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Televes
Televes
Televes
Televes

0451-2013

Filtering systems: advantages and disadvantages

Description: 

Let us have a look to the advantages/disadvantages of the different technologies used to implement filters intended to reject interferent LTE signals.


Among the most used technologies we can mention:
LC: Using discrete components (L, C) it can be achieved significant rejection to the interferent band, although at the expense of rather high insertion losses for DTT channels in the UHF band. Although these filters are, due to its price, one of the first choices, their higher insertion loss in the range of DTT signals would require a readjustment of the gain of the distribution to compensate these losses.
Ceramic resonators. The ceramic resonator filters can solve, in part, the problems of insertion loss of the LC filter. However, the choice of quality resonators is crucial to avoid filter drifting with temperature, which can vary considerably both rejection and insertion loss in adjacent DTT channels (59, 60) with the LTE frequency band.
SAW filters. The Surface Wave Filter (SAW) reaches very high rejections in small frequency ranges, but at the expense of very high insertion losses, which causes that it must be combined with additional electronics (amplification), which results in complexity in their implementation and therefore increasing manufacturing costs.

 

 

Cavity filters. Filtros de cavidad consisten en elementos sintonizables que forman las líneas de transmisión acopladas alojados dentro de cavidades resonantes de metal. These filters achieve a very significant rejection characteristics (typically 25-30 dB or even higher), maintaining a minimum insertion loss (<1 dB in the DTT frequency band, and 2 dB typical loss for channels 59, 60), thanks the high Q (quality factor) of the tunable elements.
Typically, to achieve these characteristics for insertion loss and rejection, these filters are bulky and heavy, so that although in certain documents are cited as a possible solution (LTE Study Summary Filter, DVB Technical Module), had not been considered their use in MATV solutions, but exclusively in professional solutions or very big facility headends.
This is so because each tunable element is housed in a separate cavity, and it takes a number of cavities to achieve both attenuation and rejection values, required; with the constraint that the guard band between DTT and LTE is only 1 MHz.

 

What Televes has achieved with its cavity filter (Spanish patent and future European patent extension), is to implement this technology in a small volume (10 cm x 5 cm) and a weight less than 250 grams, which enables it to be used in both indoors and outdoors. Estos elementos resonantes están separados por una distancia igual a la longitud de onda de trabajo dividida por 4 y han sido diseñados aunando el alcance de las características de rechazo y pérdidas de inserción con su implementación industrial en grandes volúmenes.
To achieve all this, Televes uses what is called microcavities; this is, small cavities housing the resonant elements of high quality factor.
Microcavities filter also presents an excellent performance against vibration and temperature, which constitute the core of the component more suitable for rejecting LTE interfering signals, maintaining the DTT coverage. They also may be used either preventively or as a mitigating element

Televes
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