1. Field of the Invention
The present invention relates to a distributed element filter used in the RF (radio frequency) stage, etc. of mobile communication equipment as a bandpass filter or the like to suppress noise and interfering signals, and more particularly to a distributed element filter which has a flat amplitude characteristic and a flat group delay characteristic in the passband, and transmission zeros in the stopbands, and is simplified in configuration in order to minimize losses for the purpose of improvement in performance.
2. Description of the Related Art
In high frequency circuit sections such as the RF stage of transmitter and receiver circuits for mobile communication equipment represented by analog or digital portable telephones or wireless telephones are often used bandpass filters (BPFs), for example, to attenuate harmonics which are caused by the nonlinearity of amplifier circuits, or to remove undesired signal waves such as interfering waves, sidebands, etc. from the desired signal waves, or when using the same antenna for both the transmitter and receiver circuits, to separate out the transmitter frequency band and the receiver frequency band.
Such bandpass filters for use in communication apparatuses are generally realized and constructed as filter circuits with desired bandpass characteristics by connecting series or parallel resonant circuits constructed with various circuit elements in a plurality of stages. Since the filter circuit blocks can be made smaller in size and have good electrical characteristics as high frequency circuits, in many cases circuit blocks are constructed using unbalanced distributed element transmission lines such as microstrip transmission lines or strip transmission lines. Generally, in a bandpass filter, as shown in FIGS. 24A, 24B, is required a complex circuit design to realize both a flat amplitude characteristic and a flat group delay characteristic, and at the same time, provide transmission zeros in the stopbands.
Procedures for directly synthesizing a bandpass filter having such characteristics based on a clear design theory have not been known as yet, and it has been practiced to construct filters empirically by using various known procedures. For example, as shown in a block diagram of FIG. 25, focusing first only on amplitude characteristics, such a filter 1 is designed from a filter of a known configuration, as has desired amplitude characteristics, namely, a flat amplitude characteristic throughout the passband and transmission zeros in the stopbands, but does not take the group delay characteristic into account yet. Next, in order that the filter 1 has a desired group delay characteristic as a whole, the filter 1 is provided with a phase equalizer 2 with all-pass characteristics, which has an effect of flattening the group delay characteristic in the passband. According to this procedure, the phase or group delay characteristic is improved by adding the phase equalizer 2 to the filter 1.
Such approach, however, has a disadvantage that the phase equalization or correction as shown in FIG. 25 has a limited effect and can not provide a sufficient equalization effect. Additionally, since the circuit design is wasteful requiring more circuit elements than would otherwise be required, the approach involves more difficulties than it solves, such as an adverse effect on the amplitude characteristic produced by the imperfect all-pass characteristics of the phase equalizer 2 and the increased loss produced by the increased complexity of the circuit.
Two procedures are well known in the art to realize transmission zeros in a filter's stopband. One is to realize transmission zeros by inserting a parallel resonator or series resonator in parallel or series in the filter or by combining these resonators. For example, as shown in the circuit diagram of FIG. 26, transmission zeros are formed on both sides of the passband by adding a combination 5 of a parallel resonator 5a and a series resonator 5b to a bandpass filter realized by resonators 3, 4.
The other procedure is to realize transmission zeros by splitting the transmission line into two paths which have the same output amplitude and differs from each other by 180.degree. in phase, and combining the two paths together. For example, as shown in the block diagram of FIG. 27, the circuit is split into two paths which are led to a two-port 6 and a two-port 7, respectively, which provide at a certain frequency the same amplitude output and differ from each other by 180.degree. in phase, and their outputs are combined to obtain an output which provides a transmission zero at that frequency.
Generally, the procedure of FIG. 27 can realize a filter with a circuit configuration easier to implement and smaller in loss than the procedure of FIG. 26 can.
Further, as a modification of FIG. 27, a procedure is known which uses a simple reactance feedback path. For this procedure, an accurate design theory or method for synthesizing the filter from the target circuit network function is not known, and an approximation or an empirical method is used. For example, as shown in the circuit diagram of FIG. 28, transmission zeros are formed by combining a filter block 8 as a conventional filter with a coupling element 9 corresponding to a branch circuit or feedback path. Because of circuit simplicity, this procedure has the effect of reducing the loss, but since no accurate design procedures are known for synthesizing the filter, the design relies on an approximation, which, therefore, has the problem that only approximate characteristics can be obtained and the obtained characteristics are not sufficient.
Another procedure known in the art is to combine a circuit of ladder structure with one of the above-described transmission zero forming procedures, and to thereafter adjust the group delay using a phase equalizer. According to this procedure, it is claimed that a filter with conventional bandpass characteristics can be obtained which has both a flat amplitude characteristic and a flat group delay characteristic throughout the passband and also has transmission zeros in the stopbands.
However, this procedure also has the problem that accurate characteristics cannot be obtained because the design relies on an approximation; furthermore, the circuit configuration becomes complex. Moreover, such filters have the problem that the transmission loss increases or only approximate and insufficient characteristics can be obtained. The problem of transmission loss is particularly pronounced when the filter is constructed of a distributed element filter such as a microstrip line circuit.