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

The paper describes the development of microelectronic active-
RC
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
LC
filters which are the duals of some
LC
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
\pm
10 percent. This adjustment procedure also provides compensation for the nonideal characteristics of the amplifiers, leading to near-perfect simulation of the nominal dissipationless
LC
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:
50 mm \times 30mm \times 5mm
; 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
f_{T}
's and/or lower
f_T
-temperature coefficients become available. Neither filter requires a channel equalizer (whereas their
LC
counterparts do), and preliminary nonlinearity, noise, and intermodulation tests had promising results.