13
LT1713/LT1714
Figure 7. LT1713 Comparator is Configured as a Series Resonant Xtal Oscillator.
LT1806 Op Amp is Configured in a Q = 5 Bandpass with f
C
= 1MHz
3V/DIV
1V/DIV
1V/DIV
200ns/DIV
171112 F08
Figure 8. Oscillator Waveforms with V
S
= 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
–
+
LT1713
2
3
6
LE
5
1
R1
1k
V
S
V
S
V
S
7
8
SQUARE
171314 F07
SINE
R3
1k
C1
0.1µF
4
R2
1k
R4
210Ω
1
6
2
3
4
7
R8
2k
V
S
1MHz
AT-CUT
R9
2k
R7
15.8k
R10
1k
R6
162Ω
R5
6.49k
C5
100pF
C2
0.1µF
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
28ps
RMS
on a 5V supply and 40ps
RMS
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.
TYPICAL APPLICATIO S
U
