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:
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
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
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