reading / British practice / Vol D - 1990 (ocr) ELECTRICAL SYSTEM & EQUIPMENT

8.39 Typical fixed

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bination of all or some of these items of equipment could be assembled to make up an antenna coupling system alitable for a particular location.

8.7.1 Fixed station transmitters

The fixed station transmitters used for power station radio system , are ,:aliclarci PMR transmitters designed [o operate at he particular band or frequencies allocated to PNIR use. The fixed stations for VHF low band systems use single-frequency simplex, amplitude modulation (AM) base stations. For the UHF band systems, the fixed stations use two-frequency simplex, frequency modulation (FM):

operation means that both the fixed station and mobiles use the same RF frequency for both transmit and receive functions. Since the same frequency is used, the fixed station and mobile are unable to operate at the same time (duplex operation). It is necessary, therefore, for the fixed station and mobile to transmit in turn, i.e., single frequency simplex operation.

operation means that the fixed station and mobile use different RF frequencies for the transmit functions. The fixed station uses one frequency for transmit which is received by the mobile receiver and the mobile uses a second frequency for transmit which is received by the fixed station receiver. Although two frequencies would allow duplex operation, simplex operation is used for speech because a user of the radio system cannot speak and listen at the same instant. Two-frequency si mplex operation also provides improved mobile to mobile communications by the use of a fixed station talkthrough facility. During talkthrough, any signal received at the fixed station receiver, e.g., from a mobile, is re-transmitted by the fixed station transmitter to other mobiles switched to the same RF channel.

The transmitter equipment has to comply with the following performance specifications issued by the Radiocotnmunications Agency (RA) of the Department of Trade and Industry (DTI):

•For frequency modulated (FM), UHF and VHF equipment — MPT 1326 — Performance Specification for Angle modulated VHF and UHF radio equipment for use at fixed and mobile stations in the Private Mobile Radio Service.

•For amplitude modulated AM, VHF equipment — MPT 1302 — Performance Specification for Amplitude modulated VHF radio equipment for use at fixed and mobile stations in the Private Mobile Radio Service.

Figure 8.40 shows a block diagram of a typical RF fixed station transmitter. The speech, plus any tone

signalling, is transmitted from the remote controller over a cable connection to the line interface and audio frequency (AF) amplifier section of the transmitter. A crystal-controlled RF oscillator generates the RF carrier wave which is connected via a frequency multiplier to the amplitude modulation (AN1) modulator or frequency modulation (FM) modulator in series with the output of the AF amplifier section.

In an AM transmitter, the amplitude of the RF carrier wave is varied in sympathy with the complex AF output from the AF amplifier section and the radio frequency remains constant. In an FM transmitter, the frequency of the RF carrier wave is varied in sympathy with the AF output from the AF amplifer section and the amplitude of the RF carrier wave remains constant.

The modulator in an AM transmitter is a non-linear device which has the RI carrier and signal voltages applied to it in series.

The modulator in an FM transmitter varies the frequency of the carrier wave to produce a FM signal which, as shown in Section 8.4.2 of this chapter, can be represented by:

V fm = Ve sin 27r (f c + 5)1

where Fe is the carrier frequency and

Sr is the frequency deviation corresponding to the frequency shift of the carrier for the peak amplitude of the information signal

Thus for a carrier of 10.7 MHz and frequency deviation of 5 kHz (0.005 MHz), the instantaneous carrier frequency would be 10.705 MHz at the positive peak of the signal and 10.695 MHz at the negative peak.

The output of the modulator is connected to a power amplifier driver section which provides the driving signal to operate the power output stage of the transmitter. Since this stage is operating at relatively high RF powers, e.g., 0.5 W for handportables and 25 W for fixed stations, use of a linear Class A amplifier stage would result in significant power losses in the internal impedance of the stage. Figure 8.41 (a) shows a typical transfer characteristic for a Class A amplifier stage. A constant bias centres the signal on the linear portion of the characteristic between the cut-off point for current through the stage and the upper point where current would be drawn by the control circuit. This causes a constant current to flow through the stage which is varied as shown when a RF signal is present at the input. This results in a stage efficiency of between 40 and 65%. For fixed station transmitters the heat generated in the stage would have to be dissipated adequately. For a handportable, this would result in an unacceptable drain on the battery as well as requiring adequate stage cooling.

Figure 8.41 (b) shows a typical transfer characteristic of two Class B amplifier stages operating in push-pull,

,Alitte one amplifier carries the positive half-cycles of the RE signal and the other the negative. The outputs of each of the two amplifier stages are connected to a orninon load which combines the two half-cycles to , t2 the RE signal output shown. No current flows in t He stage unless a signal is present in addition to the onstant cut-off bias. This improves the stage efficiency

to between 70 and 85%.

