[time-nuts] have 10MHz need 19.5Mhz

[Demian Martin]

> From: kevin-usenet at horizon.com
> To: albertson.chris at gmail.com
> Cc: time-nuts at febo.com, kevin-usenet at horizon.com
> Subject: Re: [time-nuts] have 10MHz need 19.5Mhz
> Message-ID: <20130606162408.10277.qmail at science.horizon.com>
> 
> 
> Any 2-pin crystal connection to an IC is a simple Pierce oscillator.
> There's an on-chip inverter, with a high-impedance feedback resistor to
keep
> the duty cycle balanced.
> 
> You want to disconnect the output pin XTALN, and feed the input XTALP with
> an AC-coupled waveform of about 1 V p-p.  So use a capacative divider
> between the PLL output and ground to get the voltage right.
> 

If the crystal is close to the desired frequency I have had good success
capacitively coupling an external clock to the xtal in pin on a gate
oscillator leaving the crystal in place.  I have used 100 pF or less to get
this to work. If the external oscillator is missing the circuit still runs
but if its present at some level the circuit locks to the external source.
If there is a possibility the external source is not present (GPS system is
down) the rest of the system still runs with reduced accuracy.
          Demian