Circuits and Components for System Evaluations and Design
4.2.4 FiltersWe use filters to selecto r reject certain signals. A very important task of filters is the reduction of input noise, because the noise bandwidth is rather closet o a 3dB bandwidth in most RF equipment. The main characteristics of filters are their frequency response (both attenuation and group delay) and impedance matching.
figure 4.3 This is how or why power divider phase characteristics affect antenna array performance. Any phase differences of the divider will be directly added to the phasing network values.
Often manufacturers specify the passband attenuation and stopband attenuation separately. Many filters are reflective in nature, which means that there is large mismatch for frequencies within the stopband.
The losses in the passband should normally be a lot below 1 dB, but at very high millimeter-wave frequencies this may turno ut to be difficult.
Some simple filters do not give much more tan 20 to 30 dB of attenuation in their stopband, but most are capable of 50 to 60 dB ore ven more.
Commercially available filters are found in all four main categories: lowpass, highpass, bandpass, and band reject. Normally, the frequency response is fixed. We can also purchase tunable components, which are base don pure mechanical adjustments. Sometimes the adjusting mecanism is driven by a stepper motor.
Faster and durable devices rely on electronics, which can be found as varactor diode designs and as yttrium iron garnet (YIG) blocks. Tuning typically compromisos other performance figures. Mechanically tuned filters show generally better attenuation responses but suffer from slow speed and wear in use.
Figure 4.4 suitably selected comercial lowpass and highpass filters in cascade can provide a semicustom passband for our system. This shows the response when a highpass filter with a cutoff at 530 Mhz is used in series with a low pass filter with a 630 MHz cutoff. Actually, both filters are sold as 600 MHz devices.
Libro: Circuits and Components for System Evaluations and Design
Autor: Pekka Eskelinen
Nombre: Josmar Eduardo Depablos Rodriguez
Asignatura: Circuitos de Alta Frecuencia
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