Appendix 5

Characteristics of filters

The following are parameters which are often quoted in descriptions of filters.

Bandwidth is the width of the band of frequencies that the filter has been designed to pass or reject.

Insertion Loss is the loss of wanted signal measured in dB caused by fitting the filter and should obviously be as low as possible.

Characteristic Impedance or Impedance is the source and terminating impedance br which the filter is designed. For all TV aerial filters this is 75 ohms.

Attenuation or Suppression measured in dB is the ratio of wanted to unwanted signals after filtering.

1. . High-pass filter

   The basic circuit for this type of filter is shown below. UHF signals pass through Cl because its reactance is low, whereas the reactance of Ll and L2 is high enough to be ignored. At frequencies below the turnover frequency, the reactance of Cl becomes higher preventing the passage of these frequencies, whilst LI and L2 act as low impedance shunts to earth.

   
2. . Low-pass filter

   The basic circuit is similar to the High-Pass Filter but C's and L's are transposed giving the complementary response below.

   
3. . "Bandpass" filters

   These are basically parallel tuned circuits tuned to the wanted frequency and operate in the same way as an IF transformer.

   Transfer of signals may be:

   a. by magnetic coupling of the coils. either directly or via a link.

   
4. . "Notch" or "Band-stop" filters

   These are basically combinations of the circuits below. In Fig. 1. Li and C 1 form a series resonant circuit of low impedance to the unwanted frequency acting as a shunt across the line. whereas in Fig. 2. L2 and C2 form a parallel tuned circuit of high impedance in series with the signal path.