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063
Switchable Crystal Oscillator
Xn
Rn
1k
+5V
f n
The circuit shows a switchable crystal oscillator which can
generate any one of a number of set frequencies. Using the
high-speed LT1394 comparator from Linear Technology
( www.linear-tech.com ) it is possible to build a crystal oscil-
lator by putting the crystal in the negative feedback path,
while an RC network in the positive feedback path provides
the required phase lag.
Switching between the crystals is simply achieved using
diodes (type 1N4148). The diode corresponding to the
crystal to be selected is forward biased via its 1 k
Dn
1N4148
X2
R2
1k
+5V
f 2
D2
X1
1N4148
R1
1k
+5V
f 1
D1
1N4148
+5V
R3
C2
100n
series
resistor, while all the other inputs to the circuit must remain
at ground potential. Any crystals suitable for fundamental
frequency operation in the range 1 MHz to 15 MHz can be
used. Use of a comparator guarantees that the output sig-
nal is a squarewave.
1
2
7
IC1
3
4
LT1394
R5
2k2
R4
C1
68p
(014112-1)
014112 - 11
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