[time-nuts] have 10MHz need 19.5Mhz

Kasper Pedersen time-nuts at kasperkp.dk
Sun Jun 16 10:59:22 EDT 2013


On 06/02/2013 10:21 PM, Attila Kinali wrote:
> 
> Another stupid idea: You have a 19.5MHz crystal, probably some
> odd PLL and a varicap diode lying around: Build a VCXO out of the
> crystal and lock it to the 10MHz using the PLL.
> 

It has been done with an xor gate, a couple of passives, and the varicap
diode. There processor performs as the divider:

This bit of code will configure the gpclk0 block to output 10MHz on pin
7 of the connector:

http://n1.taur.dk/pi10MHz.tgz

The signal on the pin is the 19.2MHz crystal multiplied to 500MHz (the
GPU PLL), and then divided back down to 10MHz.

Feed that signal, and the external reference signal into an xor gate or
other phase detector of choice. Put an RC filter of around 50us on the
output, and connect that to the varicap.

Picture of the thing as built:

http://n1.taur.dk/piclock.jpg

The only thing on the other side of the board is the connector and the
74HC86 in DIP. I don't know what my varicap is, it came out of an old
gps receiver.

/Kasper Pedersen


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