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


From: Ralph A. Schmid, dk5ras
Subject: Re: [Discuss-gnuradio] [USRP-users] WBX Lo leakage to LFRX
Date: Mon, 16 Dec 2013 16:36:37 +0100

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:address@hidden On Behalf Of Nemanja Savic
Sent: Monday, December 16, 2013 4:26 PM
To: Patrik Tast
Cc: address@hidden; 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 <address@hidden> 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 <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
>         >
>         >
>         >
>         >         _______________________________________________
>         >         Discuss-gnuradio mailing list
>         >         address@hidden
>         >         https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
>         >
>         >
>         >
>         >
>         > --
>         > Nemanja Savić
>         >
>         > _______________________________________________
>         > Discuss-gnuradio mailing list
>         > address@hidden
>         > 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
>         address@hidden
>         https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
>
>
>
>
> --
> Nemanja Savić

> _______________________________________________
> USRP-users mailing list
> address@hidden
> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com




--
Nemanja Savić


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