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

[Magnus Danielson]

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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