AM Stereo receiver

• US 4,370,520 A
• Filed: 05/26/1981
• Issued: 01/25/1983
• Est. Priority Date: 05/26/1981
• Status: Expired due to Fees

• Current Assignee: RCA Corporation
• Original Assignee: RCA Corporation
• Inventors: Malchow, Max E.
• Primary Examiner(s): Hickey, R. J.
• Application Number:
  US06/266,984
• Time in Patent Office: 609 Days
• Field of Search: 179/1 GS, 455/234, 455/239, 455/240, 455/241, 455/244, 455/246, 455/247
• US Class Current: 381/15
• CPC Class Codes: G01R 15/09 Autoranging circuits
• CPC Class Codes: G01R 23/15 Indicating that frequency of pulses is either above or below a predetermined value or within or outside a predetermined range of values, by making use of non-linear or digital elements (indicating that pulse width is above or below a certain limit)
• CPC Class Codes: H04H 20/49 for AM stereophonic broadcast systems

Abstract

An AM stereo receiver includes an AGC system with first and second AGC system characteristics. The first AGC system characteristic provides a relatively constant receiver output signal for increasing received signal strength, i.e., a hard AGC characteristic. The second AGC characteristic provides a somewhat increasing receiver output signal for increasing received signal strength, i.e., a soft AGC system characteristic. A tuning detector is used to detect when the receiver is being tuned. When the receiver is being tuned, the AGC system is conditioned to provide a relatively soft AGC characteristic, which facilitates accurate receiver tuning. When the receiver is not being tuned, the AGC system is conditioned to provide a relatively hard AGC characteristic, which facilitates accurate left and right stereo channel separation.

Claims

1. In an AM stereo system including tuning means for tuning said system to various RF signals corresponding to respective stations to produce an IF signal, at least one gain controlling stage responsive to an AGC signal, and AGC means for generating said AGC signal in response to the strength of said received RF signals, apparatus comprising:

tuning detector means for providing a first signal when said receiver is not being tuned and a second signal when said receiver is being tuned; and

gain control means operating in conjunction with said AGC means and responsive to said first signal for establishing a first IF amplitude versus received RF signal strength characteristic and responsive to said second signal for establishing a second IF amplitude versus received RF signal strength characteristic, which second characteristic has a greater slope than said first characteristic.

2. An AM stereo receiver according to claim 1, wherein said receiver includes a phase modulation detector, and said tuning detector means comprises:

window comparator means responsive to the output of said phase modulation detector for providing a first comparator signal when said output of said phase modulation detector is between first and second predetermined voltages, and for providing a second comparator signal when said output of said phase modulation detector is below said first predetermined voltage or above said second predetermined voltage.

3. An AM stereo receiver according to claim 2 further comprising:

pulse forming means responsive to said first and second comparator signals for providing said respective first and second signals to said AGC means, said pulse forming means further providing a predetermined time delay so that said second signal thereof is maintained for said predetermined time delay after the output of said window comparator means changes from said second comparator signal to said first comparator signal.

4. An AM stereo receiver according to claim 1 or 2 or 3, wherein said receiver includes an automatic frequency control system for automatically tuning said tuning means to the frequency of said received signal, said receiver further comprising:

AFC control means responsive to said tuning detector means for disabling said automatic frequency control system when said tuning detector means provides said second signal.

5. An AM stereo receiver according to claim 1, wherein said AGC means includes a circuit node at which said AGC signal is provided, and said gain controlling stage includes a gain control transistor responsive to said AGC signal for controlling the gain thereof, said gain control means further comprising:

means responsive to said second signal for incrementally changing said AGC signal at said circuit node by a predetermined amount for changing the bias point of said gain control transistor so as to increase the gain of said gain control stage.