FM TRANSMITTERS (AN OVERVIEW) INTRODUCTION

FM TRANSMITTERS (AN OVERVIEW)

INTRODUCTION

The new generation FM transmitters in the AIR network can be classified according to their output powers as follows :

3 kW FM Transmitter 2x3 kW FM Transmitter 2x5 kW FM Transmitter

The Salient features, principles of working and RF block schematic of these three types of FM transmitters have been outlined in this chapter.

SALIENT FEATURES

a. Completely solid state.

b. Local/remote operation capability

c. Forced air-cooled.

d. Digitally synthesized crystal oscillator which can be set in steps of 10 kHz in the frequency band of 87.5 to 108 MHz. Frequency can be selected internally by BCD switches or externally by remote control.

e. Broadband VHF Power Amplifiers require no tuning.

f. Full power output just by pressing a single button.

g. Automatic output power reduction in the following cases :-

Mismatch (VSWR > 1.5)

Excessive heat sink temperature of output RF transistors (> 80oC).

Absorber temperature 70oC due to failure of one or more power amplifier units.

h. An automatic switch-over circuit ensures operation in the passive exciter standby mode. This means that either of the two exciters can be selected to operate as the main unit and the other exciter waits to be taken over.

i. The switching and operating status of the system is indicated by LEDs.

j. RF power transistors of power amplifiers are of screw-in type and no soldering is required during replacement.

k. Additional information such as SCA or RDS can also be transmitted.

l. Parallel operation of two transmitters in active standby mode is possible using a combining unit. If one of the transmitters fails, 1/4th of the total nominal power goes on the air so that continuity in service is maintained. Fault free transmitter can then be selected manually on antenna during suitable pause in programme with the help of U-link panels provided on the combining unit front panel.

m. High overall efficiency of the order of 55 to 60%.

STI(T) Publication 25 007/2003

3

FM Transmitter

PRINCIPLE OF WORKING

The principle of working of a modern FM Transmitter is given in block diagram in figure 1.

The L and R audio signals are converted into the stereo signal by a stereo coder. The stereo signal, also called the MULTIPLEXED (MPX) signal, then frequency modulates the VHF oscillator which is a voltage controlled oscillator (VCO) of the phase locked loop (PLL). The PLL is an automatic frequency control (AFC) system in the FM transmitter is maintained within the specified tolerance limits of + 2 kHz. In this arrangement, the phase of the VHF oscillator is compared with that of a reference crystal oscillator operating at 10 MHz. The frequency of the reference oscillator is divided by 1/1000 with the help of three decade counters in cascade to bring it down to the audio range (10 kHz). The VHF oscillator frequency is also divided by a factor N to scale it down to 10 kHz. As the VHF oscillator can operate at any assigned frequency in the FM Broadcasting band of 87.5 to 108 MHz, the factor N will vary from 8750 to 10800. the phases of the outputs from the two frequency dividers are then compared in a phase comparator and the resultant error voltage is amplified, rectified and filtered to get a DC error voltage of positive or negative polarity which corrects and drift in the VHF oscillator frequency.

Fig. 1 Block Diagram of Modern FM Transmitter

The operating frequency and the variable factor N are synthesised with the help of digital frequency synthesis techniques. Thus any frequency of high stability (same as that of the reference crystal oscillator) can be generated by using the same crystal oscillator of 10 MHz.

The FM signal obtained at the output of VHF oscillator is then amplified in a VHF Power Amplifier with an output power of 1.5 kW. This amplifier is the basic building block in the series of FM Transmitters. It is a wideband amplifier so that no tuning is required when the operating frequency is changed.


FM Transmitter(An Overview)

3 kW, 2x3 kW AND 2x5 kW FM TRANSMITTER

i) 3 kW FM Transmitter

RF block schematic of 3 kW FM transmitter is shown in figure 2. low level modulation of VHF oscillator is carried out at the carrier frequency in the Exciter type SU-115. The RF output of the exciter is split up into two halves using a splitter network called input coupler. Thus two VHF power amplifiers type VU 315 are driven by one Exciter. The RF outputs of these amplifiers are passed through harmonic filters and combined in the power coupler to get an output power of 3 kW. RF switch connects the selected exciter to the input coupler and the standby exciter to dummy load and AF switch feeds the audio to the selected exciter.

Nominal output power of the Exciter in a 3 kW transmitter is 6 W. All the modules are mounted in a single rack. Transmitter output is taken from the top and can be connected either to antenna or dummy load with the help of a U-link.

Fig. 2 RF signal flow of 3 kW FM Transmitter ( A or B)

ii) 2 x 3 kW FM Transmitter

RF block schematic of 2 x 3 kW FM Transmitter is shown in figure 3. It may be seen that the outputs of two 3 – kW transmitters are combined in a combining unit to get an output of about 5.5 kW. The nominal output of exciter in this transmitter is about 10 to 12 W because there are four PA units and the input power requirement of each PA is about 2.5 to 3 W. The exciter output is split into 4 equal parts in two stages of power splitting using three couplers. These four outputs drive four power amplifiers, each amplifier developing an output of 1.5 kW which is filtered in a harmonic filter (low pass filter with cut off frequency of 110 MHz). Two 1.5 kW outputs of each transmitter are then combined in output coupler to get an output of 3 kW for each transmitter. Both the transmitter outputs (3 kW each) are then combined in the combining unit to get an output of about 5.5 kW.


FM Transmitter

Fig. 3 RF block schematic of 2 x 3 kW FM Transmitter

iii) 2 x 5 kW FM Transmitter

This transmitter has been built up using the same concepts as 2 x 3 kW FM Transmitter and can be treated as its extension. RF block schematic of a 2 x 5 kW FM transmitter is shown in figure 4.

Fig. 4 RF Block Schematic of 2 x 5 kW FM Transmitter


FM Transmitter(An Overview)

The transmitter is housed in two racks, each rack giving an output of 5 kW which are combined in a combining unit to get an output of 10 kW. Since a total of 8 power Amplifier units have to be driven in this transmitter, a RF amplifier stage with 20 W nominal output is used after each exciter. Also a 1x4 input coupler and a 4x1 output coupler have been used in each transmitter. Power combiners used in combining units of 2x3 and 2x5 kW FM transmitters are identical.

The modern FM transmitters are compact, versatile, easy to install and operate. Their design incorporates in-built flexibility to provide different output powers using identical modules. This also adds to redundancy thereby increasing the reliability of the transmitter.


FM TRANSMITTERS (AN OVERVIEW) INTRODUCTION

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