On Sep 24, 2013, at 1:54 PM, Ian Buckley <[email protected]> wrote:
> On Sep 24, 2013, at 11:41 AM, Marcus D. Leech <[email protected]> wrote: > >> On 09/23/2013 10:59 AM, Juha Vierinen wrote: >>> >>> I was playing around with the rtl_sdr dongles and came up with a trivial >>> hack to build a receiver with multiple coherent channels. I do this >>> basically by unsoldering the quartz clock on the slave units and cable the >>> clock from the master rtl dongle to the slave units (I've attached some >>> pictures). >>> >>> You still have to do sample alignment in software, but this is relatively >>> easy. There are a lot of cool applications, such as a dual frequency beacon >>> satellite receiver, interferometry, or passive radar that you can now do >>> with $16. >>> >>> juha >>> >>> >> So, what were your test conditions? >> >> I'm feeding a +3.3dBm signal from a high-precision communications test set >> at 28.8Mhz to two of those dongles. >> >> Then I'm feeding in a 45Mhz sine wave into the two devices RF input through >> a splitter and variable attenuator. >> >> The result is horrible relative-phase-noise between the two channels. They >> dance all over the place on the scope display. >> >> In comparision, a B100 with TVRX2, under the same conditions, works >> flawlessly, with no appreciable relative phase jitter between the >> two channels. >> >> -- >> Marcus Leech > > Marcus, (appreciate you may have done a lot more than your brief description > above, but just in case….) > > The type of cheap 2 pin oscillator used with the Realtek chips will be > connected across an internal inverting buffer amplifier in the IC with shunt > capacitance and all the circuit goodness that makes such thinks work. If you > are going to replace that with a buffered clock source such as a bench signal > source or expensive TXCO you're normally going to only drive the crystal > input pin and leave the other unconnected….now which pin that is I can;t tell > you because the data sheet/schematic isn't available to my knowledge…but hey, > its $8 so trial and error! > Might also want to consider series termination for each cable to the boards > to minimize SI issues also. > Of course in Juha's case he's just using the original clock-osc and getting > lucky that it's still oscillating cleanly with the two IC's driving the > crystal. > > -Ian > Couple of random application notes on the topic: http://www.maximintegrated.com/app-notes/index.mvp/id/3582 http://www.micrel.com/_PDF/App-Notes/clk/PAN0704111%20-%20Replacing%20Crystals%20and%20Oscillators.pdf
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