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

Bob Camp kb8tq at n1k.org
Tue Oct 27 00:42:30 UTC 2015


Hi

> On Oct 26, 2015, at 7:03 PM, Attila Kinali <attila at kinali.ch> wrote:
> 
> Hi,
> 
> 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?

Yes

> 
> Why use a colpitts oscillator when using the crystal as output filter?
> Wouldn't a Butler oscillator make more sense?

No

> 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?

No

> 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?

Not really

> 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 
coefficient. 

> 

There are also a number of “tweaks” (added components) typically put into that circuit for best performance.

Bob

> Thanks in advance
> 
> 			Attila Kinali
> 
> -- 
> Reading can seriously damage your ignorance.
> 		-- unknown
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.




More information about the Time-nuts_lists.febo.com mailing list