Otherwise I'd just separate my TX and RX antennas spatially. But
that
doesn't make sense in my case since the critical component is this
switch in my case.
Right.
Although antenna separation needs to happen as well, so you have at
least 35 dB loss over the air. Keep in mind that equations derived
from far field approximations don't necessarily hold in the near field.
I aim at working in the 3.7GHz band. Thus, I assume that my receive
signal goes through the VMMK-3603 LNA in the RXLNA block in the
schematic. I wonder how that component right after the RX2 SMA works?
I
assume this is a Skyworks AS236-321LF as shown on page 11 of the
schematic.
https://store.skyworksinc.com/Products/Detail/AS236321LF-Skyworks-Solutions-Inc/88944/
Take a look at the SPDT switch in Figure 1 here:
http://www.skyworksinc.com/uploads/documents/PB_RFSwitches_PB121_15B.pdf
It's not too hard to imagine what a DPDT switch looks like from there.
Are there in fact 2 switches concatenated? Would it be appropriate
to
just add up their isolation values?
Ah, yes. As long as the USRP switches this one over to RX2, (and that
sounds like your configuration,) it looks like you get an additional
13-15 dB of isolation. Still not enough to cover the whole 35 dB, but
you're up to a worst case minimum 30 dB isolation.
If all of the specifications are to be believed, and you know you'll
receive negligible energy on the RX2 port antenna, you only need set Tx
output gain to 5 dB of attenuation, and make sure you are using RX2 for
Rx.
Another thought, would it be possible to configure a USRP in GR such
that it does continuously receive on TX/RX and then switch for the
duration of a transmit burst and switch back afterwards? Of course,
preferably this happens automagically.
I'm pretty sure this already is handled automatically by the
USRP/libuhd, but it would take a bit of searching through code to
verify.
In the case of telling the USRP you're using the TX/RX port for both Tx
and Rx, you may not get that Skyworks AS236-321LF switch switched into
the position to get the additional isolation. You'll have to check the
code or check with Ettus.
This thread [0] suggests that there is some kind of control. But so
far,
I didn't find a definite way to tell if this covers my case with GNU
Radio. Also, in [1] it sounds like switching is done automagically.
In GNURadio you should be able to set a USRP Sink to "TX/RX" and a USRP
Source to "TX/RX", and libuhd should take care of the switching
automatically, if I recall correctly. Again, you might not get the
isolation of the second Rx switch using this configuration.
Though, I wonder if I can just use 'UHD: USRP Sink' and 'Source' or
if I
need to use the Async Sink?
I have no idea. Never used it.
Regards,
Andy
Cheers
Johannes
[0]
http://lists.ettus.com/pipermail/usrp-users_lists.ettus.com/2016-October/050112.html
[1]
https://stackoverflow.com/questions/39321262/switching-usrp-from-rx-to-tx-using-gnuradio
Am 25.02.19 um 20:30 schrieb Andy Walls:
Date: Mon, 25 Feb 2019 10:29:56 +0000
From: Johannes Demel
To: "discuss-gnura...@gnu.org" <discuss-gnura...@gnu.org>
Subject: [Discuss-gnuradio] TDD mode with USRPs in GNU Radio
Hi all,
I plan to implement a TDD system with GNU Radio and X310s w
UBX160s.
My
lab setup is as follows:
- multiple USRPs (start with 2, extend to more)
- each USRP shall use 1 TX and one RX port. Preferably on one
daughterboard in order to extend it to MIMO later on.
At the moment the receiver runs continuously. While TX bursts
happen
occasionally. I'd like to turn up TX power as much as possible.
But
I'm
concerned that this will damage the RX frontend. Especially if
the
RX
gain is set to some high value too. Should I be concerned about
that?
Yes.
https://kb.ettus.com/UBX
"TX Power (Max)
10 MHz - 3 GHz: 20 dBm
3 - 6 GHz: 8 - 20 dBm"
"Input Power Levels
The maximum input power for the UBX is -15 dBm."
The TX chain is isolated from the Rx chain by a SKyWorks SKY13350-
385LF
switch.
https://files.ettus.com/schematics/ubx/ubx.pdf
http://www.skyworksinc.com/Product/712/SKY13350-385LF
which claims typical isolation of 25 dB at 3 GHz on the webpage,
but
the datasheet indicates 20 dB is actually typical and 17 dB is
minimum.
UBX-160 board design, if not done right, can degrade that.
17 dB of isolation is does not cover the 35 dB difference between
max
TX power and max Rx input power.
Or
does the USRP take care of that automatically? e.g. turn down RX
gain
during transmission?
I don't think so.
Even if a transmission is received at the same receiver, I'd
expect
my
RX DSP chain to just demodulate that burst and later on this
packet
would be discarded.
That's my experience with WBX-120 boards installed in a working
system: the radio can hear itself.
Also, I'd envision a block that 'blanks' my RX
samples whenever TX is expected.
It's easy enough to throw away these packets at the MAC layer.
Receiving them is also a rudimentary check for a continuous built
in
test to detect if the transmitter has completely failed.
All in all the questions are:
- do I need to implement some logic in GR to turn down RX gain
(or
shut
off RX) during a TX burst?
Given the numbers above, I would think you need to keep the gain
down.
- does UHD take care of that?
I don't think so.
- If I need to take care of that, how did you guys handle that
problem?
Don't run the RX chain at full gain.
Put an LNA and limiter out in front of the USRP's Rx port.
And if you're using one antenna for both Tx and Rx, on the TX/RX
and
RX2 ports respectively, you'll need a PIN diode RF T/R switch out
in
front of the USRP as well.
Regards,
Andy
Cheers
Johannes
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