WBX Lo leakage to LFRX

GNU Radio
#1

Hi all,

As I described in one of my previousposts, I wanted to have several
receiving paths for 434 MHz, and my USRP is equipped owith WBX and LFTX.
So
WBX is one, and for the other, I designed small RF frontend using TI’s
CC1000 that donwnconverts the signal to 10.7 MHz which can be used with
LFRX. Now the problem is that i have like 30 - 40 dB spike in the
spectrum
of received signal at LFRX thata comes from the LO of the WBX. Is there
any
suggestion how to supress this signal, but first from phisycal point of
few, and later if not possible trying to use some DSP manipulations.
Namely, I packed new frontend in the alu box, so it’s kind of shielded,
but
it didn’t help that much.

Thanx,

#2

Hi,

The box is the first step; you also need to block the power supply line
against any incoming RF, your 10.7 MHz IF out needs a low pass filter
that cuts off all higher frequencies coming back in on this path, and
your 433 MHz in needs to be shielded 100%, with some double shielded
coax or semi rigid.

The LO strays quite a bit, it is not a very good idea to have a second
RX on the same frequency in one box…the fun with direct conversion
receivers :slight_smile:

Ralph.

From: [email protected]lid
[mailto:[email protected]lid] On Behalf Of
Nemanja S.
Sent: Tuesday, December 10, 2013 3:49 PM
To: GNURadio D.ion List
Subject: [Discuss-gnuradio] WBX Lo leakage to LFRX

Hi all,

As I described in one of my previousposts, I wanted to have several
receiving paths for 434 MHz, and my USRP is equipped owith WBX and LFTX.
So WBX is one, and for the other, I designed small RF frontend using
TI’s CC1000 that donwnconverts the signal to 10.7 MHz which can be used
with LFRX. Now the problem is that i have like 30 - 40 dB spike in the
spectrum of received signal at LFRX thata comes from the LO of the WBX.
Is there any suggestion how to supress this signal, but first from
phisycal point of few, and later if not possible trying to use some DSP
manipulations. Namely, I packed new frontend in the alu box, so it’s
kind of shielded, but it didn’t help that much.

Thanx,

#3

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?

#4

On 12/10/2013 10:41 AM, Nemanja S. wrote:

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 L.
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org

#5

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.

#6

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

#7

On 12/10/2013 11:24 AM, Nemanja S. 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.

#8

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

#9

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

#10

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:[email protected]] On Behalf
Of Nemanja S.
Sent: Monday, December 16, 2013 4:26 PM
To: Patrik T.
Cc: [email protected]; GNURadio D.ion 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 T. [email protected]
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 S. wrote:

    > coax with usrp (LFRX). The other cable, UTP, connects io

    >                 the same box, and I intended to keep them at
    >                 the LP filter on the 10.7 MHz, i think that
    >         But something you *can* do is use offset-tuning on
    >           there's only a single conversion stage, with
    >
    > --
    Principal Investigator

–
Nemanja Savić

#11

Well, maybe I can use spectrum analyser with proper antenna, but what
can I
achieve with that. If I see peak, I still have a problem to attenuate
it.

On Mon, Dec 16, 2013 at 4:36 PM, Ralph A. Schmid, dk5ras

#12

Hi,

Try a 1/4 shorted stub at 434 MHz (at the antenna).

Patrik (who said many times not my fault but usually is…)

#13

Yes I would really like to know not to guess, in that case I could
probably
be able to solve it. It is pretty like this: by disconnecting 10.7 MHz
cable the spike disappeared so I said ok it doesn’t come from inside
USRP,
but rather through coax cable. Since cable is shielded I assume it even
goes somehow into complete receiving chain. Two possibilities to come
into
the chain: usrp and external box are close enough and thus coupling
occurs.
If not, signal probably travels over control signals through UTP cable.
What I will probably try is to put rc filters on controll lines.

On Mon, Dec 16, 2013 at 5:54 PM, Ralph A. Schmid, dk5ras

#14

First of all you need to know (not guess), where it comes from,
where it goes to…

Ralph.

From: Nemanja S. [mailto:[email protected]]
Sent: Monday, 16 December, 2013 16:48
To: Ralph A. Schmid, dk5ras
Cc: Patrik T.; [email protected]; GNURadio D.ion List
Subject: Re: [USRP-users] [Discuss-gnuradio] WBX Lo leakage to LFRX

Well, maybe I can use spectrum analyser with proper antenna, but what
can I achieve with that. If I see peak, I still have a problem to
attenuate it.

On Mon, Dec 16, 2013 at 4:36 PM, Ralph A. Schmid, dk5ras
[email protected] wrote:

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:[email protected]] On Behalf
Of Nemanja S.
Sent: Monday, December 16, 2013 4:26 PM
To: Patrik T.
Cc: [email protected]; GNURadio D.ion 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 T. [email protected]
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 S. wrote:

    > coax with usrp (LFRX). The other cable, UTP, connects io

    >                 the same box, and I intended to keep them at
    >                 the LP filter on the 10.7 MHz, i think that
    >         But something you *can* do is use offset-tuning on
    >           there's only a single conversion stage, with
    >
    > --
    Principal Investigator

–
Nemanja Savić

USRP-users mailing list
[email protected]
http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com

–
Nemanja Savić