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.
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.
Thanks,
Lukas
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