AD537

REV. C

–7–

SYNCHRONOUS OPERATION

The SYNC terminal at pin 2 of the DIP package can be used to

synchronize a free running AD537 to a master oscillator, either

at a multiple or a sub-multiple of the primary frequency. The

preferred connection is shown in Figure 10. The diodes are used

to produce the proper drive magnitude from high level signals.

The SYNC terminal can also be used to shut off the oscillator.

output will go high (output NPN off).

1

2

14

13

5

6

7

10

9

8

3

4

12

11

AD537

R

1000pF

2

1N4148

10k

USE THIS LIMITER

BUF

DRIVER

CURR-

TO-FREQ

CONV

PRECISION

VOLTAGE

REFERENCE

Figure 10. Connection for Synchronous Operation

Figure 11 shows the maximum pull-in range available at a given

signal level; the optimum signal is a 0.8 to 1.0 volt square wave;

signals below 0.1 volt will have no effect; signals above 2 volts

p-p will disable the oscillator. The AD537 can normally be syn-

chronized to a signal which forces it to a higher frequency up to

30% above the nominal free-running frequency, it can only be

brought down about 1–2%.

0.2

0.4

0.6

0.8

1.0

30%

20%

10%

FREQUENCY

LOCK-IN

RANGE

Figure 11. Maximum Frequency Lock-ln Range vs.

Sync Signal

LINEAR PHASE LOCKED LOOP

The phase-locked-loop F/V circuit described earlier operates

from an essentially noise-free binary input. PLL’s are also used

to extract frequency information from a noisy analog signal. To

do this, the digital phase-comparator must be replaced by a lin-

ear multiplier. In the implementation shown in Figure 12, the

triangular waveform appearing across the timing capacitor is

used as one of the multiplier inputs; the signal provides the

other input. It can be shown that the mean value of the multi-

plier output is zero when the two signals are in quadrature. In

this condition, the ripple in the error signal is also quite small.

Thus, the voltage at Pin 5 is essentially zero, and the frequency

is determined primarily by the current in the timing resistor,

controlled either manually or by a control voltage.

1

2

14

13

5

6

7

10

9

8

3

4

12

11

AD537

CAP

0.01µF

OUTPUT

SIGNAL

INPUT

±12V PK

1

2

6

7

8

10

12

11

14

+15V

COM-

POSITE

ERROR

SIGNAL

±1V PK

–15V

10k

DEC/

SYN

FREQ

CONTROL

INPUT

0 TO –10V

RECOVERED

FREQUENCY

SIGNAL

3.9V

DRIVER

PRECISION

VOLTAGE

REFERENCE

CURR

-TO-

FREQ

CONV

BUF

LOGIC

GND

Figure 12. Linear Phase-Locked Loop

Noise on the input signal affects the loop operation only slightly;

it appears as noise in the timing current, but this is averaged out

by the timing capacitor. On the other hand, if the input fre-

quency changes there is a net error voltage at Pin 5 which acts

to bring the oscillator back into quadrature. Thus, the output at

Pin 14 is a noise-free square-wave having exactly the same fre-

quency as the input signal. The effectiveness of this circuit can

be judged from Figure 13 which shows the response to an input

of 1 V rms 1 kHz sinusoid plus 1 V rms Gaussian noise. The

positive supply to the AD537 is reduced by about 4 V in order

to keep the voltages at Pins 11 and 12 within the common-mode

range of the AD534.

Since this is also a first-order loop the circuit possesses a very

wide capture range. However, even better noise-integrating

properties can be achieved by adding a filter between the multi-

plier output and the VCO input. Details of suitable filter charac-

teristics can be found in the standard texts on the subject.

1V RMS SIGNAL

+1V RMS NOISE

OUTPUT

Figure 13. Performance of AD537 Linear Phase Locked

Loop

By connecting the multiplier output to the lower end of the tim-

ing resistor and moving the control input to Pin 5, a high resis-

tance frequency-control input is made available. However, due

to the reduced supply voltage, this input cannot exceed +6 V.

TRANSDUCER INTERFACE

The AD537 was specifically designed to accept a broad range of

input signals, particularly small voltage signals, which may be

converted directly (unlike many V-F converters which require

signal preconditioning). The 1.00 V stable reference output is

also useful in interfacing situations, and the high input resis-

tance allows nonloading interfacing from a source of varying

resistance, such as the slider of a potentiometer.