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

KA2WEU at aol.com KA2WEU at aol.com
Tue Oct 27 15:33:56 EDT 2015

I have bought and measured the hp10811 at about -174dBc/Hz.  The 
interesting  thing is  the feedback capacitor from collector  to base which changes 
Unless the circuit has a hidden Q mulitplier the PN (SSB) can never be  
better then 177 (kT) in dBm  + Pout  in dBm - NF of the  oscillator transistor. 
Many of the GB stages are potentially unstable , so the  "hopeful' best PN 
(SSB) is 177dbm + Pout !
AT 100 Mhz the leaing values are -146/100Hz offset  and - 183 far out  and 
high crystal dissipation, 2mW or so 
In a message dated 10/27/2015 4:17:16 P.M. W. Europe Standard Time,  
bruce.griffiths at xtra.co.nz writes:

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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