On 4/13/18 10:33 AM, Dana Whitlow wrote:
Jim,
I'm curious: In what RF bandwidth will you be recording?
1 MHz for now.. the RTL-SDR isn't a super flexible device - there are
apparently good and bad rates - it does a DDC with 28.8 MHz I/Q NCO
(with who knows what kind of performance), and then filter and decimate.
There's a set of taps published for a FIR filter in the thing.
Ultimately, it comes out as 8 bit I and 8 bit Q, so I figure if I do
some more decimation on the 1MHz stream, I can get a few more effective
bits.
It will run at 2 MHz sample rate without dropping samples.. I could
probably modify the rtl utility to run at a higher rate and do a first
software filter/downsample to get the data rate down..
I'm really only interested in fairly narrow detection bandwidth.
For telescope use, Jupiter is pretty bright.
For "phase array to listen to HF signals" 20kHz is probably plenty (it's
not like I'm going to be developing a 3D CWSkimmer, yet)
Mostly it's because I'm managing a project at JPL where we're flying an
interferometer to look at CMEs from the Sun
http://adsabs.harvard.edu/abs/2017EGUGA..19.5580L
and I'm intrigued by whether I can do it in the backyard..
My first thought would be to search for a cross-correlation peak
between the two antenna outputs, but quickly realized that this
does not tell you anything about the timing differences between
the two receivers. I think you need to determine that independently
(else why bother with the interferometry in the first place?)
That's a clever idea..
Each node has its own GPS receiver, but they should all (within the
tolerance of the receiver) be "ticking" at the same time.
The receive bandwidth in conjunction with your S/N on the PPS
spikes will conspire to limit your timing accuracy, although you
can improve on that by averaging over a few minutes as you
plan.
Dana
On Fri, Apr 13, 2018 at 11:52 AM, Jim Lux <jim...@earthlink.net> wrote:
I'm building a phased array receiver (actually, an interferometer) using
RTL-SDR pods, where the elements are isolated from each other - there's a
common WiFi network connection, and each node has a BeagleBone Green, a
uBlox OEM-7M-C, and the RTL-SDR V3 (which works down to HF, since it has an
internal bypass around the RF front end).
In general, I have the RTL-SDR set up to capture at 1 Megasample/second. I
fire off a capture, record it to a file in the BeagleBone's flash, then
retrieve it from my host computer using scp over the network.
What I'm trying to do is capture data from all the nodes at
(approximately) the same time, then be able to line it all up in post
processing. The GPS (or NTP) is good enough to get them all to start
recording within a few tenths of a second.
So now the challenge is to "line em up". An obvious approach is to
transmit an inband pilot tone with some sync pattern, received by all, and
I'm working on that too.
But right now, I have the idea of capacitively coupling the 1pps pulse
from the GPS to the antenna input - the fast rising and falling edge are
broad band and show up in the sampled data.
The attached pulses1.png shows the integrated power in 1 ms chunks (i.e. I
sum the power from 1000 samples for each chunk) and it's easy to see the
GPS edges. And it's easy to create a estimate of the coarse timing (to 1
millisecond) - shown as the red trace.
But then, I want to get better. So for the 20 edges in my 10 second
example, I plotted (drift1.png) the raw I/Q output of the RTL. The pulse
isn't too huge (maybe 10 DN out of the ADC's -128 to +128 range), but is
visible. Bottom trace is the first, and they're stacked up
0, 0.1, 1.0, 1.1, 2.0, 2.1, etc.
And you can see, no surprise, that the sample clock in the RTL isn't dead
on - over the 10 seconds, it looks like it drifts about 30- 50 microseconds
- that is, the RTL clock is slow by 3-5 ppm.
SO here's the question for the time-nuts hive-mind...
What's a good (or not so good) way to develop an estimator of the
timing/frequency error. Post processing minutes of data is just fine..
I'm not sure what the actual "waveform" that is being sampled is (and it
will be perturbed by the quantization of the ADC, and probably not be the
same depending on where the RTL is tuned). That is there's some sort of
LPF in the front of the RTL, the edge is AC coupled, and then it goes into
some sort of digital down converter in the RTL running at 28.8 MHz sample
rate.
But it seems that there might be some way to "stack" a series of samples
and optimize some parameters to estimate the instantaneous time error-
given that the frequency vs time varies fairly slowly (over a minute or
so). It's fairly obvious from the plot that if one looked at the "single"
sample when the edge comes in, not only does the time shift with each
pulse, but the phase rotates as well (totally expected)
this is one of those things where you could probably lay a ruler on it and
estimate it by eye pretty well, but I'd like an automated algorithm.
It would be nice to be able to estimate the timing to, say, a few
nanoseconds over a minute or so ( - that would allow a phase estimation of
1/10th of a wavelength of a 20 MHz signal (e.g. Jupiter's RF noise, or
WWVH's transmissions)
Ideas???
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