Abstract: A voltage controlled low pass filter comprising first and second circuits connected in series between a filter input and a filter output and a third circuit for adding a DC bias voltage to the filter input with AC signals to be filtered. The first circuit has a gain inversely related to the DC bias voltage and is operative to convert any signal applied to the filter input into a first circuit output signal which is a logarithmic function of the applied signal. The second circuit has a gain directly related to the DC bias voltage such that the overall gain of the first and second circuits is unity. The second circuit also has a bandwidth which is inversely related to the gain of the second circuit, and is operative to convert the first circuit output signal into a signal at the filter output which is an exponential function of the first circuit output signal.

Abstract: A voltage controlled low pass filter comprising first and second circuits connected in series between a filter input and a filter output and a third circuit for adding a DC bias voltage to the filter input with AC signals to be filtered. The first circuit has a gain inversely related to the DC bias voltage and is operative to convert any signal applied to the filter input into a first circuit output signal which is a logarithmic function of the applied signal. The second circuit has a gain directly related to the DC bias voltage such that the overall gain of the first and second circuits is unity. The second circuit also has a bandwidth which is inversely related to the gain of the second circuit, and is operative to convert the first circuit output signal into a signal at the filter output which is an exponential function of the first circuit output signal.

Abstract: A filter circuit comprising a phase advance circuit acting as a high pass filter, a phase retard circuit acting as a low pass filter, and an amplifier connected in series between an input and an output. A positive feedback loop is provided between the input and the output. The electrical characteristics of the phase advance and phase retard circuits are such that interaction between the phase advance and phase retard circuits is substantially prevented. This may be achieved by providing the phase advance and phase retard circuits with simple output buffers or providing internal closed loop negative feedback paths to provide low output impedances. The phase advance and phase retard circuits may comprise multipliers configured to compensate for variations in the values of resistive and capacitive components of the filter circuits.

Abstract: A machine for circuit design comprising a processor, a configurable logic device and a design tablet device; the processor having a graphics capability and including a display, a design compiler which compiles a graphics representation of the circuit on said display to a set of coded instructions for the setting of the logic device and the design tablet device; wherein the logic device comprises a set of configurable circuit elements in integrated circuit form for providing a hardware representation of some or all of the digital part of the circuit displayed on said display, said coded instructions being operative to control the logic device to appropriately program said circuit elements; and wherein said design tablet device includes a plug board for plugging in discrete electrical components and devices, bus lines coupled to said processor, and wherein said processor is operative to provide appropriate energizing signals to said bus lines to actuate the discrete electrical components and devices.