[time-nuts] FE 5680A "new version" - Filtering the 10 MHz

ed breya eb at telight.com
Tue Nov 15 11:21:39 EST 2011


You can add a crystal filter using a common 10 MHz crystal in series 
with a low noise, high impedance amplifier. Terminate the input side 
with about 50 ohms, and set the Q of the filter with the load 
impedance. Since you're only worried about harmonics, the Q can be 
fairly low, making the exact crystal characteristics (frequency 
precision especially) less important. Extra components can be added 
for more sophistication and compensation for parasitics (especially 
parallel Co), if needed. More stages will make it even cleaner, but 
the amplifier noises will add up. If there is enough signal power 
available in the original output, you can leave out the amplifier and 
use an impedance matching transformer instead, avoiding the amplifier noise.

  Ed





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