Datasheet
Data Sheet AD650
Rev. E | Page 17 of 20
PHASE-LOCKED LOOP F/V CONVERSION
Although the F/V conversion technique shown in Figure 13 is
quite accurate and uses only a few extra components, it is very
limited in terms of signal frequency response and carrier feed-
through. If the carrier (or input) frequency changes
instantaneously, then the output cannot change very rapidly due
to the integrator time constant formed by C
INT
and R
IN
. While it
is possible to decrease the integrator time constant to provide
faster settling of the F-to-V output voltage, the carrier
feedthrough then becomes larger. For signal frequency response
in excess of 2 kHz, a phase-locked F/V conversion technique
such as the one shown in Figure 22 is recommended.
In a phase-locked loop circuit, the oscillator is driven to a
frequency and phase equal to an input reference signal. In
applications such as a synthesizer, the oscillator output
frequency is first processed through a programmable “divide by
N” before being applied to the phase detector as feedback. Here
the oscillator frequency is forced to be equal to “N times” the
reference frequency. It is this frequency output that is the
desired output signal and not a voltage. In this case, the AD650
offers compact size and wide dynamic range.
14 13 1
8
6
54
2
3
7
9
10
1
2
5
4
3
6
+V
S
–V
S
+V
S
AD582
–V
S
1000pF
CAP
0.1µF
200kΩ
10kΩ
OUTPUT
1kΩ
1
2
3
4
14
13
12
11
5 10
6 9
7 8
AD7512
–V
S
+V
S
CONTROL
INPUT
+5V
INPUT
VOLTAGE
16.9kΩ
0.5mA
BIPOLAR
OFFSET
1mA
–V
S
+IN
–IN
+V
S
C
OS
NULL
AD650
NULL
COMPARATOR
INPUT
COMPARATOR
FREQUENCY
OUTPUT
DIGITAL
GND
ANALOG
GND
FREQUENCY
OUTPUT
ONE
SHOT
OP
AMP
–0.6 VOLT
500Ω
10µF
0.1µF 51pF
0.1µF
+
10
–15V +15V GND
1000pF
9
00797-022
3.6kΩ
–5 VOLTS VFC NORMAL
GND AUTO ZERO
1112
8
Figure 21. Autozero Circuit
C
51pF
15pF
R
140kΩ
71.5kΩ
590kΩ
–15V
1012
4 12
11
3
13
9
5
AD650
1MHz FULL-SCALE
R
IN
= 16.9k
C
OS
= 51pF
C
INT
= 1000pF
(UNIPOLAR INPUT)
FREQ
OUT
INPUT
CARRIER
VOLTS INPUT
TO AD650
NAND XOR
1
2
5
4
36
G
D
B
S
1/2 7474
1/2 7474
D
1
PR
1
D
2
PR
2
Q
1
Q
2
CLEAR
1
SD211
DMOSFET
D TYPE FLIP FLOP
1
1
F/V
VOLTAGE
OUTPUT
INPUT
CARRIER
CLOCK
1
CLOCK
2
CLEAR
2
00797-023
AD509
OP AMP
1/4 7400
7486
Figure 22. Phase-Locked Loop F/V Conversion