[time-nuts] Low noise quartz crystal oscillator by Bruce Griffiths
bruce.griffiths at xtra.co.nz
Mon Oct 26 20:48:23 EDT 2015
On Tuesday, October 27, 2015 12:03:49 AM Attila Kinali wrote:
> I've been trying to read up on low noise crystal oscillators and had
> a closer look at the design by Bruce Griffiths. There are explanations
> to how the circuit works, but I have some questions on the details.
> I would appreciate if someone could answer these questions.
>  http://www.ko4bb.com/~bruce/CrystalOscillators.html
> I will do a short recap how the circuit works, just to make sure I
> haven't misunderstood it.
> The oscillator core is the colpitts oscillator build around Q104,
> C107/108 form the driving/feedback path to form a negative resistance
> over the quartz crystal. The resistors R112 and R113 are there only
> to keep the crystal bias free and prevent charges from building up.
> The output is formed using the crystal as filter to get rid of
> harmonics and noise outside the crystal bandwidth. The "ground" point
> of the crystal is formed using the low input impedance of the common
> base amplifier formed by Q102. The output is coupled using a
> to make it DC free and for impedance transformation.
> Q103, LED102 and R116 form a constant current source for the collector
> of Q104, using the base of Q104 as control input.
> Q105 acts as a series voltage regulator, using multiple LM329's as
> reference, which are averaged for lower noise and Q106 to compensate
> for Q105's B-E voltage drop.
> Q101 is the input power supply filter.
> Now my questions:
> Doesn't the non-zero input impedance of Q102 dampen the
> crystal unnecessarily?
The effect is relatively insignificant provided the crystal esr is significantly
larger than the CB stage input R.
With an overtone crystal this is readily achieved.
> Why use a colpitts oscillator when using the crystal as output filter?
> Wouldn't a Butler oscillator make more sense? Or is there some
> disadvantage of Butler oscillator that I am not aware of?
Avoiding doubling resistance in series with the crystal due to the 2
transistors of the butler configuration.
> Why are LEDs used as voltage references? Don't they have a horrible
> temperature coefficient and bad aging characteristics?
> My guess would be that LED101 is not that critical as it will only
> result in a slight change of the collector current and thus only
> a slight change in the input impedance common base amplifier Q102.
In both cases the LED forward voltage tempco is approximately matched
by the Vbe tempco of a transistor so that the resultant dc current is
nominally temperature independent.
LEDs have relatively low noise however they are somewhat photosensitive.
Using low noise dc bias circuits like these can significantly reduce the
close in phase noise of RF amplifiers significantly compared to a bias
circuit using a voltage divider from the power supply.
> Does the constant current source (Q103, LED 102, R116) sufficiently
> stabilize the power inside the crystal, and thus the output power?
> My guess would be that changes in h_fe of Q104 will result in
> different biasing of Q104 and thus in changes of the power within the
> crystal, which then affects frequency and aging.
The colpitts oscillator transistor in this circuit operates in a discontinuous
> Can the noise induced by Q103 be further decreased by increasing
> Or is there a reason why C109 is just 10nF? Stability maybe?
> If stability is the problem, how about using an RC low pass filter?
the noise contribution by Q103 isnt significant.
Yes bias loop stability is an issue you cant just insert arbitrary low pass RC
filters some design effort is required.
> If one would want to make this circuit tunable, where would the
> varicap get connected to? My guess would be on the right side of
> the crystal, between the crystal and C105, going to ground
> The bias voltage would be then applied directly at the
> node. Is this correct or is there a better way?
In series with C105 is a far better location.
You may then need to increase the value of C105.
> What are the criteria to choose the transistors?
Low flicker noise and sufficient RF gain at the crystal frequency.
> Thanks in advance
> Attila Kinali
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