we have implemented synchronized reception with two USRP2’s using a
common
clock and the 1PPS signal to be able to use the two ethernet interfaces.
This
works quite good but we observed one thing.
When we set the decimation rate to an odd number (5 in our case), the
samples
are aligned perfectly. When we use an even numbered decimation rate, we
could
detect unalignments in some of our test runs with the same setup.
We have the idea that this is related to the way the decimation is
performed
(refer to “Re: [Discuss-gnuradio] USRP2 Filtering/Decimation Question”
for
more details) i.e. if only the CIC is involved it works fine otherwise
there
are some troubles.
Does anybody have made similar observations or is able to tell more
about the
problem?
Great news. The basic issue is that since the halfband decimators
decimate by 2, they have an internal state (which we call odd or even).
The design wasn’t syncing this state, so I added a small amount of
code to resync the filters when they are enabled. The CIC was already
doing this, so you didn’t see the problem when only using the CIC and
not the halfbands.
If you want to see exactly what I changed, it is here: