DS00795A-page 7

© 2002 Microchip Technology, Inc.

AN795

FREQUENCY SHIFT KEYING (FSK) GENERATION AND DECODING

Frequency Shift Keying (FSK) is a simple means of transmitting digital data over a signal path (two wires, telephone lines, AM or FM

transmitters).

Typically, only two frequencies are transmitted. One corresponds to a logical "0," the other to a logical "1." A TC9400 V/F converter will

generate these two frequencies when connected as shown below. The potentiometer sets the V/F converter to the lower frequency.

The digital input then determines which frequency is selected. A "0" selects the lower frequency, a "1" selects the upper frequency.

The digital frequency signal is converted back into a digital format by a TC9400 used in the F/V mode.

ULTRALINEAR FREQUENCY MODULATOR

Since the TC9400 is a very linear V/F converter, an FM modulator is very easy to build.

Center

Frequency

TC9400

V/F

Frequency Output

TC9400

F/V

Frequency

Offset

Center

Frequency

Digital

Output

Input

0

1

0 00 0

1

TC9400

V/F

1

0 00 0

1

FIGURE 16: Ultralinear frequency modulator.

FIGURE 15: Frequency Shift Keying (FSK) generation and decoding.

DC RESPONSE DATA RECORDING SYSTEM

Low-frequency analog data (DC to 10kHz) can be recorded anywhere, stored, and then reproduced. By varying the playback speed,

the frequency spectrum of the original data can be shifted up or down.

FIGURE 14: DC response data recording system.

TC9400

V/F

TC9400

F/V

TC9400

F/V

TC9400

V/F

L

R

L

R

Cassette or

Reel-to-Reel

Recorder