[Discuss-gnuradio] Multiple inputs on LFRX - phase difference and zero-crossings detection

[grc007]

Hello,
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 difference
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 ?

2. 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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