[time-nuts] Low noise quartz crystal oscillator by Bruce Griffiths
kb8tq at n1k.org
Mon Oct 26 20:42:30 EDT 2015
> On Oct 26, 2015, at 7:03 PM, Attila Kinali <attila at kinali.ch> 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 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?
Build it and see...
> Why are LEDs used as voltage references? Don't they have a horrible
> temperature coefficient and bad aging characteristics?
but the “right” tempco
> 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.
Except that Hfe is pretty stable over time
> Can the noise induced by Q103 be further decreased by increasing C109?
> Or is there a reason why C109 is just 10nF?
Look at the loop stability of the current source and the R/C between the 10K and C109
> 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
In series with C105, with a blocking cap to the crystal and buffer sides.
> The bias voltage would be then applied directly at the crystal/C105/varicap node. Is this correct or is there a better way?
Block the DC
> What are the criteria to choose the transistors?
Low noise, more specifically the combination of low 1/F noise and low noise figure at RF and a low mixing
There are also a number of “tweaks” (added components) typically put into that circuit for best performance.
> Thanks in advance
> Attila Kinali
> Reading can seriously damage your ignorance.
> -- unknown
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