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LT1713IMS8

Single/Dual, 7ns, Low Power, 3V/5V/±5V Rail-to-Rail Comparators

Linear Technology 

LT1713/LT1714

TYPICAL APPLICATIO S

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

28psRMS on a 5V supply and 40psRMS on a 3V supply. At

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-

dal oscillators are not necessary.2 The sine wave is filtered

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.

2 Amplitude will be a linear function of comparator output swing, which is supply dependent

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.

R10

1k

VS

R1

1k

R2

1k

1MHz

AT-CUT R4

210Ω

VS

2+

1

7

LT1713

3 – LE 4 8

5

6

R3

1k

C1

0.1µF

R5

6.49k

C5

100pF

SQUARE

C4

100pF

R6

162Ω

C3

100pF

R9

2k

VS

C2

0.1µF

VS R7

15.8k

2– 7

6

LT1806

3+

R8

1

4

2k

171314 F07

SINE

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

LT1806 Op Amp is Configured in a Q = 5 Bandpass with fC = 1MHz

3V/DIV

1V/DIV

1V/DIV

200ns/DIV

171112 F08

Figure 8. Oscillator Waveforms with VS = 3V. Top is Comparator Output. Middle is

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

and Bandpass Filtered with a Q = 5 by LT1806

13

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