Z8030 View Datasheet(PDF) - Zilog

Part Name

Description

Manufacturer

Z8030

Zilog SCC Serial Communication Controller

Zilog 

Z

Region of Parallel

Operation

INDUCTIVE

Application Note

On-Chip Oscillator Design

1

0

fs fp*

2

CAPACITIVE

* fs - fp is very small (approximately 300 parts per million)

Figure 3. Series vs. Parallel Resonance

However, there are several ranges of frequencies where

the reactance is positive; these are the fundamental

(desired frequency of operation), and the third and fifth

mechanical overtones (approximately 3 and 5 times the

fundamental frequency). Since the desired frequency

range in this application is always the fundamental, the

overtones must be suppressed. This is done by reducing

the loop gain at these frequencies. Usually, the amplifier’s

gain roll off, in combination with the crystal parasitics and

load capacitors, is sufficient to reduce gain and prevent

oscillation at the overtone frequencies.

The following parameters are for an equivalent circuit of a

quartz crystal (Figure 4):

L - motional inductance (typ 120 mH @ 4 MHz)

C - motional capacitance (typ .01 pf @ 4 MHz)

R - motional resistance (typ 36 ohm @ 4 MHz)

Cs - shunt capacitance resulting from the sum of the

capacitor formed by the electrodes (with the quartz as a

dielectric) and the parasitics of the contact wires and

holder (typ 3 pf @ 4 MHz).

The series resonant frequency is given by:

Fs = 1/(2π x sqrt of LC),

where Xc and Xl are equal.

Thus, they cancel each other and the crystal is then R

shunted by Cs with zero phase shift.

The parallel resonant frequency is given by:

Fp = 1/[2π x sqrt of L (C Ct/C+Ct)],

where: Ct = CL+ CS

Cs

R

L

C

Quartz Equivalent Circuit

Symbolic Representation

Figure 4. Quartz Oscillator

UM010901-0601

6-153

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