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2.1.3
RX, TX Bandpass Filters The RX and TX bandpass filters provide two functions. The first is to effectively pass the correct frequencies to the RX and TX sections of the RF module. It is important especially for the RX section that these filters have a low insertion loss in order to ensure a low front end noise figure. These filters are also designed to provide >25dB rejection for the opposite band. This means that the transmit carrier will be attenuated by at least 25dB before entering the receive section of the phone. A plot of the low band filter is shown below.
902-905MHz Passband
0dB
10dB/div
815
925-928MHz Reject Band
101
Figure 2 Low band Ceramic filer response. ������������������������������������� For this filter the insertion loss is less than 3dB at 902 to 905MHz while the 925-928MHZ band has >25dB attenuation. This filter is used for the RX filter in the handset or the TX filter in the base. The high band filter characteristic is shown below. This filter is used for the handset TX filter and base RX filter.
925-928MHz Passband
0dB
10dB/div
815
902-905MHz Reject Band
101
Figure 3. Hight band ceramic filter Response. ����������������������������������������
