1. Field of the Invention
The invention generally relates to arrangements for suppressing or rejecting “images” (energy at harmonic frequencies) in communications systems. More particularly, the invention relates to arrangements that use a current switching technique to combine a digital-to-analog converter (DAC) function with a filtering function to significantly reduce harmonic images.
2. Related Art
FIG. 1 shows a conventional transmission arrangement, in which digital data are input to a digital-to-analog converter (DAC) 100. DAC 100 outputs an analog signal that changes value at discrete times determined by a clock signal CK as it clocks D-type flip-flops (not specifically shown) at the DAC's input. Being the result of discrete time clocking, the DAC's analog output signal has sharp vertical edges in the time domain.
Undesirably, the presence of sharp edges in the DAC's analog output signal implies there is significant energy at harmonic frequencies (see FIG. 5A; which assumes a CK frequency of 1 GHz). To mask interfering emissions such as those at harmonic frequencies (as effectively mandated, for example, by the U.S. Federal Communications Commission, FCC), a low pass reconstruction filter 102 has conventionally been employed. Such a filter lowers the higher-frequency content of signals before sending them to a modulator 104 for generating a radio frequency (RF) modulated signal.
The reconstruction filter is usually an analog filter that consumes a substantial amount of power, has large process/temperature/voltage variations (approximate ±30% for integrated passive R/C components), and is very difficult to design when the DAC input/output signal frequency is high. Furthermore, the closeness of the harmonic frequencies to the desire pass band, and the prevalence of higher energy content at lower harmonics (see FIG. 5A) have required filter 102 be complex.
Conventionally, higher order analog filters (such as Butterworth filters) have been used to implement filter 102. Undesirably, such complex filters are extremely difficult to design when the corner filter frequency is high. Also, operational amplifiers required in analog filters consume large amounts of power. Moreover, the unity gain bandwidth is often impossible to achieve with a high filter corner frequency. The accuracy of the corner frequency adjustment may sometimes be achieved by trimming or by calibration, but either of these approaches significantly increases design complexity.
Accordingly, there is a need in the art for an arrangement that simply and efficiently converts digital signals into analog signals that are lower in harmonic content, thereby avoiding or minimizing the need for the costly filters found in conventional systems.