24 Oct
2012
24 Oct
'12
11:08 p.m.
There is another way, and that's find a known RF
signal that we can use for
synchronization. GPS would be ideal, because there's explicit solutions for
time offsets/drift that's referenced to atomic clocks. We could record 5
minutes or so on the L1 band before an observation, then mid-capture tune
to our frequency of interest...assuming we can shift the e4k chip without
interupting the RTL2832 in it's capture sequence.
I'm not very familiar with your application, but you may also have to do
sub-sample time offset estimation and compensation. Even though all of the
dongles are running at exactly the same sampling *rate*, their sampling *
instants* will likely be different. This is despite the fact that you used
the same length of cable to feed the clock to all the dongles. For example,
if you want a 2.88MHz sampling rate, this will involve dividing the 28.8MHz
input by a factor of 10. One dongle might use edge numbers 0, 10, 20, 30,
... of the source clock to produce the 2.88 MHz clock and another might
used edge numbers 3, 13, 23, 33, .... Their sampling frequencies will be
exactly the same, but the sampling instant will be different.
Does anyone here know if this is possible? Shift the
tuner without
interupting the capture?
I can confirm that you can do this since this is what I did in my LTE cell
scanner to improve search speed. I configured the A/D rate once, and then I
just kept changing the LO frequency.
The thing is that once you change the LO frequency, the phase rotation of
the received signal will change randomly for all the dongles. So, during
the calibration phase, you might find that dongle 1 needs a time correction
of t1 and phase correction of p1 to get its signal to align with the signal
from dongle 2. After you change the LO, dongle 1 will still need the same
time correction (t1), but you wont know what phase to use. Perhaps phase
alignment isn't necessary for your application?
BR,
James
--
*Integrity is a binary state - **either you have it or you don’t.* - John
Doerr