[time-nuts] Bye-Bye Crystals

[Bruce Griffiths]

Rick

Yes, the 10811A is a good example of the sort of kludge that is required when one tries to adapt the Colpitts XO to work with an overtone crystal.
However, apart from that, the design is still a lot better than those in most of the ham publications (eg. clamp diodes on JFET gates to limit the amplitude etc).

The E1938A bridged T oscillator along with some of Driscoll's many XO circuits are simpler and more effective.  

Bruce

> On 15 March 2017 at 06:02 "Richard (Rick) Karlquist" <richard at karlquist.com> wrote:
> 
> 
> 
> 
> On 3/14/2017 4:03 AM, Bruce Griffiths wrote:
> 
> >
> > Looking at oscillator circuits like the HP10811A will give some idea of some of the additional complexity required for a overtone operation. Dissecting a few ocxos may also be helpful. Some start with a 10MHz crystal and a Colpitts sustaining stage and use a 74HC74 or similar to  divide the 10Mhz by 2 and drive the output pin. Even when a sinewave output is required often a CMOS inverter drives the output pin via an LC filter.
> >
> > Bruce
> >
> 
> I don't agree here.  The 10811 is not a good tutorial for general 
> oscillator design.  Because it is SC cut, it has a complicated
> mode suppression network across the base emitter junction to
> suppress mode B as well as the fundamental.
> 
> The E1983A oscillator uses the same crystal (in a low profile
> package).  You can read my paper about it and see that I
> used a very simple bridged tee oscillator circuit.  That is
> all you need to select the right overtone and mode.
> 
> This is the same circuit that I used at Zeta Labs 40 years
> ago to design hundreds of custom VCXO's, up to the 9th
> overtone.  It simply worked every time, unlike various other
> designs that were in use at Zeta.
> 
> Around 1985, I got a consulting gig at Equatorial Communications
> to redesign their 5th overtone VCXO.  Only about half of the
> crystals would work in their circuit.  They had thousands
> of "reject" crystals.  I just used my old Zeta circuit and
> all the crystals started working again.
> 
> Equatorial owned the 10 meter dish that you used to see on
> your right going south on 237 just before passing over
> Central Expressway in Mountain View.
> 
> Rick N6RK
> 
>