DISTRIBUTION AMPLIFIERS FOR UHF
Apart from MATV installations in blocks of flats, hotels and shops, there is an increasing demand for multiple aerial outlet points in ordinary dwellings, where several TV sets may be found in different rooms, often requiring signal feeds from VCRs and satellite receivers as well as off-air transmissions. For this purpose a simple mains- powered distribution amplifier is used, mounted at the rear of the main TV, or (to save difficult cable-routeing and redecoration problems) in the loft or even on the aerial pole itself. From here separate cables are routed to up to six outlets in different rooms. In areas of good field strength a passive splitter may be used to provide two outlets from a single cable, but at least 6 dB attenuation is introduced in each path.
SATELLITE AERIALS
At microwave frequencies a form of dipole (in fact a probe) is still used for signal pick-up, but unaided it would intercept virtually none of the very low-level signals from space. Even the addition of parasitic elements in Yagi form would not be effective – the signal capture area would not be great enough. Instead a parabolic dish is used to intercept r.f. energy over a larger area. The surface of the dish is care- fully formed into a true uniform parabola so that the centimetric waves are all reflected, uniformly and in phase, to the focal point. Here may be mounted the pick-up probe, though sometimes a sub- reflector is fitted to redirect the energy to the centre-point of the dish, where sits a waveguide in which the pick-up probe is mounted. The difficulties of conveying SHF signals is such that a low-noise amplifier or mixer stage is connected direct to the pick-up probe itself – more on this in Chapter 4.
TUNERS
The UHF tuner has several functions. It has to reject out-of-band transmissions, amplify the incoming signal and then mix it with an internally generated c.w. (continuous-wave) signal to give an output on the difference frequency between the two UHF signals – the incoming a.m. modulated one and the local oscillator output. The tuner’s local oscillator runs (for UK receivers) at a frequency
39.5 MHz above the required vision carrier frequency, so that for instance in the case of channel 23 (Fig. 2.6(a)) whose vision carrier has a frequency of 487.25 MHz the local oscillator would need to run at precisely 526.75 MHz. The two signals come together in a mixer, a non-linear device which produces outputs at the sum and difference frequencies of its two inputs. In this case the difference frequency of 39.5 MHz (i.f., intermediate frequency) is selected by a tuned-circuit filter which rejects other frequencies. This filter and subsequent circuits have a bandwidth sufficient to embrace not only the sideband signals corresponding to high frequencies in the vision signal, but also the sound i.f. which will appear at 33.5 MHz. This arises from the difference between the channel 23 sound carrier at 493.25 MHz and the same local oscillator frequency of 526.75 MHz. What has been described is in fact the superheterodyne principle, which is used in virtually all r.f. receiving equipment from pocket radios up to satellite installations.
The great virtue of superhet operation is that by altering the local oscillator frequency any required incoming signal can be translated to a single, common frequency carrier – the i.f. Provided the local oscillator is made to run at a frequency (in this case) 39.5 MHz higher than the wanted one, the signal (complete with sidebands) is translated to a single low frequency where it can be dealt with by a fix-tuned amplifier with fix-tuned filters to shape the required pass- band. Thus the need for ‘movable’ tuned circuits is confined to three or four within the tuner itself.
Apart from offering gain to overcome the inherently noisy mixing process, and isolating the local oscillator signal from the aerial, the r.f. amplifier is required to reject the image frequency. At a given local oscillator rate (fosc) there are two input frequencies which can give rise to a 39.5 kHz i.f. signal – the wanted frequency at fosc 39.5 MHz; and an unwanted (image) frequency at fosc + 39.5 MHz. The bandwidth of the tuned r.f. amplifier (generally a two-stage sec- tion) is tailored to offer approaching 60 dB of image frequency rejection at a frequency 79 MHz above the wanted carrier. Calcula- tion shows that the sound carrier of channel n + 4 (where n is the required channel) will give rise to a spurious i.f. signal 1.5 MHz away from the required vision i.f. carrier. To avoid beat pattern effects on the picture the r.f. amplifier’s response to the n + 4 channel must be at least 55 dB down.
