Just in case someone like myself is stumbled by this in the future.
Turns out I was misinterpreting Radio Select "B" as being the second RX and
TX channel on the E312. Instead, in the RFNoC Radio block the Radio Select
A and B refer to two different daughterboards (probably designed for the
X310 which can handle up to two daughterboards). Whereas if you are using
the E312, it has a single MIMO XVCR daughterboard with two channels.
The correct thing to do here therefore would be to change "Num Channels" to
"2 Channels" on both the RX and TX blocks and then set your center
frequency appropriately. Since the RX channel and the TX channel both have
two different Local Oscillators (which are shared, i.e. TRX A and TRX B
share a local oscillator, RX A and RX B share another local oscillator),
they will have the same center frequency. If the center frequency needs to
be set independently, then the digital tune stages need to be set - this is
something I'm still trying to understand how to implement in GNU Radio.
This manual is a good resource:
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