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

Richard (Rick) Karlquist richard at karlquist.com
Wed Oct 28 09:45:54 EDT 2015

On 10/27/2015 10:11 PM, Bruce Griffiths wrote:

> The answer to this conundrum is surely that the equation for PN doesn't apply directly in this case
> for offset frequencies outside the crystal bandwidth.
> The Crystal actually bandpass filters the signal and PN noise generated by oscillator.
> For offset frequencies outside the crystal bandwidth the oscillator generated PN is greatly attenuated
> so that the noise of the buffer amplifier chain (CB stage plus output amplifiers) dominates.
> In calculating the noise floor of the buffer amplifier chain the fact that the crystal has
> a high impedance at these frequencies should be taken into account.
> Bruce

Bruce has it exactly right.  At offset frequencies beyond 1 kHz,
the source impedance for the grounded base is very high due to
the crystal impedance being very high.  As Burgoon explains,
this condition suppresses base recombination noise, and the
only noise mechanism that is significant is the collector shot
noise.  (To minimize shot noise, don't run more DC collector
current than necessary).

I read Ulrich Rohde's 1977 article showing this circuit,
before I started working at HP in 1979.  When I got to HP,
and learned about the 10811, I pointed this article out to Burgoon.
It turns out he independently reinvented the circuit,
but he was apparently the first person to realize the noise implications
of the circuit.

This buffer circuit, extended to multiple grounded base stages
in cascade for additional reverse isolation, makes so much sense that
every oscillator where phase noise floor or reverse isolation
is important should be using it, IMHO.  (Burgoon's patent expired long

A brief comment about the collector-base capacitor in the 10811 Colpitts
circuit:  this has the usual Colpitts function at 10 MHz, but it
also prevents the 2N5179 from oscillating at 1 GHz.  It must be
installed very close to the transistor or else the 2N5179 will be
unstable.  I discovered this when I copied the schematic, but
not the layout, of the 10811 for use in the 10816 rubidium.

The 2N5179 in the 10811 is selected for minimum beta and Ft at
20 mA, which is the start up condition due to the ALC being
at full gain.  It has a special HP part number, so you wouldn't
know this just looking at the parts list.

Rick Karlquist N6RK

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