On VHF bands I and III, conventional inductors and capacitors can be used in the tuned circuits required for oscillator and r.f. ampli- fier tuning. On UHF bands IV and V such inductors would consist of less than one turn, and designing a tuner along such lines would be difficult. An alternative technique is the use of distributed constants in tuned circuits making use of lecher lines printed on the surface of a low-loss insulating board. Each line is equivalent in length to an electrical half-wavelength, having one end grounded and the other end (nodal point) tuned by a variable capacitor with which its resonant frequency can be swung over the required range.
Fig. 3.3 shows a typical varicap tuner’s circuit diagram. For optimum noise performance and matching over the entire UHF band 470 to 860 MHz the input circuit is untuned. TR701 forms the first r.f. amplifier and operates in grounded-base mode, with the input signal applied to the emitter via the diode attenuator D600/D601. TR701 collector circuit incorporates a tuned load L510 whence the signal is transferred via C213 to bandpass tuned circuit L511/L512. The local oscillator is TR702, whose frequency is governed by the capacitive loading at the top end of lecher line L518. Local and broadcast signals are applied via C220 and L513 respectively to Schottky mixer diode D603. At its anode appears the wanted beat
signal, selected and filtered by the LC network en route to i.f. ampli- fier TR703, again a common-base stage. TR703 collector circuit is returned to ground externally of the tuner, and further selection and filtering takes place in L523 and associated components.
AGC is applied in two ways in this tuner. The attenuation offered by the PIN diode pair D600/601 depends on the amount of current ‘sinked’ from the gain control pin 3 by the external a.g.c. control circuit; at high current levels (9 mA) D601 is fully on and D600 off so that the full signal level is applied to TR701 emitter. As the a.g.c. current decreases D601 turns off and D600 becomes progressively more conductive, attenuating the UHF input signal. The very linear attenuator so formed offers excellent performance in the face of high levels of unwanted signal, thus very good cross-modulation perform- ance.
VARICAP TUNING
Traditionally a variable capacitor is a mechanical device in which interleaving vanes are rotated by a shaft, their degree of mesh determining the total capacitance. The same effect can be achieved by the use of a varicap diode, the junction capacitance of which can be varied between typically 20 pF and 2 pF for applied reverse-bias voltages between 1 V and 28 V. The three varicap diodes in the tuner of Fig. 3.3, D605, D606 and D607, are carefully matched and selected in manufacture to have identical voltage/capacitance curves. The two bandpass tuned circuits L510/L511–12 in the r.f. stage are thus tuned exactly in step, and the oscillator frequency maintained exactly 39.5 MHz higher in frequency – a process known as tracking. Cor- rect tracking ensures optimum gain and performance throughout UHF bands IV and V. The channel selected, then, depends on the d.c. voltage applied to tuner pin 4.
Deriving the tuner control voltage
With received channel number depending purely on the d.c. voltage applied to the varicap tuner, a wide range of options is open to the TV set designer, including self-seeking systems, remote control and station memory, all based on modern IC technology. They will be examined later in this chapter and in Chapter 22. While the simplest types of monochrome receiver use a single rotary potentiometer as tuning control, in conjunction with a stabilised 30 V source, a slightly more sophisticated tuning system is used in inexpensive TV sets and videorecorders: Fig. 3.4 shows its basis.
A specially developed two-terminal chip, IC1, acts as a temperature- compensated voltage source for a series of potentiometers in a tun-ing bank. Each slider taps off a potential appropriate to one of the local TV transmissions, and is selected by a push-button, of which there may typically be eight, marked BBC 1, BBC 2, ITV, Channel 4 etc. The tuning voltage thus set passes into the varicap tuner, having had added to it an a.f.c. control voltage derived from the i.f. carrier. The effect of the a.f.c. voltage is to correct for slight tuning errors by ‘pulling’ the local oscillator up or down to achieve an exact vision i.f. frequency of 39.5 MHz, when tuning is spot on. Because the influence of a.f.c. control can mask the correct tuning point, provision is often made to switch it off when manual tuning is carried out: the most common artifice is a switch operated by the flap or door which conceals the tuning potentiometers.