A CCD wave filter with low sensitivity for FDM telecommunication systems

Abstract

TRANSFORMING A doubly-terminated Chebyshev LC ladder filter, a CCD wave filter has been implemented by using CCD resonators and charge amplifiers as basic building blocks. The bandpass filter which was realized on an experimental test chip has a center frequency of 50kHz, together with a relative Chebyshev bandwidth of 3%, 5dB insertion loss and 60dB stopband attenuation. Compared to known SC or CCD bandpass filters the advantages of the approach are low sensitivity of the center frequency, which is controlled by an external clock frequency, and a relative bandwidth which does not depend on the center frequency, but is controlled by capacitance ratios. These specifications are needed to meet the requirements for filters in FDM telecommunication systems.A doubly-terminated LC Chebyshev lowpass filter which, as is well known, exhibits low sensitivity with respect to element tolerances was used as an intial structure for implementing the CCD wave filter. The lowpass filter was converted first to a bandpass filter and then t o a reference wave filter by replacing resonators by hj2 transmission lines.By interpreting the incident and reflected waves of the reference filter as signals of a flow graph it was possible to develop an equivalent structure. Finally, the transmission lines were replaced by CCD delay lines. The simulation of the wave graph instead of voltages and currents is advantageom because of low signal peaking. The circuit diagram of the CCD wave filter, which is well suited for an implemantation in NMOS technology, is shown in Figure 1. Figure 2 shows a photomicrograph of a test chip which implements the bandpass filter. The new structure contains two oneport resonators, the principle of which is described e l~e w h c r e "~'~ and one two-port resonator. The two-port resonator is needed t o handle the incident and the reflected waves in the filter. The center frequencies of the three resonators are identical. The SCCD resonators are realized with a standard two-level-polysilicon gate technology, as used for high volume fabrication, resulting in SCCDs with a transfer inefficiency of the order of 5 x lo-'.Four charge amplifiers are needed in the coupling sections between the resonators to realize the three conjugate complex poles of the Chebyshev transfer characteristic. The charge-sensing and -ESSClRC SO Digest of Technical amplifying circuit which contains a single-stage voltage amplifier is shown in Figure 3 . The voltage amplifier, which has a settling time of about 200ns, is needed to get good linearity. The standard deviation of the voltage gain amounts to 0.5%.The measured attenuation characteristic of the sixth-order Chebyshev bandpass filter is shown in Figure 4. The resonators show a precise resonance frequency controllcd by the clock frequency f,. Due to n = 20 delay elements in the loop of one resonator, resonance does not only occur at fc/n but also at multiples i fc/n. The unwanted passbands have t o be suppressed by preceding and/or following filters. For a clock frequency f, = lMHz the fund...