Polyphase filters receive an N-phase or polyphase input signal and create an N-phase output signal. Quadrature filters, a particular type of polyphase filters, are well-known four-phase polyphase filters. Generally speaking, the inputs of a quadrature filter consist of four signal voltages of mutually equal value. The four signal voltages jointly constitutes a signal vector group with individual signal vectors succeeding one another in a given direction of rotation through phase angles of 90 degrees. The frequency of the polyphase signal may be positive or negative depending on a counter-clockwise or clockwise rotation. Typically, the 0 degree and 180 degree signal vectors are designated the +I and −I signals, and the 90 degree and 270 degree signal vectors are designated the +jQ and −jQ signals.
Polyphase filters are widely used in high frequency clock data paths to generate quadrature signals and enhance image rejection of a clock signal. Initially, many polyphase filters were designed using a combination of LC filters. Today, with the prevalence of integrated circuits and difficulties of incorporating inductors into integrated circuits, a combination of RC filters are commonly used in polyphase filters. Most polyphase filters in use are passive circuits. A characteristic of passive circuits is that it will induce an energy loss that in general is proportional to the number of stages. Consequently, extra buffers are often used at the outputs of the passive polyphase filters which add cost and complexity to a circuit design.
FIG. 1 is an example of a prior art single stage polyphase filter 10. The single stage polyphase filter 10 is composed of a plurality of passive capacitors and resistors. Input Iin− 102 is coupled to a positive node of C4 104 and a first side of R3 106. Output Iout− 108 is coupled to the other side of R3 106 and a negative node of C3 110. Input Qin+ 112 is coupled to a positive node of C3 110 and a first side of R2 114. Output Qout+ 116 is coupled to the other side of R2 114 and a negative node of C2 118. Input Iin+ 120 is coupled to a positive node of C2 118 and a first side R1 122. Output Iout+ 124 is coupled to the other side of R1 122 and a negative node of C1 126. Output Qout− 128 is coupled to a first side of R4 130 and the negative node of C4 104. Input Qin− 132 is coupled to a positive node of C1 126 and the other side of R4 130. The single stage polyphase filter 10 includes inputs Iin− 102, Iin+ 120, Qin− 132, and Qin+ 112. Outputs of the s polyphase filter 10 include Iout− 108, Iout+ 124, Qout− 128, and Qout+ 116. The inputs and four outputs and is commonly known as a quadrature filter. Since the single stage polyphase filter 10 includes only a plurality of passive components, the filter undesirably attenuates the signals at the outputs. Moreover, as more stages are added, the energy loss is proportional to the number of stages added to the filter.
Efforts to overcome the undesirable passive characteristic of polyphase filters have included the use operational amplifiers to provide active RC filters. Incorporating operational amplifiers have afforded polyphase filters with some gain at the filter output. Moreover, feedback loops between the inputs and outputs of the active RC filter reduce sensitivity to component variations in integrated circuits. However, a major drawback of operational amplifier based polyphase filters is the limited bandwidth which limit operation to low frequencies.
Particularly with the advent of high frequency signals above the range of 1 gigahertz, operational amplifier based polyphase filters do not have the frequency response. Concededly, circuit designers have been limited to using passive polyphase filter designs in high frequency applications. Even though the passive polyphase filter designs heavily attenuate signals and require extra buffers with large gain, circuit designers have had little choice but to add the extra components to compensate for the signal loss of the passive polyphase filter designs.
Accordingly, what is needed is a method and apparatus that combines the advantageous characteristics of passive polyphase filters and active RC filters. The improved polyphase filter should include features such as ease of manufacturing using existing integrated circuit manufacturing processes, being able to operate in the gigahertz range, and be an active circuit with gain to reduce buffering requirements.