Re: [Discuss-gnuradio] dual coherent channel rtl_sdr

[Juha Vierinen]

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 <address@hidden> wrote:

On Sep 24, 2013, at 11:41 AM, Marcus D. Leech <address@hidden> 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

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