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

[Nemanja Savic]

I have a following doubt here: there are two cables, one that brings 10.7 MHz if signal from external rf frontend to the usrp, and utp cable that provides power and control signals for rf frontend. My first doubt was that WBX LO signal is directly induced in the lines on LFRX board, but as soon as I disconnected signal cable from rf frontend to usrp the spike dissapeared, so it comes for sure from rf frontend. The problem is how. Should i put rc filters at the endof every controll signal? The other question is how to connect shields of both cables in proper way? I don't want to make ground loop.

thanx

On Tue, Dec 10, 2013 at 5:32 PM, Marcus D. Leech <address@hidden> wrote:

On 12/10/2013 11:24 AM, Nemanja Savic wrote:

But I think that I have problem with RX LO of WBX, because I don't use TX in my application, I have two receivers. External RF frontend brings HF to IF and is connected using coax with usrp (LFRX). The other cable, UTP, connects io pins as well as power of LFRX to external frontend. Interesting thing is that when I unplug my frontend the spike doesn exist any more in the spectrum of LFRX signal, which means that maybe noise comes via UTP cable, because it's shield is not bonded to ground.

Well, again, you can offset the RX LO.

On Tue, Dec 10, 2013 at 5:12 PM, Marcus D. Leech <address@hidden> wrote:

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

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--
Nemanja Savić

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

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Nemanja Savić