Abstract: A diplexer comprises a first frequency selective filter coupled between its first input terminal and a circuit point, a second frequency selective filter coupled between the circuit point and its output terminal, and third frequency selective filter coupled between its second input terminal and the circuit point. The first filter selects frequencies higher than a first frequency and the second filter selects frequencies lower than a second frequency which is higher than the first frequency. The third filter selects frequencies lower than a third frequency which is lower than the first frequency. In a television receiver, the first and second filters serve as a bandpass filter for the UHF band, and the third filter passes the VHF and CATV bands.

Abstract: A tracking filter suitable for use in the IF stage of FM receiver circuitry incorporates a bandpass filter the components of which yield a characteristic that permits the filter to provide substantially the entirety of a required 90.degree. phase shift for phase detection control circuitry that rapidly actively tunes capacitive components of the filter without imparting unacceptable group delay behavior to the configuration. The filter is a two-pole pair, single zero at the orgin, bandpass filter inserted in the signal path between a pair of signal power splitters. Part of the input signal is applied from the first splitter to one input of a phase comparator the other input of which is derived from the output signal via the second power splitter. The output of the phase comparator is applied through a broadband control amplifier to actively adjust the center frequency of the bandpass filter.

Abstract: A frequency translating circuit such as an intermediate frequency circuit comprises a wide band transformer having a two-terminal secondary and first and second frequency passing filters coupled respectively to one and the other of the secondary terminals. Each of the filters is arranged to pass a different subband of frequencies within the wide band of frequencies passed by the transformer and to act as substantially a short circuit to the other frequency subband.