTP CAT 2 cable, 
> which is kind of shielded, and I have a few capacitors on the other 
> end for filtering supply line. As for the LP filter on the 10.7 MHz, i 
> think that LFRX itself has cuoff frequency of arround 50 MHz or so. 
> Will it help a bit if I put some alu plates over WBX board?
>
> --
> Nemanja Savić
The problem is that even with 50dB LO suppression in the TX mixer, there 
will still be some LO energy leaking out the antenna port.

But something you *can* do is use offset-tuning on the TX side to move 
the LO off to the side.  It'll still be there, but outside of your RX 
passband.

In "built for a specific purpose" radios, it's often the case that the 
last conversion stage uses a fixed LO that is offset from the final 
frequency, and
  there's a deep notch filter on the output of the final mixer.  That 
strategy isn't possible in designs like these that aren't "for a 
specific purpose", since
  there's only a single conversion stage, with variable LO--you'd have 
to put in a notch yourself.




--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org