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

[KA2WEU at aol.com]

Yes, more the termination .... Ulrich 
 
 
In a message dated 10/28/2015 9:02:56 P.M. W. Europe Standard Time,  
magnus at rubidium.dyndns.org writes:

Hi,

It is well worth mentioning that a crystal filter on the  output can 
become a challenge, as the source impedance can be far from 50  Ohm, and 
thus a bit of a challenge depending on how you  measure.

Cheers,
Magnus

On 10/28/2015 06:11 AM, Bruce  Griffiths wrote:
> Ulrich
>
> Surely you meant to  write
>
> PN(SSB) = -177 -Pout + NF
>
> If we attempt  to apply this equation to the 10811A for which you 
measured a PN floor of  -174dBc/Hz
> this implies that
>
> NF - Pout =  3dB
>
> Best case (NF = 0dB - unlikely! Pout would need to be much  higher for 
nonn zero NF)
>
> Pout =-3dBm or 500uW.
>
>  The question is identifying this power.
> The crystal dissipation is  50uW (HP Journal March 1981 p24)
> The signal power dissipated in the CB  stage input R is around 10% of 
this or about 5uW.
>
> 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
>
>
>       On Wednesday, 28  October 2015 8:34 AM, "KA2WEU at aol.com" 
<KA2WEU at aol.com>  wrote:
>
>
>   I have bought and measured the  hp10811 at about -174dBc/Hz. The 
interesting  thing is  the feedback  capacitor from collector to base which 
changes Rin=1/gm. 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  
Ulrich  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.
>  Bruce
>
>
>       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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