On 09/24/2013 04:57 PM, Ian Buckley wrote:
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:
Just tried a series termination on each dongle, consisting of a
1000pF cap in series with a 200Ohm resistor on each arm. It still
is "sane" with a
+3.3dBm sinusoidal input, but there's no difference in the
relative phase-noise between both channels.
--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org
|