Nexperia
74HC6323A; 74HCT6323A
Programmable ripple counter with oscillator; 3-state
12.2 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.
12.3 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.
74HC_HCT6323A All information provided in this document is subject to legal disclaimers. © Nexperia B.V. 2018. All rights reserved.
Product data sheet Rev. 4 — 9 July 2018
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