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?
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. 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.
If you're operating at frequencies below perhaps 100MHz, you can simply
put +10dB attenuation in-line with your RX chain, improving
the TX/RX isolation by that amount. Ettus quotes -15dBm because
several of the USRP devices have front-ends that don't want more than
-15dBm at the input terminal--some of them are more robust, by about
10dB or so. But it's just a "prudent" level for a receiver intended
for over-the-air work. In normal situations, a signal of -15dBm at
the input terminals to the reciever is "barn burner" loud, and as I
indicated
for receive amplifiers, as you go much about -10dBm input power,
there's a risk of damaging the 1st-stage input amplifier. That's just the
nature of VHF/UHF small-signal transistors.
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