SATELLITE RECEIVERS
There are two basic functions within an analogue satellite receiver: to select the required channel and to demodulate it to produce video and audio signals. There are many secondary functions – powering the LNB, selecting polarisation, status indication, provision for descrambling, selection of sound carrier, remodulation to UHF and others – but most of them have counterparts in other home video equipment.
The first i.f. frequency, a block-converted group of channels containing the entire spectrum received by the dish ensemble and placed anywhere from 950 MHz to 2.15 GHz, enters the receiver and is amplified (Fig. 4.15) and a.g.c.-controlled. Image-rejection filter- ing is also carried out here. R.f. tuning is by lecher lines and varicap diodes, as in the conventional tuners examined in Chapter 3. The selected signal is applied to a mixer along with a second input from an oscillator whose frequency is varicap-controlled by a PLL tuning system of the type we have already met. The 2nd i.f., generally at about 480 MHz, is selected and extracted by a SAW filter with a bandwidth of 27 MHz. The filtered signal is amplified to overcome the SAW filter losses. After further amplification and close a.g.c. control the signal is brought to a level suitable for application to the vision i.f. demodulator. Before we leave this ‘tuner-heart’ section, note the prescaler and programmable divider within the dotted box. These are the essence of the tuning circuit: programme tuning data from the station memory enters the module at pins C, D, L (Clock, Data, Load/enable).
FM vision demodulator
There are several ways to demodulate f.m. signals. The quadrature detector has already been described in Chapter 3, and other forms of modern f.m. demodulators will be examined in videorecorder applications in Chapter 14. An IC-based technology for recovering f.m. signals is the phase-locked-loop (PLL) system, and this is commonly used in f.m. vision demodulators.
A simplified block diagram of a PLL demodulator appears in Fig.
4.16. The incoming carrier is applied as one input of a phase detector whose error output governs the frequency of a VCO (Voltage- Controlled Oscillator). The second input of the phase detector is the VCO output, and since the phase detector’s error voltage steers the VCO until its two inputs coincide in frequency and phase, the oscillator (provided a short enough time-constant is present in the error- voltage filter) exactly follows the frequency of the carrier signal, including its deviation due to the modulating (vision) signal. Thus the error – or correction – signal, in pulling the VCO continually in line with the f.m. carrier, reflects (in its amplitude and polarity respectively) the amount and direction of all excursions of the car- rier from its nominal centre frequency. In doing so it forms a perfect facsimile of the original modulating signal, which is what we want from a demodulator: in this case a video waveform similar in form to that of Fig. 2.5. PLL demodulators are also used in f.m. radio sets for broadcast and commumications reception, and in broadcast stereo decoders.
Integrated receiver/decoder
Fig. 4.17 shows a block diagram of an IRD (Integrated Receiver/ Decoder) for reception of Astra transmissions. The tuner heart combines the functions of tuner, i.f. and vision demodulator, so that baseband video and sound signals are passed to the video input block, wherein is a de-emphasis circuit together with a clamp (to remove the
energy-dispersal waveform), a sound-rejection trap, and output buffers. The main output goes from here to the video switch block which selects a direct signal or a decrypted one as necessary, passing it out to the SCART socket and the UHF remodulator. In the absence of a satellite carrier signal a tuning/identification pattern is automatically switched in, primarily to assist UHF tuning of the associated TV set. A second feed from the video input block passes to the sound demodulator section. It consists of a pair of f.m. demodulators, one each for left and right sound channels, working on carriers 180 kHz apart. These carriers (at 10.52 and 10.7 MHz for R and L respectively) are the products of a superhet process in which a local oscillator beats against the incoming audio f.m. carriers at (e.g.) 7.02/7.20 MHz. By controlling the frequency of the local sound-detector oscillator different stereo carrier pairs can be brought into line with the audio demodulators to select the required sound channel per transponder. It is done in preset software and selected by the control microprocessor IT01. Demodulated sound, after de-emphasis, is passed out of the box through the SCART interface, while the L and R signals are mixed for application to the mono-only UHF remodulator.
There are two further outputs from the ‘video input’ block. The composite video/baseband out can be used to feed a MAC decoder or a special decoder for extra subscription programmes etc. A feed is also taken to the Video crypt board, on which the scrambled (subscription) transmissions are restored to normal video signals – so long as the fee has been paid! The sync separator stage, IL01, is a simple processor from which line, field and coincidence pulses are derived for use in the descrambler, which is digital in operation, and whose precise method of operation is a part of a confidentiality agreement between broadcaster and receiver manufacturer. A ‘smart- card’, purchased by the subscriber, is fed into the card reader, and so long as the data in the card is compatible with the transmission at the time, descrambling takes place. In this particular design the commands from the local and remote-control keyboards (the latter via an infra-red link) are also processed on the Videocrypt board – in the microcomputer control system, which governs all the functions of the receiver via the I2C bus examined in Chapter 22.
The power supply section of a satellite receiver is similar to that of a TV or VCR, and will be dealt with in detail in Chapter 11. For cool running and efficiency, switch-mode types are currently used. The PSU in Fig. 4.17 is of the switch-mode type and has seven output lines, not all of which are switched off by the control microprocessor during standby mode, when the r.f.-through amplifier, the remote control receiver/decoder and the display system must be kept in operation. The LNB is powered at all times to prevent thermal cycling and thus maintain stability and reliability, and also to receive the authorisation codes transmitted to each individual receiver from time to time; this also involves keeping much of the receiver section alive too.
Satellite receivers without integral descramblers often have a connection socket for a separate card reader/descrambler, taking the form of a SCART socket or the 15-pin D-type connector system shown in Fig. 4.18.