Unfortunately that approach degrades the phase noise floor due to crystal
dissipation limitations as well as degrading the phase noise in the flicker
region due to the crystal itself,Not only is a clean (harmonic and subharmonic
free) sine wave desirable so is low phase noise.Thermal drift of the crystal
filter phase shift will also be problematic.Cascaded low Q filters suffer less
from this than a single high Q filter.
A low pass filter combined with series resonant shunt traps will have fewer
issues with phase instability at 10MHz than using a 10MHz bandpass filter.
Bruce
On Wednesday, 4 February 2015 9:53 AM, "Fuqua, Bill L" <[email protected]>
wrote:
Push-Push Jfet amplifier with parallel inputs and a Toroid output
transformer, no secondary along with a
simple filter using a 10 MHz series resonate crystal connected to one drain and
an adjustable capacitor connected
to the other would work fine. You connect the other ends of the two together
and a loading resistor to ground.
The capacitor is used to neutralize or null out the shunt capacitance of the
crystal so that a capacitive path
for the other frequencies , 5, 15, 20, etc is eliminated. Then follow up with
your linear class A amplifier.
The loading on the output of the crystal filter will determine it's Q and is
not real critical, but should be
perhaps around 10-100 times the series resistance of the crystal. Since most
readily available crystals
are not exactly on frequency a lower Q, higher R would be desired, but that
will not greatly affect the
5 MHz or undesired harmonic attenuation. Perhaps one or two kHz bandpass would
be just about right.
Just don't overdrive the crystal. Also, for the price of $1 or less you may
get 10 or so for further experimentation.
This combination of doubler and crystal filter should provide a very nice
sinewave output
73
Bill wa4lav
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