MC912D60ACPVE8 Datasheet, PDF (183/460 Pages) Freescale Semiconductor, Inc – MC68HC912D60A MC68HC912D60C MC68HC912D60P Technical Data

Oscillator
MC68HC912D60C Colpitts Oscillator Specification
12.4.1.4 Input ESD Resistor Path Modification
To satisfy the condition of oscillation, the oscillator circuit must not only
provide the correct amount of gain but also the correct amount of phase
shift. In the Colpitts configuration, the phase shift due to parasitics in the
input path to the gate of the transconductance amplifier must be as low
as possible. In the original configuration, the parasitic capacitance of the
clock input buffer (OTA), automatic Loop Control circuit (ALC), and input
resistors (RFLT and RFLT2) reacted with the input resistance to cause
a large phase shift.
To reduce the phase shift, the input ESD resistor (marked RESD in the
figure above) was changed from a single path to the input circuitry (the
ALC and the OTA) and oscillator transconductance amplifier (marked
GM in the figure above) to a parallel path. In this configuration, the only
capacitance causing a phase shift on the input to the transconductance
device is due to the transconductance device itself.
MC68HC912D60A — Rev. 3.1
Freescale Semiconductor
Oscillator
Technical Data
183