[time-nuts] Bye-Bye Crystals

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Mar 14 07:03:10 EDT 2017

For a sinewave oscillator


is a start for fundamental crystals.

However the npn transistor operates a little too close to saturation for my liking.

A small change to the biasing of the npn will fix this.

With overtone crystal operation mode suppression is necessary to ensure the crystal operates on the desired overtone.

Amateur literature on crystal oscillators and even LC oscillators includes quite a few relatively poor circuits.

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.


>     On 14 March 2017 at 21:44 Hal Murray <hmurray at megapathdsl.net> wrote:
>     artgodwin at gmail.com said:
>         > > 
> >         I'm not after quality - I do have an application in mind but it doesn't need
> >         to compete with mass production. Just wondering if it's feasible to make
> >         something crude that will resonate.
> > 
> >     > 
>     Are you doing this for fun or ???
>     Feasible? Sure. Cheaper? That depends.
>     The cost difference between a complete oscillator package and a simple
>     crystal is tiny. The osc is often cheaper if you include board space or
>     engineering time.
>     Is your background digital or analog? Do you want a sine wave or a clock?
>     My background is primarily digital. If the chip you are using has 2 pins
>     setup to drive a crystal, you can probably get it to run reliably by
>     following the data sheet and/or app notes. The usual recipe is 2 tiny caps
>     and a big resistor. (big in resistance, not physically big)
>     An advantage of using a crystal with the on-chip amplifier that I didn't
>     mention last time is that you save the osc power if you power down that
>     corner of the chip.
>     If you want a sine wave, you are out of my comfort zone. I'd probably look
>     in ham radio literature.
>     They make logic chips like a 74HCU04, U for unbuffered. One of their uses is
>     for making oscillators. I've never done it. Try google.
>     --
>     These are my opinions. I hate spam.
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