=09
=09
=20
Switchable Output Crystal Oscillator
This oscillator circuit =
permits=20
crystals to be electronically switched by logic commands. The circuit is =
best=20
understood by initially ignoring all crystals. Furthermore, assume that =
all=20
diodes are shorts and their associated 1kO resistors open. The two 1kO =
resistors=20
at the non-inverting input of IC1 (LT1016) set the output to half the =
supply,=20
ie, +2.5V. The RC network from the output to pin 3 sets up phase-shifted =
feedback and the circuit looks like a wide-band unity gain follower at =
DC. When=20
crystal X1 is inserted (remember, D1 is temporarily shorted) positive =
feedback=20
occurs and oscillation commences at the crystal=E2=80=99s resonant=20
frequency.
Circuit =
diagram:
If D1 and its 1kO resistor are then =
considered=20
to be part of the circuit, oscillation can only continue if logic input =
A is=20
biased high. Similarly, the circuit can only operate at crystal =
X2=E2=80=99s frequency=20
if logic input B is high. Additional crystal/diode/1kO resistor branches =
permit=20
logic selection of the crystal frequency. For AT cut crystals about a=20
millisecond is required for the circuit output to stabilise due to the =
high Q=20
factors involved. Crystal frequencies can be as high as 16MHz before =
propagation=20
delays in the comparator prevent reliable operation.