FORSTERKAR- OCH VAGFORMARKRETS

摘要

A circuit arrangement for digitally indicating the frequency to which an AM/FM radio receiver is tuned in which the oscillations of the AM and FM oscillators are amplified and converted to rectangular oscillations whose frequency is counted by means of electronic counters and then digitally displayed. The oscillations of the AM oscillator and the oscillations of the FM oscillator are amplified and converted to rectangular signals by means of a common broadband amplifier which has a sufficiently broad bandwidth so that the oscillator frequencies for the AM range as well as those for the FM range are both passed and amplified, and the coupling of the input of the broadband amplifier to the outputs of the tuning oscillators is so loose that the tuning oscillators are not noticeably detuned by changes in capacitance of the broadband amplifier. The common broadband amplifier includes two inverters which are each designed as an integrated circuit, and have a fast rise-time. The two inverters are connected in series with one another via a capacitance and each inverter is biased at its input with a biasing voltage so that its operating point lies on the steep portion of the curve indicating the relationship between the input voltage and the output voltage of the inverter.