AN501

Vishay Siliconix

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Document Number: 70608

03-Aug-99

FSK

Frequency shift keying (FSK), commonly used in data

transmission networks, relies on representing the digital code

with frequency sine wave bursts. An FSK multiplexing system

block diagram is shown in Figure 14. Each digital level has a

specific signal frequency. The DG535/536 can be used to

multiplex

16

different

digital

channels

into

a

single

transmission line or into a transmitter. Similarly, a DG535/536

may be used to demultiplex the data at the receiving end.

Since the device can manipulate higher frequency sine waves,

data can be transmitted at a higher rate than with a

conventional multiplexer.

PCM

A more commonly used and faster form of digital data

transmission is known as PCM (pulse coded modulation).

Used in telecommunications systems, PCM converts analog

speech signals into 8-bit digital words for serial transmission.

The data transfer rate used (for 4 kHz bandwidth voice signals)

is up to 274.176 Mbps.

The DG535/536 can be used to route PCM signals in main

telephone exchanges, replacing bulky hard-wired distribution

frames. PCM highways can thus be rerouted remotely, under

computer control, rather than manually.

RZ (returns to zero) PCM data consists of three discrete

(ternary) levels to overcome long periods of zeroes (Figure

13). Digital signals can degrade beyond legibility after only a

few hundred yards of travel down a transmission line.

Therefore, the PCM signals must be regenerated at regular

distances to avoid excessive distortion.

Figure 16 shows the architecture of a conventional binary

distribution

frame

in

a

telephone

exchange.

Signal

regeneration is applied to handle degradation during

transmission and routing. Code converters are required to

change the ternary PCM into binary PCM for routing within the

distribution frame. Similarly, code converters are required to

reconvert the binary PCM into ternary PCM for transmission.

Unlike digital switches which require specific digital signals,

using the DG535/536 in the distribution frame (Figure 17)

eliminates the need for code conversion and meticulous

regeneration because it can handle analog signals.

Programmable Gain Video Amplifier

The circuit shown in Figure 19 uses the DG536 as a binary

gain select for a video/wideband op-amp (CLC410).

The gain of the Si582 is set by:

A

R

f

= 470

W.

TABLE 2.

Gains For The Circuit of Figure 19

Logic Input

0000

47 k

W

1.0

0001

4.7 k

W

1.1

0010

2.4 k

W

1.2

0011

1.6 k

W

1.3

The low on-resistance of the DG535/536 gives good gain

stability, and the resistor tolerances mainly determine the gain

error of the circuit.

The wideband qualities of the CLC410 allows this circuit to be

employed for digital level correction in any video systems

including broadest quality specifications.

Digital

Input

Channels

e.g. 1011

FSK Output

Address

Bus

Output

to Transmission

Line or

Transmitter

Input

Stage

FSK

Generator

FSK

Generator

FSK

Generator

FSK

Generator

D

Frequency

Synthesizer

DG535/

Binary

PCM

a) Analog voice signal converted to digital signal using

8 bits per sample

b)

Digital signal transmitted and clock regenerated

from digital signal to get synchronization

Ternary

PCM

FIGURE 14. FSK Multiplexing System Block Diagram

FIGURE 15.

DG536