Re: [Discuss-gnuradio] dual coherent channel rtl_sdr

[Marcus D. Leech]

On 09/25/2013 02:01 PM, Juha Vierinen wrote:

Hi guys,

Based on my very limited understanding on electronics, osclilators and other such things, I would expect oscillator effects to cancel out when looking at the relative phase of two channels. I would expect the phase jitter on the relative phase to be dominated by what happens in the ADC and the digital down conversion. That is why I am running the two dongles off the same clock.  

I have some measurements of the relative IQ signal (z_1/z_2) at 100 kHz, 10 Hz, and 1 Hz sample rates. I did a power spectrum estimate using a Hanning window to look at relative phase noise. I didn't do any incoherent averaging on any of these, so there is some statistical noise. I only see some "imperfections" in the 10 Hz and 1 Hz spectrum.  However, I see these kinds of effects in 100 times more expensive receivers too. For my purposes, the relative phase behaviour of the two channels is good enough. 

I have attached the spectra. I have also attached the 1 Hz IQ signal, which shows a small systematic wiggle, but very little phase difference between the two channels.

The samples also are aligned over two hours of sampling, which means that there cannot really be a very large amount of clock drift between the two systems.

Sure, the thing has it's faults. But with $16, you really can't beat the price. 

juha

On Wed, Sep 25, 2013 at 1:21 AM, Marcus D. Leech <address@hidden> wrote:

On 09/25/2013 12:04 AM, Jared Clements wrote:

Hi Marcus,

Interesting discussion. I was wondering how well synchronized clocking might work.  Was hoping it would work better than it sounds.  Could you quantify the level of phase noise you're seeing, like with a power spectral density  plot?  Just for curiosity's sake?

I know of a recently updated OOT module that should have the blocks you need ;-)

I was hoping to build a correlating interferometer by modifying several of these dongles to run off the same clock.  I may need to rethink the plan.

Jared

I'll get to that tomorrow night.

I just ran a simulation of what I was observing (just because the lab is downstairs, and I'm upstairs).

Looks like the best it does is about -40dB/c  75kHz from the carrier.  That's bluddy awful.



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Here are some plots I made.

Along with the .grc file I used to produce them.   The RTL has 30dB worse phase noise, under the test conditions, at 100kHz offset.

Not wonderful, but not entirely unusable either.

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

phase_test_b100.png
Description: PNG image

phase_test_rtl.png
Description: PNG image

phase_test.grc
Description: application/gnuradio-grc