Electrical – How to connect the crystal to generate a simple pulse

Choice depends on MANY tradeoffs such as: cost, volume, stability, temperature range, frequency, package size, power consumption, phase noise, etc You have to specify all or we make assumptions.

• The "sweet spot" for fundamental AT cut Xtal's in micro-slice low cost EMD package is 4 or 8MHz to divide down to 1MHz. Lower is bigger and more expensive, much higher tends to be overtone harmonic and less stable.

• 50 ppm stability is standard, 30 ppm is avail for -20~+70'C, much less is not possible unless you choose a VCXO 1ppm or a narrow temperature range.

• 50 ppm tolerance is standard at room temp. design can null this but costs more than sorting if you can tolerate 30 ppm or 15 ppm as cost goes up with small sort bins. 50ppm tolerance is $0.15 @1k and 30 ppm is $0.20 @1k assuming SMD 4 or 8MHz.

• Standard CMOS parallel resonant oscillator is easiest and lowest parts count, but use NPO caps to create parallel load of 15 to 20pF typ as specified with 2 caps on either side.

Although you can get better phase noise results with a discrete filter Pierce oscillator design, the standard CMOS inverter works well for most.

• C1 + C2 = Cload
• R = self bias 1~10MΩ
• R1 = limit power dissipation in Xtal (uW) is usually 3~10KΩ

Of significant importance is the capacitor on the input to the inverter. The phase shift between output and input has to be 180 degrees for oscillation, and the XTAL and capacitor together produce this phase shift. If you look at the equivalent circuit of the XTAL and fed a signal in at the right frequency then looked at the output across said capacitor you'll see that phase shift. That's why you can push up or down the frequency a little bit when you vary the cap: -

In the above example I've set the equivalent circuit of the XTAL to be: -

• L = 8.45mH
• Cs = 0.03pF
• Cp = 6pF

As you can see in the lower graph the phase shift between the limits of the series and parallel resonance points falls to 180 degrees.

There is a lot more subtlety to it than what I've shown but hopefully this is enough to help.