[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
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