Switching between signal and reference source

I’m buliding a system that switches the RF input, several times per
second, between the actual
signal, and a stable reference source.

On a USRP2, what kind of latency is there between the antenna input, and
data arriving at the
head of the signal processing chain?

Small errors at the ‘edge’ of the switching time are acceptable, but I
don’t want to get into a situation
where my notion of whether the input is connected to the source or
the reference is horribly
out of what.

Can I use the GPIO stuff to give me better confidence of adequate
synchronization between the
actual data, and the state of the input?


Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org

On Thu, Apr 15, 2010 at 16:52, Marcus D. Leech [email protected]
wrote:

Can I use the GPIO stuff to give me better confidence of adequate
synchronization between the
actual data, and the state of the input?

If you are willing to give up an LSB, you can turn on GPIO streaming
with enable_gpio_streaming(). Then io_rx[15] will show up in the lsb
of I, and io_rx[14] will show up in the lsb of Q. You need to use the
usrp2.source_16sc source to get the raw samples, then gr_and_const to
separate the digital bit from the rest of the sample.

The io bits get stuffed as the samples come out of the DDC, so there
is a fixed offset (you’ll need to measure it) based on the group delay
through the whole analog front-end and FPGA DDC. The delay is
decimation dependent, but deterministic.

Johnathan

On Thu, Apr 15, 2010 at 17:19, Marcus D. Leech [email protected]
wrote:

OK, so Josh, how do you get access to usrp2.source_16sc from GRC?

There isn’t a source for that. You would need to create a
hierarchical block with the needed blocks and wrap in a GRC wrapper.

Johnathan

The io bits get stuffed as the samples come out of the DDC, so there
is a fixed offset (you’ll need to measure it) based on the group delay
through the whole analog front-end and FPGA DDC. The delay is
decimation dependent, but deterministic.

Johnathan

OK, so Josh, how do you get access to usrp2.source_16sc from GRC?


Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org

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