In article , Paul Keinanen
writes:

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.

That should not normally be a problem. The basic formula for
RMS noise voltage is SQRT(4 k T Bw R) with k = 1.38 x 10^-23,
T is temperature in Kelvin, Bw is bandwidth of noise, R is Ohms
of the resistor.

Assuming a VCO control loop bandwidth of 5 KHz, temperature on
the warm side at 320 K, and a resistor value of 10 KOhms, the
RMS noise voltage would be about 0.94 microVolts.

If the VCO control voltage range is 4 VDC and the tuning range is
approximately linear over 4 MHz, that 0.94 uV will produce a phase
noise of 0.94 Hz RMS. Not a great deal...:-)

It's more than likely that stray voltage garbage in the circuit from
other sources (such as inadequately bypassed supply rails) would
be a potential problem. [pun intended]*

Len Anderson
retired (from regular hours) electronic engineer person













* when puns are outlawed, only outlaws will have puns...