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

[Bruce Griffiths]

dk4xp at arcor.de wrote:
>
>
>
> ----- 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.
>
>    
Using a transistor with a higher ft than necessary in an oscillator 
circuit isnt usually a good idea.
> 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.
>
>
>    
Even without significant inductance between the oscillator transistor 
base and ground the shunt capacitance of the crystal itself can cause 
parasitic oscillations to occur.
A high frequencies the base is grounded via the tank capacitor, and the 
emitter impedance exhibits a negative resistance in series with an 
inductance.

Driscoll actually used ferrite beads on base and collector leads to 
suppress such oscillations.
The location of these beads is clearly shown in the original paper.
Driscoll used a capacitively split tank so that there is a capacitor 
from the oscillator transistor base to ground.
>> 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.
>
>    
A symmetric clipper (easily implemented by dc biasing an AC coupled 2 
diode pp detector) has some advantages.
> 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
>
>
>
>    
Some of Driscoll's later low phase noise OCXOs employ an MMIC as the 
oscillator and use the crystal together with a diode limiter and 
matching circuits to match the crystal to the 50 ohm input and output 
impedance of the MMIC. A splitter at the MMIC output is used. One 
splitter output drives the feedback loop whilst the other is used to 
drive the ooutput buffer.

Bruce