Take a handheld scanner or a UHF walkie talkie, tune it to the LO frequency, use a paperclip or some similar 2cm piece of wire as antenna, and start sniffing with this improvised probe for the leakage.
Ralph. From: USRP-users [mailto:usrp-users-boun...@lists.ettus.com] On Behalf Of Nemanja Savic Sent: Monday, December 16, 2013 4:26 PM To: Patrik Tast Cc: usrp-us...@lists.ettus.com; GNURadio Discussion List Subject: Re: [USRP-users] [Discuss-gnuradio] WBX Lo leakage to LFRX Hi, i don't understand what can I do in this way with balun. Antenna is matched very well I think, since I copied reference design from TI. I am sure that spike comes from WBX LO because when I change center frequency the spike also shifts. I'll remind about the configuration again: USRP1 with WBX and LFRX. I want two 434 MHz receivers. One receiver is made with WBX as RF frontend, while the second receiver uses TI CC1000 transciever as RF frontend. CC1000 provides 10.7 M IF signal. From USRP to CC1000 there are two cables, one is coax for IF signal and another is UTP for power and controlling. I am pretty sure that leakage signal comes through utp cable and disturbs ground or i don't know what. Power supply is decoupled many times with caps. This now the point where experience of the engineer comes into the came, but unfortunately I am not that experienced. I would like to know what would you do with the shield of coax cable and UTP cable? How probable is that I will make a ground loop in that way? Thanx On Mon, Dec 16, 2013 at 4:09 PM, Patrik Tast <pat...@poes-weather.com> wrote: Hi, Try finding ferrites for your frequency (11 MHz) and google on BalUn (balanced to unbalanced), http://en.wikipedia.org/wiki/Balun You could also do it with COAX, impedance match using the Smith Chart, if your antenna impedance is known eg http://en.wikipedia.org/wiki/Smith_chart Very usual interference at low freqs are AC power sources...and are harmless (you wont see the *spike* when you see a proper signal) Patrik On Mon, 2013-12-16 at 14:38 +0100, Nemanja Savic wrote: > 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 <mle...@ripnet.com> > 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 > > <mle...@ripnet.com> 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 > > > > > > > > _______________________________________________ > > Discuss-gnuradio mailing list > > Discuss-gnuradio@gnu.org > > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > > > > > > > > > > -- > > Nemanja Savić > > > > _______________________________________________ > > Discuss-gnuradio mailing list > > Discuss-gnuradio@gnu.org > > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > > > -- > Marcus Leech > Principal Investigator > Shirleys Bay Radio Astronomy Consortium > http://www.sbrac.org > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > > > > > -- > Nemanja Savić > _______________________________________________ > USRP-users mailing list > usrp-us...@lists.ettus.com > http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com -- Nemanja Savić
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