[USRP-users] Isolating the grounds of LFTX and LFRX in N200

khalid.el-darymli khalid.el-darymli at mun.ca
Mon Oct 20 07:25:16 EDT 2014

Hi Ralph,

Yes, I understand that the proximity of both PCBs and the lack of shielding
is a part of the problem, but I think the common ground too plays a role
into this. As mentioned in my earlier email, I personally verified this for
the case of two separate N200 devices.

Please see this,

All the best,

On Sat, Oct 18, 2014 at 2:31 AM, Ralph A. Schmid, dk5ras <ralph at schmid.xxx>

> Not the ground is the problem, but the proximity of both PCBs, and the
> lack of shielding. Guess it will be a bigger task to shield and isolate
> them, using separated devices is better.
> Ralph.
> *From:* USRP-users [mailto:usrp-users-bounces at lists.ettus.com] *On Behalf
> Of *khalid.el-darymli via USRP-users
> *Sent:* Friday, October 17, 2014 7:45 PM
> *To:* usrp-users at lists.ettus.com
> *Subject:* [USRP-users] Isolating the grounds of LFTX and LFRX in N200
> Hi,
> I have *two N200 *devices. One of which is used as a Tx and Rx
> simultaneously and the other is only used as Rx. The LFRX and LFTX
> daughterboards are used for Rx and Tx, respectively. In my present test, I
> am using FMCW chirp with a BW of 50 KHz.
> I noticed that if the grounds of the two N200 units are connected with
> each other (e.g., through using external LNAs with a common ground) I can
> see the FMCW sweeps with a power around 10 dB above the noise floor in the
> Rx only N200 unit; without having nothing connected to its Rx input. I
> managed to absolutely cancel out this effect through simply isolating the
> common grounds between the two N200 units.
> The issue I am having now is with the N200 unit that has both the LFTX and
> LFRX daughterboards inside it. In this unit, I am seeing the FMCW sweeps at
> the Rx (without having nothing connected to Rx) with a power of around 20
> dB above the noise floor.
> It seems that since the LFTX and LFRX daughterboards are plugged-in to the
> same motherboard, they both share the same ground!
> My question is, *how do I isolate the grounds of the LFTX and LFRX
> daughterboards connected to the same N200 unit to minimize the coupling
> between two of them?*
> I will appreciate any help on this.
> Thanks.
> Best regards,
> Khalid
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