74HC6323A Datasheet, PDF (15/15 Pages) NXP Semiconductors – Programmable ripple counter with oscillator; 3-state

Philips Semiconductors
Programmable ripple counter with
oscillator; 3-state
Product specification
74HC/HCT6323A
Typical Crystal Oscillator
In Fig.13, R2 is the power limiting
resistor. For starting and maintaining
oscillation a minimum
transconductance is necessary, so
R2 should not be too large. A practical
value for R2 is 2.2 kΩ.
The oscillator has been designed to
operate over a wide frequency
spectrum, for quartz crystals
operating in the fundamental mode
and in the overtone mode. The circuit
is a Pierce type oscillator and requires
a minimum of external components.
There are two on-chip capacitors, X1
and X2, of approximately 7 pF.
Together with the stray and input
capacitance the value becomes 12 pF
for 8-pin SO packages. These values
are convenient and make it possible
to run the oscillator in the third
overtone without external capacitors
applied. If a certain frequency is
chosen, the IC parameters, as
forward transconductance, and the
crystal parameters such as the
motional resistances R1
(fundamental), R3 (third overtone)
and R5 (fifth overtone), are of
paramount importance. Also the
values of the external components as
R_s (series resistance) and the crystal
load capacitances play an important
role. Especially in overtone mode
oscillations, R_b (bias resistance) and
the load capacitance values are very
important.
Considerations for Fundamental
Oscillator:
In the fundamental oscillator mode,
the R_b has only the function of biasing
the inverter stage, so that it operates
as an amplifier with a phase shift of
approximately 180°. The value must
be high, i.e. 100 kΩ up to 10 MΩ. The
load capacitors C1 and C2, must
have a value that is suitable for the
crystal being used. The crystal is
designed for a certain frequency
having a specific load capacitance.
C1 can be used to trim the oscillation
frequency. The series resistance
reduces the total loop gain. One
function of it is therefore to reduce the
power dissipation in the crystal. R_s
also suppresses overtone oscillations
and introduces a phase shift over a
broad frequency range. This is of less
concern provided R_s is not too high a
value.
Note
A combination of a small load
capacitor value and a small series
resistance, may cause a third
overtone oscillation.
Considerations for Third-overtone
Oscillator:
In the overtone configuration, series
resistance is no longer applied. This
is essential otherwise the gain for
third overtone can be too small for
oscillation. A simple solution to
suppress the fundamental oscillation,
is to spoil the crystal fundamental
activity. By dramatically reducing the
value of the bias resistor of the
inverting stage, and applying small
load capacitors, it is possible to have
an insufficient phase in the total loop
for fundamental oscillation. However
the phase for third overtone is good. It
can be explained by the R_b × C_l time
constant. During oscillation the
crystal with the load capacitors cause
a phase shift of 180°. Because R_b is
parallel with the crystal (no R_s), R_b
spoils the phase for fundamental.
R_b × C_l must be of a value, that it is
not spoiling the phase for third
overtone too much. Because third
overtone is a 3 times higher
frequency than the fundamental, the
R_b × C_l cannot 'maintain' the higher
third overtone frequency, which
results in a less spoiled overtone
phase.
PACKAGE OUTLINES
See “74HC/HCT/HCU/HCMOS Logic
Package Outlines”.
September 1993
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