16

LT1394

1394f LT/TP 0499 4K • PRINTED IN USA

©LINEAR TECHNOLOGY CORPORATION 1998

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

Voltage-Controlled Crystal Oscillator (VCXO)

Figure 20, a variant of the basic crystal oscillator, permits

voltage tuning the output frequency. Such voltage-con-

trolled crystal oscillators (VCXO) are often employed

where slight variation of a stable carrier is required. This

example is specifically intended to provide a 4

× NTSC

sub-carrier tunable oscillator suitable for phase locking.

The LT1394 is set up as a crystal oscillator, operating

similarly to Figure 3 (a). The varactor diode is biased from

the tuning input. The tuning network is arranged so a 0V

to 5V drive provides a reasonably symmetric, broad tuning

range around the 14.31818MHz center frequency. The

indicated selected capacitor sets tuning bandwidth. It

should be picked to complement loop response in phase

locking applications. Figure 21 is a plot of tuning input

voltage versus frequency deviation. Tuning deviation from

the 4

× NTSC 14.31818MHz center frequency exceeds

±240ppm for a 0V to 5V input.

PART NUMBER

DESCRIPTION

COMMENTS

LT1016

UltraFast Precision Comparator

Industry Standard 10ns Comparator

LT1116

12ns Single Supply Ground-Sensing Comparator

Single Supply Version of LT1016

LT1671

Fast Single Supply Comparator

60ns, 450

µA Single Supply Comparator

LT1720

UltraFast Dual Single Supply Comparator

Dual 4.5ns, 4mA Single Supply Comparator

RELATED PARTS

Figure 21. Control Voltage vs Output Frequency for Figure 15.

Tuning Deviation from Center Frequency Exceeds

±240ppm

INPUT VOLTAGE (V)

0

2

4

5

9

8

7

6

5

4

3

2

1

0

1394 F21

13

14.314.0MHz

14.31818MHz

14.3217MHz

Figure 20. A 4

× NTSC Sub-Carrier Voltage-Tunable Crystal Oscillator. Tuning Range

and Bandwidth Accommodate Variety of Phase Locked Loops

1394 F20

–

+

LT1394

5V

390

Ω

200pF

* 1% FILM RESISTOR

** NORTHERN ENGINEERING LABS C-2350N-14.31818MHz

*** C SELECT SETS TUNING BANDWIDTH. SET TO COMPLEMENT

LOOP RESPONSE IN PHASE LOCKING APPLICATIONS

5V

FREQUENCY

OUTPUT

3.9k*

0V TO 5V

1M

2k

2k

100pF

15pF

Y1**

100pF

C SELECT***

0.047

µF

1M

1M

1M*

1N4148

LT1004-2.5

47k*

1k*