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Switched reactances in VCO

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November 8th 03, 11:07 AM

Paul Keinanen

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On Sat, 08 Nov 2003 06:08:28 GMT, wrote:

Thanks. I'll have to experiment with resistors vs inductors.
I guess I always assumed you *had* to use inductors to keep
the RF off the supply line.

There is only one problem that I can think about when using only
resistors to isolate the diode control voltage is the thermal noise
voltage generated by any resistor. Especially in VCO control voltage
lines, the thermal noise voltage generated by the resistor will add up
to the control voltage, changing the capacitance and hence generate
phase noise in the oscillator.

However, a reverse biased switching diode at 12 V should have a quite
low absolute capacitance and thus, any switching voltage noise would
have a very minimal effect on the capacitance.

Paul OH3LWR

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