Re: [Discuss-gnuradio] dual coherent channel rtl_sdr

[Ian Buckley]

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