On 07/07/2021 02:34 PM, Wolsieffer, Carl L. ERDC-RDE-CRL-NH CIV wrote:
Part of the good news there is I'm utilizing the full 200 Msps clock speed so
I've been skipping both duc on TX and ddc on RX, thus hopefully minimizing any
front end delays to be measured
I'm slightly confused about the Rx metadata portion: I know in the uhd
namespace there is the rx_metadata_t struct, but as far as I can tell, I as the
user have been setting those values in the code in an effort to help
synchronize both Tx and Rx. Are you saying this value I've been setting for
time_spec will be the most accurate approximation of the first received sample
(and when first sample was tx'd for that matter?)
Thanks as always,
Casey
-----Original Message-----
From: Marcus D Leech <patchvonbr...@gmail.com>
Sent: Tuesday, July 6, 2021 5:25 PM
To: Wolsieffer, Carl L. ERDC-RDE-CRL-NH CIV <carl.l.wolsief...@erdc.dren.mil>
Cc: usrp-users <usrp-users@lists.ettus.com>
Subject: Re: [USRP-users] UHD Transmit and Receive times
The RX metadata includes time stamps. The time stamp clocks (and this event
timing) will be as good as your common reference clock.
There will be an inevitable latency due to the DSP group delay inherent in the
DUC, which you will have to measure. It’s usually on the order of 10s of sample
times but can vary from version to version of the FPgA code and sample rate
settings.
Sent from my iPhone
On Jul 6, 2021, at 5:16 PM, Wolsieffer, Carl L. ERDC-RDE-CRL-NH CIV via USRP-users
<usrp-users@lists.ettus.com> wrote:
Hello,
I am using two x310s both hosted from a common PC. I've stitched together a few UHD
examples to transmit a pre-recorded waveform from one x310 and receive at the other
at the exact same time. I'm using an FMCW waveform and would like to run a
correlation on the received signal to make an attempt at measuring distance between
the Tx and Rx. I used a common reference and PPS between the two x310s and scheduled
a transmit and receive by passing a time_spec in the tx_metadata variable for the
tx_stream->send() function as well as the time_spec variable of the
rx_stream->issue_stream_cmd function.
My question is: how much stock do I put in these time_spec values? Is there
some timestamp I can retrieve that will tell me something closer to the truth
about when the first sample was emitted from the transmitter or was recorded at
the receiver? Or can I rely on the SDR adhering pretty strictly to the
time_spec value it was passed provided it was set sufficiently far in future to
give SDR time to schedule commands?
Thanks!
Casey
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The rx_metadata_t includes a "timespec" member which is the hardware
timestamp of the first sample in the buffer associated
with this metadata (in a recv()) call.
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