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List:       usrp-users
Subject:    Re: [USRP-users] Impulse Response Experiment
From:       Martin Braun via USRP-users <usrp-users () lists ! ettus ! com>
Date:       2016-10-31 20:31:55
Message-ID: 8bf346e8-e555-5c0d-b0e8-8a1f81211a31 () ettus ! com
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Sam,

I can't see any immediate issues with your code. How much is the
slippage though? Is it on the order of a bunch of samples, or is it way off?

You can further reduce randomness by aligning receives relative to PPS
edges. Before going into the loop, you can set the time yourself (say to
zero) relative to a PPS edge (set_time_next_pps()). Then, from there,
you can do receives at specific times rather than polling
get_time_now(). At least that would reduce one source of randomness.

Cheers,
M

On 10/31/2016 08:19 AM, Sam Carey via USRP-users wrote:
> Howdy,
> =

> I=92m performing an experiment to obtain the impulse response of the X310
> from one UBX daughterboard to another.
> =

> I use timed commands to set the TX and RX LOs to the same frequency at
> the same time.
> I use time specifications for the transmit meta data and for the receive
> stream command.
> =

> I started by heavily adapting the txrx_loopback_to_file example in uhd.
> I implemented a program that generates a burst of 10,000 samples,
> setting one of them, near the middle, to 1 and the rest to zero (like an
> impulse). Then this burst gets transmitted at the same time as I receive
> 10,000 samples. The TX port of daughterboard A is connected directly to
> the receive port of daughterboard B through a coaxial and attenuation.
> Then I check the received waveform, which should essentially be the
> impulse response of the system. I see a very sharp peak, which
> corresponds to the single, non-zero sample I sent. The phase of this
> sample is constant, even if I rerun the program, so I think this proves
> the LOs are properly synchronized. The location of the peak is
> consistent from burst to burst (the test repeats in a while loop), but
> this location will change randomly if I rerun the program, as if an
> extra delay is randomly chosen when the program starts.
> =

> I think the peak should always be in the same location. Does anyone know
> what=92s going on here?
> =

> Here=92s the code for reference:
> https://github.com/samcarey/SharedCode/blob/master/txrx_impulse_test.cpp
> =

> Thanks,
> Sam
> =

> =

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