Hey Marcus,
> I think I'm missing some context here. What is the goal?
I was wondering that as well, but I leapt to conclusions; or, call it requirements
engineering with incomplete information if you will :D
Oğuzhan said that the switch U805, which spec-wise only has < 27 dB isolation, wasn't
isolating enough for their purposes. And that's why they wanted to bypass it. What I added
as interpretation is of course that they wanted to remove the IC from the signal chain –
otherwise there couldn't be any isolation gain.
Then I further interpreted they actually cared about RX/TX isolation in full-duplex
operations (because that's what they've been working on in public on this list and/or
discuss-gnuradio). Now, the thing is that there's *two* switches between the TX1 signal
and the RX1 input, U805 and U807, and their isolation of course adds up. And RX/TX
isolation is hence the "common port->disabled switchable port" isolation of U805, and the
"disabled switchable port->common port" isolation of U807 combined, so, as you said,
pretty much within what the rest of the board can do.
I don't actually know what TX/RX isolation is possible on the AD9361; if I'd have to take
a guess from the RX EVM figures in the presence of close-by blockers, it would indeed be
in the range of 2×24 dB.
Best,
Marcus
On 18.09.23 15:13, Marcus D. Leech wrote:
On 18/09/2023 06:11, Marcus Müller wrote:
Hi Oğuzhan,
we of course haven't run any matching tests on what happens in that bypass mode; it
would still seem to me that if you bypass U805, you'd want to remove the in- and output
capacitors, indeed. I'd strongly advise against doing this modification, for the
following technical reason:
24 to 27 dB isolation is indeed what the datasheet of the SKY13335-381LF says, but
don't forget that you have another 24 to 27 dB of isolation between TX and RX in U807,
adding up to 48 to 54 dB of isolation. That's in the same ballpark as crosstalk through
the board, power supply, adjacent traces etc. The B2xx was designed by RF engineers who
kind of knew what they were doing :D
So, don't expect to win much by removing U805. Especially paired with the fact that we
have no characterization of matching in the bypassed mode, I'd assume you really won't
end with a cleaner RX, but just more problems
Best,
Marcus
On 18.09.23 08:11, ouzan...@hotmail.com wrote:
Hello, in my case, the function performed by the "SKY13335−381LF" IC switch is not
working for me. And as far as I can see, the isolation of this component is low
(24dB-27dB). The card I have is B200. Schematic B210 given in the link
"https://files.ettus.com/schematics/b200/b210.pdf";. I want to bypass this component.
As far as I can see, there are pads for this on the card. I think it is bypassed by
activating C847 and C849 in the relevant part of the B210 schematic that I sent you in
attachment.
The main question I want to ask is, should I connect the 220pF capacity seen in C810
to C847, and the 470pF capacity seen in C814 to C849? Or should I short circuit directly?
I am aware of all the risks. I'll be happy if you can help me. Have a nice day.
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IF the goal is really really high TX/RX isolation, you're not going to achieve that with
this design. Since the TX and RX signals
go through the AD9361 RF chip, which is tiny, very high isolation will never
be achieved.
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