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:
This is a continuation of application Ser. No. 07/728,861 filed Jul. 11, 1991, now abandoned, which was a continuation of application Ser. No. 07/343,320 filed Apr. 26, 1989, also now abandoned. 
    
    
     TECHNICAL FIELD 
     This invention relates generally to radio frequency receivers and more particularly to receivers with sensitivity improvement. 
     BACKGROUND ART 
     One measure of a receivers 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 receivers sensitivity. However, existing receiver sensitivity improvement techniques (such as &#34;click swallowing&#34;) 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 3 dB signal-to-noise ratio, which is often called the noise range. Therefore, a need exists for a simple method to improve the receiver&#39;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 the sensitivity of the receiver. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a receiver in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 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 (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 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 signal is amplified in an optional audio amplifier 26 before being presented via a speaker 28. 
     According to the present invention, the signal or carrier strength of the desired received IF signal (processed at the IF frequency) is determined and compared to a signal 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 signal threshold (which may be set at the 3 dB signal-to-noise ratio). If the RSSI output (30) indicates that the received IF signal is at or below the signal threshold (set 3 dB approximately above 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 5 dB above the signal threshold (which is set 3 dB approximately above the noise level) or 8 dB above the noise level. It will 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&#39;s amplitude resides at a level below the signal threshold. As a result, the recovered signal has a much greater signal-to-noise ratio. Therefore, sensitivity of the receiver improves. It is to be appreciated that the unmodulated signal could be added with the received signal before the preselector 14, the mixer 16 of anywhere else after the antenna 12 and before the detector 24 to obtain the same benefits of an improved receiver sensitivity.