The invention, in a demodulator circuit such as a pager, relates to a control circuit of threshold levels for a multi-level comparator in the demodulator circuit for detecting a multi-level signal with the use of the comparator.
In a demodulator circuit such as a pager, a multi-level comparator is used for receiving and demodulating a multi-level signal and it is practiced therein to compare the signal transmitted through the process of detection and predetermined threshold levels to obtain the desired signal.
FIG. 1 and FIG. 2 are circuit diagrams explanatory of related art examples. In the circuit shown in FIG. 1, a current output of a detector is input to two comparators 1 and 2 through a low-pass filter (LPF) to be compared therein with their respective different threshold levels and, thereby, output data corresponding to the multivalued data is obtained. Further, in the above described circuit, it is adapted such that the average voltage of the detected output is detected in an average voltage detector circuit 4 and the same is fed back to a comparator 5 as its threshold level and, thereby, the operating point of the detected output is shifted.
In the circuit shown in FIG. 2, it is adapted such that a detector output is input to two comparators 11 and 12 through a low-pass filter (LPF) to be compared with their respective different threshold levels and, thereby, output data corresponding to the multivalued data is obtained. In this circuit, the average voltage of the detected output is detected in an average voltage detector circuit 13 and, with the use of it, the threshold levels for the comparators 11 and 12 are shifted.
However, there have been problems with such circuits that the signal level is shifted by a deviation in the intermediate frequency and thereby lowering of the sensitivity is incurred, or that the threshold levels for the multi-level comparator become unable to follow changes in the level of the detected output and therefore lowering of the sensitivity is incurred when there arise fluctuations in the devices, changes in the temperature, or changes in the input level.
Further, when identical data or biased data are continuously included in the received data, the average value of the level of the detected output changes and an offset is produced between the threshold levels for the comparators and the operating point of the level of the detected output, and hence problems of lowering in the sensitivity or inability of signal reception arise. Still further, in the case of intermittent signal reception, to decrease the time constant in obtaining the average voltage at a rise of the received signal (quick charging), then, by the setting to shorten the rise period, an error voltage is left according to the timing of the quick charging. A problem then arises that the duty cycle of the output data changes at each instant intermittent signal reception is made.