MC145170-2 Technical Data, Rev. 5

18

Freescale Semiconductor

Design Considerations

4

Design Considerations

4.1

Crystal Oscillator Considerations

The following options may be considered to provide a reference frequency to our CMOS frequency

synthesizers.

4.1.1

Use of a Hybrid Crystal Oscillator

Commercially available temperature-compensated crystal oscillators (TCXOs) or crystal-controlled data

clock oscillators provide very stable reference frequencies. An oscillator capable of CMOS logic levels at

For additional information about TCXOs, visit www.freescale.com on the world wide web.

4.1.2

Use of the On-Chip Oscillator Circuitry

The on-chip amplifier (a digital inverter) along with an appropriate crystal may be used to provide a

reference source frequency. A fundamental mode crystal, parallel resonant at the desired operating

frequency, should be connected as shown in Figure 20.

for lower frequencies. Assuming R1 = 0

can be estimated to be:

where

C1 and C2 = external capacitors (see Figure 21)

The oscillator can be “trimmed” on-frequency by making a portion or all of C1 variable. The crystal and

distortion, stray capacitance, stray inductance, and startup stabilization time. Circuit stray capacitance can

A good design practice is to pick a small value for C1, such as 5 to 10 pF. Next, C2 is calculated. C1 < C2

results in a more robust circuit for start-up and is more tolerant of crystal parameter variations.

CL

CinCout

Cin Cout

+

----------------------------Ca Cstray

C1 C2

×

C1 C2

+

---------------------

++

+

=