[time-nuts] Notes on the Driscoll VHF Overtone Crystal Oscillator

[dk4xp at arcor.de]

----- Original Nachricht ----
Von:     Bruce Griffiths <bruce.griffiths at xtra.co.nz>
An:      Discussion of precise time and frequency measurement <time-nuts at febo.com>
Datum:   28.12.2009 06:52

> 
> An inductor in series with the 220 ohm emitter resistor will improve the
> phase noise floor.

In theory, yes. But already with only 220 Ohms, Q3 will oscillate wildly
at a few hundred MHz.

The mechanism is this: Somewhat hot RF transistor NE688, collector at
RF ground, emitter at high-ish impedance ---> When you measure into 
the base, you see a negative resistance in series with a few pF.

Add L6 = 82 nH with the other side at RF ground and you have built 
the usual negative-impedance VCO for VHF/UHF.
The crystal and the intended feedback network just don't matter any more.

I should have re-read my own Dubus article on oscillator simulations from 
6 years ago before I tried the Distaw. :-(   
Other people have observed the wild oscillations, too.


> The MMIC output amplifier has a wider bandwidth than necessary and
> doesn't have a particularly high reverse isolation.

Also, it has 20 dB gain, that alone guarantees a less than ideal
far-off noise level. The BAS70 clips at less than 1 V pp, this
should be more. Could be easily done in the Rohde style with a decoupled
DC divider and one Schottky that points from the divider to the
collector circuit.

I have changed my own locked VHF crystal oscillator back to Butler - this time
single stage with 3* cheap NXP BF862 in parallel, common gate.
The gate can be grounded directly, needs no voltage divider and decoupling.
Input impedance of the 3 FETs is abt. 7 Ohms, which brings us close to
the point of diminishing returns for the usual 45 Ohm crystal.

The BF862 works to 700 MHz, so it is just fast enough and won't surprise
me at 3 GHz.


regards, Gerhard  dk4xp