Figure 8.41 (c) shows a typical transfer characteristic fkLf. a Class C amplifier stage. In this case, the stage is biased by a multiple of the cut-off bias voltage value. large signal is applied to the input of the stage from the driver stage of the transmitter which results

m the flow of RF signal pulses through the load of He Class C output stage. As shown in the figure, the pulses occur at alternate half-cycles of the input signal and are present for a period less than the time of a bill-cycle. The output load of the Class C stage is a parallel tuned circuit, tuned to the RF carrier wave. The Transmitter is designed so that the size of the RF pulses 1, large enough to contain sufficient energy to ensure tie tuned circuit continues to oscillate between pulses, arid contains sufficient harmonic information to carry

!lie modulating information signal. Class C operation unproves the stage efficiency to between 95 and 98 07o.

1s can be seen from the transfer characteristic, a Class C staee operates on the non-linear portion of tHe characteristic. Therefore it requires only a small interfering signal fed back from the antenna to produce

ii m‘ anted intermodulation products between the trans- mitted carrier frequency and the interfering signal.

Figure 8.42 shows typical intermodulation char-

ILIcristics due to the non-linearity of the Class C output 'rage of a transmitter at 150 MHz.

The graph shows the level of third order intermodulation relative to the interfering signal level. As ■ otilci be expected, the level of the third order intermodulation decreases with an increase in frequency ,eparation between the source of intermodulation, i.e., he transmitted frequency and the interfering signal.

The highest level of third order intermodulation for adjacent transmit channels, i.e., 12.5 kHz or

25 kHz channel spacing. This level of between —7 and —8 dB is sometimes referred to as the 'conversion figure' of 7 or 8 dB.

Except where poor engineering practices have been adopted in the assembly of transmitters in the fixed station cubicle/cabinet, most intermodulation generation will be caused by coupling between antennas, either directly or via the feeder runs, or due to a combination of both — see Section 8.6 of this chapter on antennas.

Care must be taken in the design of the coupling equipment (which interfaces the power amplifier and the antenna system) to reduce the signal levels which are received by the output stage from the antenna.

Modern nuclear and large fossil-fuel power stations require a number of fixed stations to provide full radio cover of the power station. In order to use the same radio channel at a number of fixed stations, the transmitters are operated in a quasi-synchronous mode. This is necessary because it is usually not possible completely to confine the transmitted RF signal to one particular zone of the power station.

A handportable user receiving similar signal strength RF carrier waves from two transmitters operating on the same frequency, would experience erratic reception of the radio message as he moved in the vicinity of overlap. This would be due to the addition, carried out in the handportable receiver, of all signals being received. The received signals would include direct and reflected signals from each of the transmitters. In some locations, the prominent signals from each transmitter would be out-of-phase causing a cancellation of signal, while elsewhere they would be in-phase.

By off-tuning one of the signals by a very small amount, e.g., 2 or 3 Hz in 460 MHz, a quasi-synchro- nous mode of operation is achieved.

The signals now add but result in a 2 or 3 Hz beat signal being produced which is sub-audible to the handportable user. In practice, a hissing beat is heard but it does not detract from the intelligibility of the message.

In order to provide quasi-synchronous operation, it is necessary to have a very stable RF oscillator in the

STAGE 2 CURRENT —1

STAGE 1 CURRENT

4-1

transmitter. A crystal-controlled oscillator is therefore provided with a frequency stability ageing rate of the order of 5 parts in 10 10 per day. The crystal used is contained in a temperature-controlled oven. The quasisynchronous systems marketed by Philips Telecommunications Ltd use a high stability drive unit Type HS400, a simple block diagram of which is shown in Fig 8.43.

The latest transmitters being introduced for PMR systems use synthesising techniques. The crystal-con- LOAD trolled oscillator is used to provide the stabilised RE generator but individual channel frequencies are derived digitally by means of multiplier and divider circuits.

This approach has the advantage that one transmitter can be programmed to produce any one of 256 channels.

(b) Cass B Amplification

STAGE CURRENT, I t +1

NEGATIVE BIAS,V -4—

• INPUT

CI Class C Amplification

F[G, 8.41 Amplifier classifications

8.7.2 Fixed station receivers

The fixed station receivers used for power station radio systems are standard PMR receivers designed to operate at the particular band of frequencies allocated to PMR use.

As explained in Section 8.7.1 of this chapter for fixed station transmitters, the VHF low band systems used in power stations employ single-frequency simplex AM operation and the UHF systems two-frequency simplex FM operation. The receiver equipment has to comply with the performance specifications issued by the Radiocommunications Agency of the DTI as described in the section on transmitters, i.e., MPT 1302 and 1326. Figure 8.44 shows block diagrams of typical AM and FM RF fixed station receivers.

AM and FM systems both use similar antenna coupling equipment and RF amplifier sections.

The AM oscillator and mixer stage connects the RF output of a local oscillator and the received RF signal in series to a non-linear device which produces three signals, fe , fc — f o and fc 1.0 , e.g., for a received frequency (f,) of 461.5 MHz and a local oscillator