Indeed, a 900Mhz duplexor with a 400Khz split is an extreme challenge. If the TX and RX really do need to operate at the same time on more or less the same frequency, then you have some plumbing to do for sure.
Sent from my iPhone > On Jan 5, 2021, at 12:07 AM, Marcus D Leech <patchvonbr...@gmail.com> wrote: > > To do a little expectation-setting, the narrowband MDS of a modern, not that > spectacular, UHF RX chain of better than -140dBm is not that unusual. So when > I talk about a sensitive RX chain, I’m talking about those sorts of levels. > > In that kind of context, -24dBm seems deafeningly loud, and -80dBm seems kind > of normal. One must of course make adjustments for PSD and decide what > constitutes “narrowband”. > > Sent from my iPhone > >> On Jan 4, 2021, at 11:49 PM, Marcus D Leech <patchvonbr...@gmail.com> wrote: >> >> 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
