[time-nuts] Bye-Bye Crystals
Richard (Rick) Karlquist
richard at karlquist.com
Tue Mar 14 13:02:49 EDT 2017
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.
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.
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