If the TX and RX frequencies are a few 100kHz different then what you want is a duplexor arrangement where the TX frequency is strongly attenuated ahead of the RX.
This is how repeaters work when multicoipled to a single antenna. The TX frequency is often attenuated 90dB in the RX path. Sent from my iPhone > On Jan 4, 2021, at 11:33 PM, Lukas Haase <lukasha...@gmx.at> wrote: > > Hi Marcus, > >> On 01/04/2021 12:25 PM, Lukas Haase wrote: >> In an RFID system, the RX is *designed* to be up-close-and-snugly with >> the other ends TX. There's NO reason to have an input >> stage that is sensitive in the usual RF sense. So, completely >> different engineering goals than in a typical RF system. > > That's not quite true. Sensitivity *is* important (in the RF sense) because > it defines operating distance. > Yes, many people claim RFID systems are forward link limited (which was true) > but state-of-the art tags can have a sensivivity of -24dBm which corresponds > to over 20m free space, 900 MHz. > The response from the tag at the reader is around -80dBm. The R2000 chip that > I mentioned has a sensitivity in excess of that. > > Indeed, this is the reason why *sensitivity* (due to self interference) is > the limiting factor in RFID. > >>> 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? >> Any limiter diode has shunt capacitance. Which means that the degree to >> which input power is shunted to ground is proportional >> to the input frequency and shunt capacitance. ANY attenuation >> (whether resistive dissipation or shunt-to-ground pathways) in >> front of the first gain stage adds *DIRECTLY* to the noise figure of >> that stage. So, let's say you have a nice small-signal LNA with >> a notional noise figure of 0.5dB, and you put 10dB of loss in front >> of it--the noise figure now becomes 10.5dB. For RFID type applications >> this doesn't matter that much--see my "up close and snugly" >> comments. But for "ordinary" RF receive chains, you generally want >> to minimize noise figure while maximizing gain and linearity. >> >> There are exceptions--for example at HF (below 30MHz or so), the input >> noise is *utterly dominated* by galactic background noise and >> atmospherics--there's no point in having an input stage with a noise >> figure below perhaps 5-10dB. So for HF, input stages tend to >> be optimized for linearity at higher input levels--because even >> distant signals can be quite strong at HF--particularly on the lower end. >> >>> - The large self-interfere would result in clipping (hard nonlinearities). >>> Is this any problem for the LNA (gain desensitivisation etc.) >> Well. Yeah. That's what the P1dB parameter is all about--the input >> level at which gain is compressed by 1dB. >> >> The overall take-away here is that generic radios (whether they be SDRs >> or others) should be thought of as *components* in an >> overall *engineered RF system*. That may mean things like relays to >> shunt the RX pathway during TX, circulators, attenuators, >> diplexors, filters, RF-plumbing-in-general. > > Yes. > > To clarify limiter vs non-limiter. > My self interfering signal from TX can be up to 20-25dBm. The desired signal > is a modulation bandwidth (few 100 kHz) away and the receiver should be as > sensitive as possible to that signal. > > Now I have the two options: > > 1.) Adding a normal attenuator of 40dB. This ensures the USRP input is always > less than 25-40=-15dBm but it also reduces my desired signal by 40dB. In > other wirds, my noise figure worsens by 40dB, as you mentioned. > > 2.) Adding an RF limiter with flat leakage +5dBm (I found SKY16602-632LF). > Afterwards I add a 20dB attenuator to get the max to 5-20=-15dBm. > > If I understand your explanation correctly, there is no real difference > between these two (because the limiter would still crushes signal and with it > desensitizes the desired signal on top). > Is this correct? > > Would you prefer one option over the other? > > Thanks again, > Lukas > > _______________________________________________ USRP-users mailing list USRP-users@lists.ettus.com http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
