Z85233 Datasheet, PDF (290/317 Pages) Zilog, Inc. – The Zilog SCC Serial Communication Controller

Application Note
On-Chip Oscillator Design
Power
Supply
1
RTxCB (SCC)
EXTAL (Z180)
Probe (in)
470 pf
Vcc
Gnd
I.C.
Under Test
(All Unused
Inputs: 10kΩ To Vcc)
SYNCB (SCC)
XTAL (Z180)
22 pf
Frequency
Generator
1V P-P/Sine
Figure 6. Gain Measurement
Probe
(out)
Load Capacitors
The effects/purposes of the load caps are:
Cap C2 combined with the amp output resistance provides
a small phase shift. It also provides some attenuation of
overtones.
Cap C1 combined with the crystal resistance provides
additional phase shift.
These two phase shifts place the crystal in the parallel
resonant region of Figure 3.
Crystal manufacturers specify a load capacitance number.
This number is the load seen by the crystal which is the
series combination of C1 and C2, including all parasitics
(PCB and holder). This load is specified for crystals meant
to be used in a parallel resonant configuration. The effect
on startup time; if C1 and C2 increase, startup time
increases to the point at which the oscillator will not start.
Hence, for fast and reliable startup, over manufacture of
large quantities, the load caps should be sized as low as
possible without resulting in overtone operation.
Amplifier Characteristics
The following text discusses open loop gain vs. frequency,
open loop phase vs. frequency, and internal bias.
Open Loop Gain vs. Frequency over lot, VCC, Process
Split, and Temp. Closed loop gain must be adequate to
start the oscillator and keep it running at the desired
frequency. This means that the amplifier open loop gain
must be equal to one plus the gain required to overcome
the losses in the feedback path, across the frequency band
and up to the frequency of operation. This is over full
process, lot, V_CC, and temperature ranges. Therefore,
measuring the open loop gain is not sufficient; the losses
in the feedback path (crystal and load caps) must be
factored in.
Open Loop Phase vs. Frequency. Amplifier phase shift at
and near the frequency of interest must be 180 degrees plus
some, minus zero. The parallel configuration allows for
some phase delay in the amplifier. The crystal adjusts to this
by moving slightly down the reactance curve (Figure 3).
Internal Bias. Internal to the IC, there is a resistor placed
from output to input of the amplifier. The purpose of this
feedback is to bias the amplifier in its linear region and to
provide the startup transition. Typical values are 1M to
20M ohms.
UM010901-0601
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