HEAD-END UNITS
The head-end unit is mounted at the focal point of the dish, and consists of two components: a feedhorn and an LNB. Both are weatherproofed for their outdoor environment.
Principle
Reception at SHF is a heterodyne process, but involves two i.f. frequencies in a double superhet configuration. The first ‘local oscillator’ is installed at the dish antenna assembly in order to convert to a lower (and easier to handle) frequency as early in the signal chain as possible. The oscillator in the LNB is not adjustable: it runs at a constant and very stable frequency of about 10 GHz. Each incoming carrier beats against the oscillator to produce difference frequencies which represent the first i.f. so that, assuming an oscillator frequency of 10 GHz, an incident 11.650 GHz signal gives rise to a ‘difference’ i.f. of 1.650 GHz, an incident 11.674 GHz signal, an i.f. of 1.674 GHz, an 11.175 GHz signal, an i.f. of 1.175 GHz and so on. Thus the satellite transponders’ signals are ‘block converted’ to a lower band and appear in reverse order on the new, lower-frequency carriers which pass down the cable to the indoor unit. There they are tuned by a second superhet unit with variable local oscillator and fixed i.f. as already described in Chapter 3. The basic principle is shown in Fig. 4.13, though in practice there is a multistage r.f. amplifier between the pick-up probe and the mixer diode.
SHF front end
The signal radiation reflected from the dish is collected by a waveguide at its focal point. The open end of the waveguide is fluted to provide an approximate match between its characteristic impedance and that of ‘free space’; the front end of the horn thus formed is protected against water and other ingress by a sealed cap, transparent to SHF radiation. The shape of the waveguide horn is a matter of careful design since it determines not only the efficiency with which signals are collected, but also the shape and amplitude of the side- lobes in the directivity diagram (Fig. 4.7), and the bandwidth and noise performance of the ensemble. In commercial kits the dish, feed- horn and LNB are all matched, but care must be taken when assembling an outdoor unit from unrelated components.
The feedhorn/waveguide is bolted to the front of the LNB, which contains the pick-up device in which the SHF magnetic energy is converted to an electrical signal current. It is at this point that provi- sion must be made for selecting the required polarisation of radia- tion, and rejecting signals arriving in the wrong polarisation. The two most common methods of selection involve no moving parts. One design has (printed on a glass fibre board) a micropatch on which the SHF signal is intercepted, with two adjacent printed ‘probes’ set at 90°. Each has its own r.f. amplifier which also acts as an electronic switch to select the required vertical or horizontal mode.
The amplifier in use, and thus the polarisation selected, is determined by the LNB operating voltage sent up the cable by the receiver: 13 V selects vertically polarised transmissions, 17 V selects horizontally polarised ones, while the 12 V or so required to power the LNB is cut off the bottom as it were.
An alternative system, not used for new designs of domestic receivers, uses a needle-probe mounted vertically in the LNB’s front aperture, capable only of intercepting vertically polarised signals. In the feedhorn is an electromagnetic polariser consisting of a coil wound on a ferrite core through which the incoming r.f. wave passes. Depending on the current flowing in the coil the wave’s polarisation is ‘twisted’ and at an optimum current exact alignment of the wave with the probe is achieved. Typically needing a d.c. current range of 70 mA to achieve 0–90° polarisation twist, these devices have low insertion loss but are frequency-dependent in their operation. A compromise setting can be made for a single narrow range of transponder frequencies, but where the system embraces a wide range of incoming signals – especially in a polar-mount system, where many satellites are addressed – provision must be made to optimise polariser current for each transponder.
DBS transmissions use circular polarisation, but all the transpond- ers on each satellite have the same (either RH or LH) characteristic, so that LNBs designed for single-satellite use have fixed polarisa- tion, and their design is simplified thereby.
LNB arrangement
Fig. 4.14 shows a functional diagram of a typical low-noise block. The pick-up probe is directly connected to an FET (Field Effect Transistor) made of gallium arsenide, GaAs, with tiny printed strip- lines 2–3 mm long as resonant circuits. One or two further stages of SHF amplification are provided to bring the signal level up to a point where it can be applied to a Schottky mixer diode, whose non- linearity ensures a strong beat signal, selected by filters and further amplified on its way to the output socket at the rear of the LNB module.
The local oscillator is also based on a GaAs FET: its output must
be pure and noise-free and its frequency very stable with time and temperature. The tuned circuit is formed by a ceramic-based dielectric resonator mounted on the PCB between the drain and gate leadouts of the transistor, and not necessarily having any electrical connec- tions to the circuit at all. The resonator may well have a screw-disc with which its frequency is preset at the factory.
The most important aspect of an LNB is its noise figure, normally quoted in dB, and indicating the relative amount of noise added to the signal in its passage through the device. Commercial LNBs for home use typically have a noise figure of around 0.8 dB, and current designs using HEMT (high electron mobility transistor) devices can achieve noise figures better than 0.5 dB. The overall gain of an LNB is about 60 dB, sufficient to launch the 1st i.f. signal into the down- lead at a high enough level to overcome the losses in the cable.
The power requirement for an LNB is generally 13–17 V at about 200 mA, and is fed via the downlead, with signal and power separated at either end by L/C components.
Band- and satellite-switching systems
The 13V/17V polarisation switching arrangement is supplemented in modern installations by additional control signals coming up the cable from the receiver. ‘Universal’ LNBs are capable of receiving a signal spectrum of 10.7–12.75 GHz, switching between low-band (10.7–11.7 GHz) and high-band (11.7–12.75 GHz) operation by means of a dual-frequency local oscillator. It is switched between
9.75 GHz and 10.60 GHz by a 22 kHz tone, off for low band, on for high band; the tone level is about 600 mV peak-to-peak, superimposed on the supply voltage. In practice it is more commonly used to select LNBs than to change receiving bands, using commercially available tone-triggered changeover switches.
To address the problem of band- and LNB-selection Eutelsat designed a control system called DiSEqC, based on modulating the 22 kHz tone with digital data: software commands permit the choice of several LNBs as source feeds. The simpler variant can switch up to four LNBs and select their oscillator frequency and polarisation by means of data bytes in the control signal, while a later variant (Version 2.0) is bi-directional to permit feedback from the head-end to the receiver’s control system for status checking and (e.g.) control of a motorised dish.