EXCITER

INTRODUCTION Exciter type SU-115 generates FM signals in the frequency range 87.5 to 108 MHz inof 10 kHz. Its normal RF output power is 10 Watt. However, output power of 30 MW ocan also be obtained with the help of internal links and switches. A simplified Block Diagram of the Exciter is shown in given fig. 1. L and R audio signafed to stereo coder card. These are pre-emphasised as per 50 u sec Pre-Emphasis The pre-Emphasis can be switched OFF if required with the help of a switch provided ofront panel. These signals are then stereo encoded. Output of stereo coder cordmultiplexed (MPX) signal which modulates the RF signal generated in the VCO. modulated output is passed through a PIN diode regulator and then amplified in three sto get an output of 10 W. Harmonics are removed by 110 MHz Low Pass Filter.Directional coupler senses are forward and reflected voltages which are processComparison & Logic circuits. Automatic level control (ALC) circuit stabilizes the opower of the Exciter under adverse operating conditions of mismatch (VSWR >temperature and power supply variations. SUB-UNITS The main sub-units of the Exciter are :- S.No. Sub-Unit R & S PART No.

1. Display Board 1 & 2 674.0345 2. Input Board 674.0316 3. Stereo Coder 674.0216 4. Power Supply & AF Section 674.0574 5. Synthesiser 594.2059 6. Filter Board 674.0597 7. 10 Watt Output Stage 674.0451 8. 28 V Regulator Board 674.0716

Physical layout of these sub-units is shown in Fig. 2. Functional block diagram of the Exciter is shown in Fig.3. After proper impedance matching, the L & R audio signals are controlled by BCD swprovided on the front panel. In mono operation, the audio signal on the left chanamplified to a level of +6 dBm and taken to the front panel through Display Board 2. fine adjustment of level by + 0.5 dB is possible. The signal is then pressed through a 1Low Pass Filter in the Power Supply and AF section to suppress the 19 kHz Pilot Tonthe (L-R) signal.

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FM Transmitter

For stereo operation, the L and R signals available at the output of Input Board are fed to two identical circuits in the stereo coder unit. Here Pre-emphasis is introduced which can be switched ON or OFF from the front panel. There stereo or multiplexed (MPX) signal is fed to a summing amplifier to which pilot tone (19 kHz) is added. SCA and traffic signals can also be added here if desired. The summed output is fed to a MPX Low Pass Filter which suppresses the harmonics of the 38 kHz sub-carrier. The MPX signal is amplified to a level of +6 dBm. The audio level (mono or stereo) going into the synthesizer is sampled and applied to a peak detector for metering of DEVIATION (∆ f) on the front panel. L and R audio levels and pilot level are also metered. Output of the synthesizer is FM signal. The modulated signal is amplified to a level of about 2 V and fed to the Filter Board which consist of

1 W RF Amplifier stage. Switching Facilities for getting three output powers of 30 mW, 1 w and 10 W. Output filter. Directional coupler

The 10 watt amplifier is a separate stage mounted behind the front panel. Its heat sink is visible on the front side and is convection cooled. The 10 watt output is taken out at the 50 ohms connector at the rear. OPERATING INSTRUCTIONS

1. No manual adjustments are required during normal operation. 2. Functions of front-panel controls and displays are shown in fig. 4 (a) & (b) and pin

assignments of rear panel in fig. 5. 3. The output power of the transmitter can be set with level adjuster (12) on the front

panel. If the set value exceeds the internally set threshold value, the “RF Present” LED (13) lights.

4. The output power, L & R audio levels, frequency deviation and pilot tone level can be read on the meter with the help of selector switches (1) and (2).

5. The audio level and hence frequency deviation of the transmitter can be adjusted with input-signal BCD switches (4).

No particular maintenance is required under normal operating conditions. A simplified interconnection diagram of all the cards of exciter is shown in figure 6.

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Exciter

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FM Transmitter

1. Power Supply 2. Regulator Board for 10-W stage

3. AF Section 4. Synthesizer (Swung out)

5. Filter Board and 1-W output stage 6. 10-W Output Stage

7. Coder 8. Input Board

9. Display Board-1(Display board 2 is a card behind display board 1)

Fig. 2 Physical Layout Diagram

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Exciter

Fig. 3 Functional Block Diagram

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FM Transmitter

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(b) Rear side of Exciter

(a) Front side of Exciter Fig. 4 Front and Rear View

Exciter

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Fig. 5 Pin Assignment of Rear Panel

FM Transmitter

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Fig. 6 Exciter Interconnection Diagram (Simplified)