[time-nuts] Crystal Drive Level

Bruce Griffiths bruce.griffiths at xtra.co.nz
Tue Aug 12 18:53:43 EDT 2008


Pete wrote:
> Peter,
>
> Bruce's text explains that R103 sets the crystal current to
> about 1.8mA. This setting would result in operating a 10 ohm
> crystal at about 32uW. You would have to adjust R103 to
> yield about 1mA through your 10 ohm crystal.
>
> Pete Rawson
>
>   
Pete

1) The 3 MHz crystal ESR almost certainly isn't 10 ohms.
The specification is actually for no more than 10 microwatts dissipated 
in a 10 ohm resistor connected in series with the crystal.
This is a convenient way of monitoring the crystal current but the 10 
ohm resistor shouldn't be permanently connected in series with the 
crystal as it degrades the Q.

2) The value of R103 needs to be adjusted to achieve the required output.
The value required depends on the values of C101 and C102.
Start with a high value for R103 (use a pot) and adjust the value until 
the required output is achieved.
Measure the resistance of the pot and replace it with a fixed value 
resistor.

Alternatively an AGC circuit could be used to regulate the crystal 
current by adjusting the base current of the oscillator transistor.

Bruce



More information about the time-nuts mailing list