xr215a Exar Corporation, xr215a Datasheet - Page 22

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

FSK Demodulation
Figure 21 contains a typical circuit connection for FSK
demodulation. When the input frequency is shifted,
corresponding to a data bit, the DC voltage at the phase
comparator outputs (pins 2 and 3) also reverses polarity.
The operational amplifier section is connected as a
comparator, and converts the DC level shift to a binary
output pulse. One of the phase comparator outputs (pin 3)
is AC grounded and serves as the bias reference for the
operational amplifier section. Capacitor C
PLL loop filter, and C
Range select resistor, R
adjustment to set the VCO frequency.
XR-215A
Rev. 1.01
Figure 20. Demodulated Output Waveforms for Time-Multiplexed Operation
Channel Select Pulse
Demodulated Output
2
and C
X
, can be used as a fine-tune
3
as post-detection filters.
1
serves as the
22
Typical component values for 300 baud and 1200 baud
operation are listed below:
Note:
For 300 Baud operation the circuit can be time-multiplexed be-
tween high and low bands by switching the external resistor R
in and out of the circuit with a control signal, as shown in
Figure 12.
FSK Generation
The digital programming capability of the XR-215A can be
used for FSK generation. A typical circuit connection for
this application is shown in Figure 22.
frequency can be shifted between the mark (f
(f
circuit can provide two separate FSK outputs: a low level
(2.5 Vp-p) output at pin 15 or a high amplitude (10 Vp-p)
output at pin 8. The output at each of these terminals is a
symmetrical squarewave with a typical second harmonic
content of less than 0.3%.
300 Baud
Low Band: f
High Band: f
1200 Baud
1
Table 1. Typical Component Values for Modems
) frequencies by applying a logic pulse to pin 10. The
Conditions
Operating
f
f
f
f
1
2
1
2
1
2
= 1070Hz
= 1270Hz
= 2025Hz
= 2225Hz
= 1200Hz
= 2200Hz
R
C
C
R
C
R
C
C
0
1
3
0
1
0
1
2
= 5k , C
= C
= 0.033 F
= 8k , C
= C
= 2k , C
= C
= 0.01 F
Typical Component
3
2
2
= C
= 0.003 F
= 0.047 F,
Values
3
0
0
0
= 0.033 F
= 0.1 F
= 0.12 F
= 0.17 F
2
) and space
The VCO
X