LT1711 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LT1711' PDF : 12 Pages View PDF
LT1711/LT1712
TYPICAL APPLICATIO
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
R10
R5
1k
6.49k
C4
100pF
VS
R1
1k
R2
1k
C5
100pF
1MHz
AT-CUT R4
210Ω
R6
162Ω
VS
2+
1
7
LT1711
3 – LE 4 8
5
6
R3
1k
SQUARE
R9
2k
VS
C2
0.1µF
C3
100pF
R7
15.8k
VS
2– 7
6
LT1806S8
3+
R8
1
4
2k
171112 F07
SINE
C1
0.1µF
Figure 7. LT1711 Comparator is Configured as a Series
Resonant Xtal Oscillator. LT1806 Op Amp is Configured
in a Q = 5 Bandpass with fC = 1MHz
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.
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
LT1711 (Note the Glitches). Bottom is Buffered, Inverted
and Bandpass Filtered with a Q = 5 by LT1806
RELATED PARTS
PART NUMBER
LT1016
LT1116
LT1394
LT1671
LT1713/LT1714
LT1719
LT1720/LT1721
DESCRIPTION
UltraFast Precision Comparator
12ns Single Supply Ground Sensing Comparator
7ns, UltraFast Single Supply Comparator
60ns, Low Power, Single Supply Comparator
Single/Dual 7ns, Low Power, 3V/5V/±5V, R-R Comparator
4.5ns, Single Supply 3V/5V/±5V Comparator
Dual/Quad, 4.5ns, Single Supply Comparator
COMMENTS
Industry Standard 10ns Comparator
Single Supply Version of the LT1016
6mA Single Supply Comparator
450µA Single Supply Comparator
7ns/5mA versions of the LT1711/LT1712
4mA Comparator with Rail-to-Rail Outputs and Level Shifting
Dual/Quad Version of the LT1719
12 Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 q FAX:(408)434-0507 q www.linear-tech.com
171112f LT/TP 0401 4K • PRINTED IN USA
© LINEAR TECHNOLOGY CORPORATION 2001
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