13

LT1713/LT1714

Figure 7. LT1713 Comparator is Configured as a Series Resonant Xtal Oscillator.

3V/DIV

1V/DIV

1V/DIV

200ns/DIV

171112 F08

Xtal Feedback to Pin 2 at LT1713 (Note the Glitches). Bottom is Buffered, Inverted

and Bandpass Filtered with a Q = 5 by LT1806

–

+

LT1713

2

3

6

LE

5

1

R1

1k

7

8

SQUARE

171314 F07

SINE

R3

1k

C1

0.1

µF

4

R2

1k

R4

210

Ω

1

6

2

3

4

7

R8

2k

1MHz

AT-CUT

R9

2k

R7

15.8k

R10

1k

R6

162

Ω

R5

6.49k

C5

100pF

C2

0.1

µF

C3

100pF

C4

100pF

–

+

LT1806

1MHz Series Resonant Crystal Oscillator

with Square and Sinusoid Outputs

Figure 7 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 resonant 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

(top trace of Figure 8) with jitter measured at better than

Pin 2 of the comparator, on the other side of the crystal, is

a clean sine wave except for the presence of the small high

frequency glitch (middle trace of Figure 8). This glitch is

caused by the fast edge of the comparator output feeding

back through crystal capacitance. Amplitude stability of

the sine wave is maintained by the fact that the sine wave

is basically a filtered version of the square wave. Hence,

the usual amplitude control loops associated with sinusoi-

and buffered by the fast, low noise LT1806 op amp. To

remove the glitch, the LT1806 is configured as a bandpass

filter with a Q of 5 and unity-gain center frequency of

1MHz, with its output shown as the bottom trace of

Figure 8. Distortion was measured at – 70dBc and – 60dBc

on the second and third harmonics, respectively.

and therefore adjustable. The important difference here is that any added amplitude

stabilization or control loop will not be faced with the classical task of avoiding regions of

nonoscillation versus clipping.

TYPICAL APPLICATIO S