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 fi nd a stable DC operating point and refuse to
oscillate. The comparator output is a 1MHz square wave
with a measured jitter of 28ps
RMS
with a 5V supply and
40ps
RMS
with a 3V supply. On the other side of the crystal,
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 fi ltered version of the square
wave; the usual amplitude control loops associated with
sinusoidal oscillators are not immediately necessary.
1
One can make use of this sine wave by buffering and
fi ltering it, and this is the combined task of the LT1806. It
is confi gured as a bandpass fi lter 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 Confi gured as a Series Resonant Crystal Oscillator.
The LT1806 Op Amp is Confi gured in a Q = 5 Bandpass Filter with f
C
= 1MHz
–
+
8
SQUARE WAVE
V
S
V
S
V
S
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
–
+
V
S
7
6
1 (NC)
4
18067 F08
V
S
= 2.7V TO 6V
3
2
LT1806
LE
R6
162Ω
100pF
SINE WAVE
200ns/DIV
1V/DIV
1V/DIV
3V/DIV
18067 F09
1
Amplitude will be a linear function of comparator output swing, which is supply dependent and
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.
Figure 9. Oscillator Waveforms with V
S
= 3V. Top Trace is Comparator Output.
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
