[time-nuts] Low noise quartz crystal oscillator by Bruce Griffiths

Attila Kinali attila at kinali.ch
Mon Oct 26 19:03:49 EDT 2015


I've been trying to read up on low noise crystal oscillators and had
a closer look at the design by Bruce Griffiths[1]. 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.

[1] 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 transformer
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?

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?

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.

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.

Can the noise induced by Q103 be further decreased by increasing C109?
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?

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 crystal/C105/varicap node. Is this correct or is there a better way?

What are the criteria to choose the transistors?

Thanks in advance

			Attila Kinali

Reading can seriously damage your ignorance.
		-- unknown

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