Crystal calculations

A project log for Pulse to Tone Converter

Converts pulse dialing of your POTS telephone to DTMF dialing allowing you to use it with your VOIP's FXS-port.

Christoph TackChristoph Tack 02/11/2018 at 20:400 Comments

What crystal package to use : small 3.2x2.5mm or big HC49/US?  One parameter determining the choice is the drive level of the crystal.  If the drive level is too high, i.e. >200µW typically, the small crystal can't be used.  

The drive level is calculated as follows:

Quartz crystal equivalent circuit (reference:



Calculation of CL

Calculation of drive level


The smaller crystal package can be used safely.

Another parameter that determines the choice is availability of the crystals.  For 8MHz, HC49US crystals have better specs, are more widely used and are cheaper than the 3.2x2.5mm package crystals.