[time-nuts] Low noise quartz crystal oscillator by Bruce Griffiths
bruce.griffiths at xtra.co.nz
Tue Oct 27 03:03:56 EDT 2015
As Rick has pointed out numerous times when the output signal is extracted via the crystal by a CB stage (or cascade thereof) the PN floor is determined by the ratio of the amplifier equivalent input noise current to the crystal current. That is the amplifier equivalent input noise current at frequencies for which the crystal impedance is high. If one neglects this crucial point one comes to the conclusion (e.g. see Eq 4.-1 page 274 of Ulrich Rohde's: Microwave and Wireless Synthesisers Theory and Design.) that with a crystal current of 1.4mA rms and a crystal esr of 50 ohms that the XO PN floor cannot be lower than -154dBc/Hz. Even the XO circuit in the ARRL handbook (attributed to Ulrich) using this method of signal extraction has a measured PN floor of -168dBc/Hz. Many other XO's (including the 10811A which uses a crystal current of 1mA ) have an actual PN significantly lower than this. One would have thought that this glaring discrepancy between "theory" and practice would have been noticed and corrected by now.
On Tuesday, 27 October 2015 6:01 PM, Richard (Rick) Karlquist <richard at karlquist.com> wrote:
The oscillator transistor and buffer amplifier are basically
the same as the HP 10811, except for the absence of mode
suppressors. The difference here is that the oscillator
self limits in the oscillator transistor, whereas the 10811
has ALC. The discontinuous operation of the transistor,
as explained by Driscoll some 45 years ago, is undesirable
because it increases the load resistance the crystal sees.
The 2 transistor "Driscoll oscillator" fixes this problem
by using an additional stage that limits instead of the
oscillator transistor. This has been widely used for
decades. It is interesting to note that the 10811 ALC
works by varying the DC bias current in the oscillator
transistor. This is in contrast to the elaborate DC
bias current stabilization here.
I have demonstrated that the close in phase noise in
the 10811 is entirely due to the flicker noise of the
crystal. The only place where the 10811 circuit comes
into play is beyond 1 kHz from the carrier, where the
Burgoon patent circuit (which apparently has prior art
from Ulrich Rhode) reduces the phase noise floor. I
have built two different oscillator circuits for 10811
crystals and have measured the flicker noise as being
the same as the intrinsic noise of the crystal.
Thus, obsessing over noise in oscillators circuits may
be overkill, unless you are planning to use a much
better crystal (BVA, etc). OTOH, it might be advantageous
to improve the reverse isolation by adding additional
grounded base buffer stages. There are various NBS/NIST
papers where several grounded base stages are cascaded.
I did this in the HP 10816 rubidium standard.
It is good to see time-nuts learning about oscillator
circuit by building them.
Rick Karlquist N6RK
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