XR215A Datasheet, PDF(22/32 Page) Exar Corporation

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XR215A Datasheet, PDF (22/32 Pages) Exar Corporation – Monolithic Phase-Locked Loop

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XR-215A

Demodulated Output

Channel Select Pulse

Figure 20. Demodulated Output Waveforms for Time-Multiplexed Operation

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 C1 serves as the

PLL loop filter, and C2 and C3 as post-detection filters.

Range select resistor, RX, can be used as a fine-tune

adjustment to set the VCO frequency.

Typical component values for 300 baud and 1200 baud

operation are listed below:

Operating

Conditions

300 Baud

Low Band: f1 = 1070Hz

f2 = 1270Hz

High Band: f1 = 2025Hz

f2 = 2225Hz

1200 Baud

f1 = 1200Hz

f2 = 2200Hz

Typical Component

Values

R0 = 5kW, C0 = 0.17mF

C1 = C2 = 0.047mF,

C3 = 0.033mF

R0 = 8kW, C0 = 0.1mF

C1 = C2 = C3 = 0.033mF

R0 = 2kW, C0 = 0.12mF

C1 = C3 = 0.003mF

C2 = 0.01mF

Table 1. Typical Component Values for Modems

Note:

For 300 Baud operation the circuit can be time-multiplexed be-

tween high and low bands by switching the external resistor RX

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. The VCO

frequency can be shifted between the mark (f2) and space

(f1) frequencies by applying a logic pulse to pin 10. The

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%.

Rev. 1.01

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