Microelectronic active RC channel bandpass filters in the frequency range 60-108 kHz for FDM SSB telephone systems

Abstract

The paper describes the development of microelectronic active- <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RC</tex> 4-kHz-spaced channel bandpass filters (CBFls) for the frequency range 60-108 kHz, suitable for the formation and decomposition of the 12-channel bank (primary group) in direct-modulation-type FDM SSB telephone systems. The active filters are designed by simulation (essentially by Gorski-Popiel's method) of low-sensitivity <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</tex> filters which are the duals of some <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</tex> CBF's currently used in FDM systems in the U.K. Our circuit development is firmly based on the microelectronic technology adopted by us, in which naked-chip dual- or quad-operational amplifiers, laser-adjustable thick- or thin-film resistors, and NPO ceramic-chip capa- th p citors are used. In order to minimize cost, the circuits are designed to accept, as far as possible, capacitors with preferred (and preferably also equal) nominal values, and a resistor adjustment procedure has been developed which permits wide capacitor manufacturing tolerances of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\pm</tex> 10 percent. This adjustment procedure also provides compensation for the nonideal characteristics of the amplifiers, leading to near-perfect simulation of the nominal dissipationless <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</tex> prototype filters. Microelectronic models of channel 12 CBF (60-64 kHz) and channel 1 CBF (104-108 kHz) have been built (size of each filter: <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">50 mm \times 30mm \times 5mm</tex> ; dc power 400 mW). Channel 12 CBF meets the loss-frequency specification over the required temperature range 10-40°C. For channel 1 CBF the specification is met at ambient temperature (25°C) but not at 10°C or 4O°C; this will be remedied when amplifiers with higher <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f_{T}</tex> 's and/or lower <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">f_T</tex> -temperature coefficients become available. Neither filter requires a channel equalizer (whereas their <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</tex> counterparts do), and preliminary nonlinearity, noise, and intermodulation tests had promising results.