[USRP-users] usrp b210 and mimo

Juha Vierinen x at mit.edu
Fri Dec 5 12:25:17 EST 2014

> Is the B200 timing offset always before (or after) the N200, or does it
randomly bracket the N200?

The b200 signal starts consistently about 20 ms +/- 100 us earlier than the
n200 signal. The standard deviation of the offset is about 100 us.

I'm using the noise injection of our radar signal, and various power line
interference spikes seen by a n200 and a b210 simultaneously as an external
timing reference.

> There are a couple of things I'll mention up front that come to mind
because of what you are doing…but they don't explain your issue:
> 1)N2x0 will always sample the PPS with a 100MHz clock, B200 will sample
PPS with whatever rate is configured as the master clock rate. If that
value is 50MHz then there's a 10nS ambiguity introduced …if 100/master_rate
is not an integer then the sampling clock edges are going to constantly
shift phase w.r.t each other.

I'm using 1 MHz. The B210 is using 32 MHz I think. Both are integer
decimation rates. Once the B210 streaming is started, there is no relative
drift as long as the samples keep coming in.

> 2) Group delay through the DSP's is quite different…it will in general be
larger for B200 I suspect…but it will depend heavily on what the configured
decimation is.

I kind of figured there would be a constant group delay offset, and I think
it is about 20 ms for 1 MHz.

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