10

LT1671

Crystal Oscillators

Figure 7 shows a crystal oscillator circuit. In the circuit, the

resistors at the LT1671’s positive input set a DC bias point.

The 2k-0.068

µF path sets up phase shifted feedback and

the circuit looks like a wideband unity-gain follower at DC.

The crystal’s path provides resonant positive feedback

and stable oscillation occurs.

Figure 7. 1MHz to 10MHz Crystal Oscillator

Figure 8. Switchable Output Crystal Oscillator. Biasing A or B

High Places Associated Crystal in Feedback Path. Additional

Crystal Branches Are Permissible

–

+

LT1671

2k

5V

2k

1MHz TO 10MHz

CRYSTAL (AT-CUT)

0.068

µF

OUTPUT

1671 F07

2k

1671 F08

–

+

LT1671

1k

5V

1k

1k

75pF

D1

OUTPUT

B

A

LOGIC INPUTS

AS MANY STAGES

AS DESIRED

XTAL A

1k

XTAL B

XTAL X

D2

2k

= 1N4148

GROUND XTAL CASES

Switchable Output Crystal Oscillator

Figure 8 permits crystals to be electronically switched by

logic commands. This circuit is similar to the previous

examples, except that oscillation is only possible when

one of the logic inputs is biased high.