Hi, I modified my clock sharing so that I only insert a signal in the Xtal_In pin of other dongle. This way I won't have two circuits driving the same crystal, as Ian pointed out. The pin next to the edge of the dongle turned out to be the Xtal_In pin (the input of the opamp on the slave dongle). The dual coherent rtlsdr dongle still works the same. I guess I was lucky to get it working the first time.
I am working towards setting up a fanout buffer, to do this properly. juha On Tue, Sep 24, 2013 at 4:54 PM, Ian Buckley <i...@ionconcepts.com> wrote: > On Sep 24, 2013, at 11:41 AM, Marcus D. Leech <mle...@ripnet.com> 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 > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio >
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