> Date: Sun, 03 Jan 2021 21:12:18 -0500 > From: "Marcus D. Leech" <patchvonbr...@gmail.com> > To: usrp-users@lists.ettus.com > Subject: Re: [USRP-users] X310/UBX as a monostatic transceiver (e.g. RFID > reader)? > Message-ID: <5ff27982.9050...@gmail.com> > Content-Type: text/plain; charset=windows-1252; format=flowed > > On 01/03/2021 07:17 PM, Lukas Haase via USRP-users wrote: > > Hi, > > > > I would like to use an X310 with UBX in a monostatic antenna configuration > > (same frequency range). This is for example common in RFID readers. > > > > My transmit power is 30dBm (ZHL-25W-272+ after the USRP), fed into a 6dBi > > antenna. > > > > Now if I use a circulator, I find isolation of around 15dB (e.g. > > https://www.everythingrf.com/products/rf-microwave-circulators/mesa-microwave/769-185-mmccc700a). > > This means my RX input sees 15dBm. Stupidly, the UBX datasheet wants a > > maximum input signal of -15dBm. So it's 30dB too strong! > > > > Alternatively, I could use a directional coupler. However, the directivity > > is typically around 20dB (e.g. > > https://www.everythingrf.com/products/directional-couplers/rf-lambda/45-89-rfdc8m3g10d) > > which brings me to +10dBm at the RX input. If I assume directivity is not > > an issue, the signal at RX is limited by the S11 of the antenna (typically > > 15dB). With 10dB of coupling: 30dBm-15dB-10dB=5dBm. Still too high and a > > incurs a 10dB noise figure penalty. > > > > As another option, I could add 30dB attenuator after the circulator. But > > this results in an unacceptable 30dB noise figure penalty. > > > > Do I understand anything wrong or is a monostatic TRX just not possible > > with USRP? > >
Hi Marcus, Thanks for the insight(s)! > It's not even *slightly* "stupid". An RX front-end that is sensitive > enough to connect directly to an antenna and be able to receive low signal > levels (typical of over-the-air work) CANNOT have excessive input > power. If it does, the coupla-molecules-layer-thick gate layer in the > input amplifier transistor will simply cease to exist. This is not > peculiar to USRPs--ALL over-the-air receivers designed to be sensitive in > the VHF bands and above have this problem. Hmm, I had thought so but this is a very common scenario. How do RFID readers do this then? See for example the state of the art R2000 (https://support.impinj.com/hc/article_attachments/360000749340/Impinj_INDY_R2000ProductBrief_9.6.18.pdf) ? It specifies an RF input power of +15dBm. It specifies a noise figure of 18dB. Which means if there is resistive attenuation, it can be a maximum of 18dB. > You can put in limiter > diodes, but such diodes are usually "limited" to a lowest power level > of perhaps +5dBm, and will inherently add to the noise-figure of the > RX chain. I have thought of a limiter already. This could be an option. It's true, I haven't found limiters with lower power levels. Two questions here: - How/why would they add to the noise figure? - The large self-interfere would result in clipping (hard nonlinearities). Is this any problem for the LNA (gain desensitivisation etc.) Thanks, Lukas _______________________________________________ USRP-users mailing list USRP-users@lists.ettus.com http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
