Tuning detection circuit for a high-frequency receiver, and receiver including such detection circuit

An in-lock tuning detector for use in an FM radio receiver for detecting when a selected tuning frequency window of the receiver includes a tuning frequency of a received frequency modulated radio frequency (RF) signal;

• the receiver having a selectable tuning frequency and comprising FM detecting means having an S-curve frequency detection characteristic for deriving a demodulated signal from the received RF signal, means responsive to the demodulated signal to set the tuning frequency of the receiver in accordance with the tuning frequency of the received RF signal, and a level detector for detecting a signal level of the received RF signal;
• said tuning detector comprising:
• a window comparator coupled to said FM detecting means to derive from said demodulated signal first and second logic signals indicative of whether the tuning frequency of the received RF signal is below or above said selected tuning frequency window;
• a signal level comparator coupled to said level detector for producing a third logic signal indicative of whether a signal level of the received RF signal exceeds a predetermined threshold;
• and
• a first combinational circuit coupled to said window comparator and to said signal level comparator for combining said first, second and third logic signals to derive a fourth logic signal which is indicative of whether the selectable tuning frequency of the received RF signal is within said selected tuning frequency window, said fourth logic signal being subject to error for tuning frequencies of the received RF signal for which said FM detecting means operates on an incorrect slope portion of said S-curve;
• characterized in said tuning detector further comprises:
• phase detecting means coupled to said window comparator to detect a phase difference between said first and second logic signals, and based thereon to produce a fifth logic signal indicative of whether said demodulated signal was produced by operation of said FM detecting means on a correct portion of said S-curve;
• and
• a second combinational circuit coupled to said phase detecting means and to said first combinational circuit for combining said fourth and fifth logic signals to derive therefrom a lock-in detection signal indicative of whether the received RF signal is within said selected frequency window, and which is based on operation of said FM detecting means on said correct slope portion of said S-curve.