Selective call communication system

Abstract

The encoder in such system generates a sequence of tones in which every other tone is selected from a first group of tones in one band of frequencies, and the remaining tones in the sequence are selected from a second group of tones in another band of frequencies. The tones in the second group are respectively harmonically related to the tones in the first group, such as having frequencies double the respective frequencies of the tones in such second group. The tones are generated by an oscillator in the encoder, every other tone being multiplied by 1/2, for example. Circuitry, coupled to the oscillator and to the multiplier, alternately delivers the divided and undivided tones, to create the afore-mentioned sequence of tones. The oscillator has associated therewith a switchable impedance, predetermined amounts of which are sequentially switched into the oscillator to cause it to produce the sequence of tones. To minimize energy loss, circuitry is provided to insure that the phase of the tones is substantially continuous as the different amounts of impedance are switched into the oscillator. A programming device enables internal fixing of the frequencies of certain tones, which tones represent the identification of the encoder. A manual selector enables external selection of the frequencies of other tones which represent other information relative to the encoder.