xr215a Exar Corporation, xr215a Datasheet - Page 24

• Manufacturer Part Number: xr215a
• Description: Monolithic Phase-locked Loop
• Manufacturer: Exar Corporation

Frequency Synthesis
In frequency synthesis applications, a programmable counter or divide-by-N circuit is connected between the VCO
output (pin 15) and one of the phase detector inputs (pins 4 or 6), as shown in Figure 23. The principle of operation of the
circuit can be briefly explained as follows: The counter divides down the oscillator frequency by the programmable
divider modulus, N. Thus, when the entire system is phase-locked to an input signal at frequency, f
at pin 15 is at a frequency (Nf
of discrete frequencies can be synthesized from a given reference frequency. The low-pass filter capacitor C
normally chosen to provide a cut-off frequency equal to 0.1% to 2% of the signal frequency, f
XR-215A
+5V
Rev. 1.01
0V
Keying
Input
s
), where N is the divider modulus. By proper choice of the divider modulus, a large number
R
Figure 22. Circuit Connection For FSK Generation
x
11
12
10
6
V
9
U1
EE
13
0.1 F
Phase
Comp.
VCO
5K
C
0
14
10K
24
0.1 F
5
2
5K
16 V
3
+12V
XR-215A
1
Op
Amp
CC
0.1 F
7
3K
15
4
8
5K
10K
s
.
FSK Output
s
FSK Output
(Low Level)
, the oscillator output
F1
F1
F2
F2
2.5V
10V
PP
PP
1
is