Automatic threshold squelch

• US 4,411,021 A
• Filed: 07/27/1981
• Issued: 10/18/1983
• Est. Priority Date: 07/27/1981
• Status: Expired due to Term

• Current Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: Yoakum, John H.
• Primary Examiner(s): Ng, Jin F.
• Application Number:
  US06/287,335
• Time in Patent Office: 813 Days
• Field of Search: 455/212, 455/218, 455/219-225, 455/303, 455/312, 455/220, 375/114, 328/163
• US Class Current: 455/222
• CPC Class Codes: H03G 11/04 Limiting level dependent on strength of signal; Limiting level dependent on strength of carrier on which signal is modulated H03G11/008 takes precedence
• CPC Class Codes: H03G 3/344 Muting responsive to the amount of noise (noise squelch) H03G3/345 takes precedence

Abstract

An automatic threshold squelch circuit, responsive to noise signals from the receiver demodulator, having automatic means for setting the squelch level. A noise detector is coupled to the demodulator and to a comparator and sample and hold circuit such that a predetermined portion of the detected noise signal level is stored in the sample and hold circuit when no signal is being received. The stored noise level is then compared to the detected noise signal from the noise detector and a mute signal is generated in response to the detected noise level exceeding the stored signal level.

Claims

1. An automatic threshold squelch circuit, for radio receiver having a demodulator responsive to noise signals from the receiver demodulator for generating a mute signal when no transmission is being received by the receiver, said squelch circuit comprising:

noise detector means for producing a unidirectional signal level as a function of the noise signal from the demodulator;

memory means, coupled to the noise detector means, for sampling and storing the unidirectional signal level when no transmissions are being received by the receiver;

comparator means, coupled to the detector means and the memory means, for comparing the unidirectional signal level from the noise detector means to the stored signal level, and generating an unmute signal when the uni-directional signal level is less than the store signal level.

2. The automatic threshold squelch circuit of claim 1, further comprising attenuation means, coupled to the noise detector means and the memory means, for attenuating signals coupled therethrough from the noise detector means by a predetermined amount.

3. The automatic threshold squelch circuit of claim 1 or 2 further comprising timing means, coupled to the receiver, for disabling reception of transmissions for a predetermined period of time following receiver turn on, thereby permitting the memory means to store an initial unidirectional signal level.

4. The automatic threshold squelch circuit of claim 3, wherein the memory means comprises an analog sample and hold circuit.

5. The automatic threshold squelch circuit of claim 3, wherein the memory means comprises a digital sample and hold circuit.

6. The automatic threshold squelch circuit of claim 1, further comprising filter means, coupled to the demodulator and the noise detector means, for filtering the noise signals and coupling the filtered noise signals to the noise detector means.

7. Communications receiving apparatus which operates in response to incoming signals, comprising:

radio receiver apparatus including RF, mixer, IF, demodulator, and audio sections;

noise detector means for producing a unidirectional signal level in response to noise signals from the receiver demodulator;

memory means having an input coupled to the noise detector means, for storing the unidirectional signal of the highest level applied to the memory means input;

comparator means, coupled to the detector means and the memory means, for comparing the unidirectional signal level from the noise detector means to the stored signal level and for generating an unmute signal when the uni-directional signal level is less than the stored signal level.

8. The communications receiving apparatus of claim 7, further comprising timing means coupled to the mixer section for disabling the mixer for a predetermined period of time following receiver turn on.

9. The communications receiving apparatus of claim 7 or 8 further comprising attenuation means, coupled to the noise detector means and the memory means, for attenuating signals coupled therethrough from the noise detector means by a predetermined amount.

10. The communications receiving apparatus of claim 7 or 8, further comprising a squelch gate, for coupling the demodulated audio signals from the demodulator section to the audio section, in response to the unmute signal and for inhibiting said demodulated audio signals in the absence of the unmute signal.

11. In communications receiving apparatus having a demodulator, a method of providing automatic squelch, comprising the steps of:

(a) producing a unidirectional signal level as a function of a noise signal level from the receiver demodulator;

(b) storing the unidirectional signal level in response to a predetermined condition producing a stored signal level;

(c) comparing the unidirectional signal level to the stored signal level and generating a squelch control signal when the unidirectional signal level is less than the stored signal level.

12. The method of claim 11 wherein the storing step (b) comprises storing the unidirectional signal level when there is no transmission being received by the receiver.

13. The method of claim 12 further comprising the step of attenuating the unidirectional signal level by a predetermined amount.