On 18/09/2023 09:43, Marcus Müller wrote:
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
Near as I can tell, ADI don't actually specify the RX<-->TX isolation,
but if it's better than 60dB I'd be very surprised--with
the caveat that I'm not an RF chip designer, and sometimes intuition
fails you...
Even if it's 70dB, and you're transmitting at +10dBm, that means the RX
is "seeing" about -60dBm of your TX signal.
That's about 60-70dB above the MDS threshold for narrowband signals
at max RX gain. So hoping to use RX/TX isolation
to not "see" your transmit signals is wishful thinking in an RF
platform design like this. Although, obviously for most
full-duplex schemes, the RX and TX frequencies are separated, and the
TX-to-RX leakage is only a concern if it contributes
to loss of linearity/sensitivity in the RX pathway.
In systems like simple analog repeaters (with split frequencies), cavity
duplexers are used to isolate the TX/RX frequencies, although
one *still* has the in-the-radio leakage to be concerned about. In
the olden days, the RX and TX assemblies were often on separate
boards, in their own sub-system enclosures to improve this. Hard to
do that on a board where the RX and TX paths go through
the same chip...
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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