Re: [Discuss-gnuradio] UHD recv timestamp differs from stream command time spec

[Matt Ettus]

On 09/02/2010 10:39 AM, Marc Epard wrote:
> I'm using a USRP2 with UHD and starting a receive a short time in the
> future. I have an external reference and PPS. Below is the pertinent
> code, slightly simplified.
>
> The timestamp returned in the metadata by recv is always off a little
> from the time I requested in the stream command. The amount it is off
> seems to be consistent and related to the sampling rate. For 25 Msps,
> it's always off by 230 nanoseconds, for 12.5 Msps, it's off by 270
> nanoseconds, for 6.25 Msps, it's off by 350 nanoseconds. If I receive
> one packet at a time using RECV_MODE_ONE_PACKET, the first packet's
> metadata is off by the same amount.
>
> I'm using the latest gnuradio and uhd (the one with USRP1 support).
>
> Should I expect the timestamp in the received metadata to match the
> requested time in the stream command?


The timestamp that you request is used as the time to turn on the 
receiver.  The timestamp returned with the data is the time of those 
samples at the output of the receive DSP chain.  Since there is 
decimation in the receive chain, there will be some delay between when 
you enable the receiver and when the DSP spits out its first sample. 
When using our standard DSP units that we provide in the code, this 
delay is a function of the decimation rate.

If you really need your first sample to be at time X, you will need to 
send a command which starts the receive chain a little early.

There are two related issues.  First, if you are using the standard ATR 
settings, the receiver is turned off when you are not receiving. 
Nothing powers up instantly, so if you want to have useful reception at 
a particular time, you will also need to turn it on early.

The second issue is group delay through the RF, ADC, and DSP.  We 
demarcate time at the output (decimated) side of the DSP.  That data 
came in over the air some time earlier, but that time delta is a 
function of the daughterboard design and settings (particularly the 
filters), the ADC design, and the DSP design and settings.  The DSP 
delays can be calculated.  In any application where you need to know to 
the nanosecond the time a particular sample represents at the antenna 
port you will need to calibrate the delays.

All of this applies to the TX side as well, in reverse.

Matt