Transmitters and receivers

11 Transmitters and receivers The previous chapters have covered all the circuit functions used in transmitters and receivers, but when putting them together into a TX or RX equipment, or indeed a T/R (transmitter/receiver, e.g. Figures 11.7 and 11.8), then certain additional considerations arise. These are considered below. Figure 11.1 a shows the block diagram of a 1 kW HF transmitter, such as might be used in commercial or military point-to-point communications. The block diagram of a low power solid state VHF FM transmitter, such as might be used as a 'fill-in' transmitter where the signal from the main transmitter is inadequate, would be very similar. The baseband signal would consist of the programme input material, speech or music, nowadays often in stereo. Baseband signal processing produces the mono-compatible sum signal, the stereo difference signal which is modulated onto a suppressed subcarrier, and the stereo pilot signal at half the frequency of the subcarrier. Often also, RD (radio data) information at a low bit rate is modulated onto an additional subcarrier. This carries a variety of information such as station identity, other frequencies on which the same programme can be received (useful for auto-searching FM receivers in cars), etc. The composite baseband signal is modulated onto a carrier at a suitable IF frequency such as 10.7 MHz and then, after filtering to the final bandwidth, translated in a mixer stage to the final transmit frequency. In the USA, the serasoidal modulator was at one time popular, but this has a maximum phase deviation less than ___180 ~ Frequency multiplication was therefore necessary to obtain the required deviation, making it difficult to achieve an acceptable signal to noise ratio even with a mono signal. In a broadcast transmitter, the transmit frequency is seldom if ever changed, so tuning arrangements are much simpler than those commonly found in receivers. However, sophisticated protection arrangements for safety purposes are necessary, including interlocks to prevent the equipment being accidentally powered up whilst personnel are servicing it, and trips to protect the PA in the event of an antenna fault, etc. In one sense, a good transmitter is easier to design than a good receiver, since the only signal it has to handle is the wanted signal. This is especially true of a transmitter working over only a fairly narrow percentage bandwidth such as the 88-108 MHz VHF FM broadcast band, as it is then easy to arrange that no mixer spurious outputs fall on or close to the wanted output in the transmit band. In an HF communications transmitter covering the band 1.6-29.999 MHz, the problem is more acute. A double conversion scheme would therefore be used with the modulation typically taking place at 1.4 MHz, the signal then being translated to an IF of (say) 45 MHz before down conversion to the final transmit frequency. Low-power UHF transmitters used in walkie-talkies, portable telephones, etc., operating in parts of the 470-960 MHz spectrum