RECEIVER THRESHOLD EXTENSION

Abstract: A receiver (10) is provided where an information signal (11) is received (12) and examined to determine (20) its signal strength. When the signal strength is at least equal to a threshold, an unmodulated signal (40) is added to the received signal to improve the sensitivity of the receiver.

Description: RECEIVER THRESHOLD EXTENSION Technics F iς. This invention relates generally to radio frequency receivers and more particularly to receivers with sensitivity improvement. Ba ground Art One measure of a receiver's performance is its ability to reject noise while providing a clear desired signal. Typically, this performance is rated by a signal-to-noise ratio. The higher the signal-to-noise ratio, the better the sensitivity (and performance) of the receiver. Others skilled in the art have used complicated detector and compensation arrangements that attempt to improve the receiver's sensitivity. However, existing receiver sensitivity improvement techniques (such as "click swallowing") do not function well when the desired information signal has a received amplitude that is near the noise level. This noise barrier resides approximately below a 3dB signal-to-noise ratio, which is often called the noise range. Therefore, a need exists for a simple method to improve the receiver's sensitivity below the noise range. Summary of the Invention Accordingly, it is an object of the present invention to improve sensitivity to a receiver when the information signal is near the noise level. Briefly, according to the invention an information signal is received and examined to determine its signal strength. When the signal strength is at least equal to a threshold, an unmodulated signal is added to the received signal to improve 0 the sensitivity of the receiver. Brief Description of the Drawings Figure 1 is a block diagram of a receiver in accordance with the present invention. 5 Detailed Description of the Preferred Embodiment Referring to Figure 1, a receiver 10 is shown having an information signal 11 being received by an antenna 12. The received information signal (and received noise) is filtered 0 (bandlimited) via a preselector 14 before being mixed (mixer 16) with an injection signal from an oscillator 18. The oscillator 18 is programmed (34) to provide a suitable injection signal by a controller 32 so that a desired information signal can be received. The mixer 16 provides an intermediate frequency (IF) signal to an 5 IF amplifier 20 to be amplified prior to being filtered by an IF filter 22. The filtered signal is processed by a detector 24 to provide a recovered signal. It will be appreciated that an additional intermediate frequency stage can be added before the mixer 16 along with additional bandpass filters. The detected or recovered fj signal is amplified in an optional audio amplifier 26 before being presented via a speaker 28. According to the present invention, the signal strength of the desired received IF signal (processed at the IF frequency) is determined and compared to a threshold. Preferably, signal strength detection is accomplished in the IF amplifier 20, which has a receiver signal strength indicator (RSSI) output. Preferably, the IF amplifier 20 comprises an MC 3362 manufactured by Motorola Inc. or its functional equivalent. The present invention adds an unmodulated signal to the received IF signal when the received IF signal is below a predetermined threshold (which may be set at the 3dB signal-to- noise ratio). If the RSSI output (30) indicates that the received IF signal is at or below the threshold (set near the noise level), the controller 32 programs (42) the frequency of another oscillator 40 to produce the unmodulated signal. Having its frequency set at the same frequency of the received IF signal, the unmodulated signal is amplified in an amplifier 38 controlled by the controller 32 (via line 36) before being added (summed 39) to the received IF signal at the input to the filter 22. The gain of the amplifier 38 is set (36) such that the amplitude of the unmodulated carrier signal is preferably at least 8dB above the threshold (which is set near the noise level). It will be appreciated that the summing node 39 may occur before or after the filter 22. To compensate for the suppression of the desired signal due to the injection of the unmodulated signal, additional amplification is provided by the audio amplifier 26 by adjusting (44) its gain. The injection of the unmodulated signal suppresses the noise more than the desired signal when the received signal's amplitude resides at a level below the noise threshold. As a result, the recovered signal has a much greater signal-to-noise ratio. Therefore, sensitivity of the receiver increases. What is claimed is:

Claims: Claims 1. A method of receiving an information signal, comprising the steps of: (a) receiving said information signal to provide a received signal; (b) determining when said received signal has a signal strength at least equal to a threshold; and (c) adding an unmodulated signal to said received signal when said signal strength is at least equal to said threshold. 2. The method of claim 1 , further comprising the step of controlling a gain of an amplifier stage in response to said step (b). 3. The method of claim 1 , further comprising the step of detecting said received signal and said unmodulated signal to provide an output signal. 4. The method of claim 3, further comprising the step of amplifying said output signal. 5. The method of claim 1 , further comprising the step of synthesizing a frequency of said unmodulated signal. 6. The method of claim 5, wherein said unmodulated signal has a frequency substantially the same as said information signal's frequency. 7. The method of claim 1 , further comprising the step of adapting an amplitude of said unmodulated signal. 8. The method of claim 7, wherein said amplitude of said unmodulated signal is approximately 8dB above a noise level. 9. A receiver for receiving an information signal, comprising: means for receiving said information signal to provide a received signal; means for determining when said received signal has a signal strength at least equal to a threshold; and means for adding an unmodulated signal to said received signal when said signal strength is at least equal to said threshold. 10. The receiver of claim 9, further comprising means for controlling a gain of an amplifier portion of said receiver as a function of response to said signal strength. 11. The receiver of claim 9, further comprising means for detecting said received signal and said unmodulated signal to provide an output signal. 12. The receiver of claim 11 , further comprising means for amplifying said output signal. 13. The receiver of claim 9, further comprising means for programming a frequency source to provide said unmodulated signal. 14. The receiver of claim 13, wherein said unmodulated signal has a frequency substantially the same as said information signal's frequency. 15. The receiver of claim 9, further comprising means for adapting an amplitude of said unmodulated signal. 16. The receiver of claim 15, wherein said amplitude of said unmodulated signal is approximately 8dB above a noise level.

Inventors: MARKO PAUL DAVID, STENGEL ROBERT E, MARKO, PAUL, DAVID, STENGEL, ROBERT, E.
Applicants: MOTOROLA INC, MOTOROLA, INC.
Application year: 1990
Main classification: H04B15
IPC codes: H04B1