The present invention relates to a circuit board for mounting a band-pass filter and, more particularly, to the land configuration of the area of a circuit board where a band-pass filter is to be located.
A band-pass filter is extensively used with, for example, a mobile radio communication apparatus since it has a strict electric characteristic and is highly resistive to environments, small size, and light weight. Generally, a band-pass filter has a casing made of metal or similar conductive material and accommodating major circuitry therein, and an input terminal, an output terminal and a ground terminal extending out from the bottom of the casing. The input, output and ground terminals are respectively soldered to an input line, an output line and a ground surface provided on a circuit board. This configuration, however, has a drawback that unexpected coupling is apt to occur between the input and output terminals of the filter, preventing a desired frequency band characteristic or similar electric characteristic from being achieved. To eliminate such coupling, it is a common practice to locate a shield between the input and output terminals on the circuit board. It has been reported that assuming specifications prescribing an attenuation at an image frequency, i.e., a simple characteristic of greater than 56 dB, the attenuation achievable with the shield is 56.0 dB at an image frequency of f0(21.7 MHz)-910 kHz, which is higher than 40.0 dB available with a circuit board lacking the shield.
However, the unexpected coupling between the input and output terminals also occurs within the circuit board, and this cannot be coped with by the shield. Moreover, since the casing of the filter is connected to the ground surface of the circuit board only at the ground terminal thereof, an impedance exists between the casing and the ground surface. This impedance is likely to degrade the desired electric characteristic of the filter.