Applocation of SAW – Filters in the ethereal Television Systems in the city conditions
Abstract - The results of investigation and practical application of "low-loss" filters on surface acoustic waves (SAW) in ethereal collective television systems in city conditions are given. Use of the circuit of channel amplifiers in which frequency selectivity is carried out by the high-frequency SAW- filters included on an input and an output of the amplifier is analysed.
Now in Russia the rough growth of information technologies is observed. The important place in this process occupies the decision of a problem of multichannel television reception in cities, settlements and individual structures, there the qualitative reception and distribution in a collective network of ten - fifteen television programs probably at performance of the following conditions:
- Allocation of a spectrum of a signal of each received television channel;
- Equalization of a signal level before summation and supply in a distributive network.
The given work is devoted to the solution of a problem of a filtration and equalization of television channels (TVC) by means of the high-frequency channel SAW-filters being a the basic part of channel amplifiers (CA).
II. Main part
In this work the selectivity of CA is achieved by high-frequency low-loss SAW-filters, which are established on an input and an output of each amplifier. The block-diagram of construction is shown on fig.1
The requirements to the electrical characteristics of such SAW-filters following :
- The insertion loss is no more than 1… 6 dB;
- The nominal frequencies are from 48 MHz (1-TVC) up to 860 MHz (69-TVC);
- The width of passbands is 8 MHz (the relative width of passbands - from 1 % up to 15 % for various TVC);
- The selectivity on the nearest non-adjacent channel - not less than 40 dB;
- The input and output resistance - 75 Ohm;
- The passband variation is no more than 1,5 dB.
The major problem by development of these filters was the finding of the compromise  between such parameters as insertion attenuation, selectivity and width of a passband. The relative width of a passband should vary over a wide range for various TVC thus selectivity on next TVC must be maximal.
For realization of required parameters three types of designs are investigated: longitudinally - coupled resonator filters (LCRF), SAW-filters exploiting U-shaped multistrip couplers and SAW resonator filters exploiting reversive multistrip couplers. The choice is made for the benefit of structure of the first type. The design of the given type of SAW-filter is shown on fig. 2 and experimental characteristics - on fig. 3 and 4 (8-th and 29-th TVC accordingly).
In work the way of a filtration and equalization of signals of television broadcasting channels is investigated by means of high-frequency channel SAW-filters.