LT1806/LT1807

24

18067fc

TYPICAL APPLICATIONS

1MHz Series Resonant Crystal Oscillator with Square

and Sinusoid Outputs

Figure 8 shows a classic 1MHz series resonant crystal

oscillator. At series resonance, the crystal is a low imped-

ance and the positive feedback connection is what brings

about oscillation at the series resonance frequency. The

RC feedback around the other path ensures that the circuit

does not find a stable DC operating point and refuse to

oscillate. The comparator output is a 1MHz square wave

however, is an excellent looking sine wave except for the

fact of the small high frequency glitch caused by the fast

edge and the crystal capacitance (middle trace of Figure 9).

Sinusoid amplitude stability is maintained by the fact that

the sine wave is basically a filtered version of the square

wave; the usual amplitude control loops associated with

One can make use of this sine wave by buffering and

filtering it, and this is the combined task of the LT1806. It

is configured as a bandpass filter with a Q of 5 and does

a good job of cleaning up and buffering the sine wave.

Distortion was measured at –70dBc and –60dBc on the

second and third harmonics.

Figure 8. LT1713 Comparator is Configured as a Series Resonant Crystal Oscillator.

–

+

8

SQUARE WAVE

1

7

4

5

R3

1k

R4

210Ω

R1

1k

R8

2k

R9

2k

R5

6.49k

1k

R7

15.8k

R2

1k

1MHZ

AT-CUT

C1

0.1μF

6

3

2

C2

0.1μF

C4

100pF

C3

100pF

LT1713

–

+

7

6

1 (NC)

4

18067 F08

3

2

LT1806

LE

R6

162Ω

100pF

SINE WAVE

200ns/DIV

1V/DIV

1V/DIV

3V/DIV

18067 F09

therefore controllable. The important difference here is that any added amplitude stabilization loop

will not be faced with the classical task of avoiding regions of nonoscillation versus clipping.

Middle Trace is Crystal Feedback to Pin 2 at LT1713. Bottom Trace is Buffered,

Inverted and Bandpass Filtered with a Q of 5 by the LT1806