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AN501 Datasheet(PDF) 8 Page - Vishay Siliconix |
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AN501 Datasheet(HTML) 8 Page - Vishay Siliconix |
8 / 11 page AN501 Vishay Siliconix www.vishay.com S FaxBack 408-970-5600 6-8 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 V + 1 ) R f Rg For example, listed below in Table 2 are the results when Rf = 470 W. TABLE 2. Gains For The Circuit of Figure 19 Logic Input Rg Gain (AV) 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 S1 S2 S3 S16 A0 to A3 DG535/ (f0) (f1) 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 |
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