Multiple inputs on LFRX - phase difference and zero-crossings detection

I now have a project with the following specifications:
I need to send 2 input-signals parallel over USRP, then find out the
zero-crossings (time) of each signal so I can calculate the phase
of them.
The 1st signal S1 is up to 1MHz
The 2st signal S2 is about 50 Hz

  1. Sending 2 input signals parallel to USRP:
    My idea is:
    • Sending these 2 Signals in 2 separate daughter-boards LFRX
    • (Or) Put them two on only one LFRX through the 2 SMA connectors.
    • How about using digital I/O pins ?
    • What is the maximum voltage can I feed the daughter-board through
      SMA ?
    • Will the inputs be synchronously sent and sampled ?

Zero-crossings detection:

  • How can I find out the zero-crossing time, esp. of the signal S2 ?

Thanks in advance for your reply.

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