Abstract:
Squelch circuitry for narrow band audio receivers which uses a sub-audible tone or coded signal to modulate the intermediate frequency in accordance with the presence of a desired broadcast signal and then detects the sub-audible tone frequency or coded signal in the modulated IF frequency to control the audio output of the receiver to an off condition when there is excessive noise.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The present invention relates to squelch systems which operate to turn off the audio output of a radio receiver when there is no received signal and to turn it on when the desired signal is present. 
     2. Description of the Prior Art 
     Squelch systems are well known in the art. An example of a present squelch system is shown in FIG. 1 wherein an AM receiver is shown having an input antenna  12  receiving broadcast signals and presenting them to an RF filter  14 . Filter  14  removes all but a selected frequency and this signal is amplified by an RF amplifier  16  to produce an amplified selected RF signal. A first mixer  20  in conjunction with a first local oscillator  22  receives the amplified selected RF signal and converts it to a first intermediate frequency signal which is presented to an IF filter  24  to pass only the selected first IF frequency. This signal is amplified by IF amplifier  26  and presented to a second mixer  30  which, in conjunction with a second local oscillator  32 , converts the first selected IF frequency to a second IF frequency. The second selected IF frequency is filtered by an IF Filter  34  and amplified by IF amplifier  36  to produce the desired AM signal on output line  37 . It should be noted that a “single conversion” receiver architecture could be used in which case the first mixer  20  and oscillator  22  could produce the selected IF frequency to filter  34  and amplifier  36  and the second mixer  30  and local oscillator  32  would not be required. However, the dual conversion (or even a triple conversion) receiver architecture has certain advantages (unrelated to the present invention) and is used in this embodiment. In any event, the signal on output line  37  is presented to envelope detector  40  to produce the audio signal to be amplified by an audio amplifier  42  and filtered by audio filter  44  which removes some of the noise and non-speech audio that may exist in the detected signal. This signal is presented to an audio switch which, as will be explained, operates as a squelch switch to provide an audio output on a line  47  when there is a signal and to prevent the output on line  47  when there is no signal. More particularly, to provide the squelch function, audio switch  46  is enabled and disabled by the output of an OR gate  48 . When OR gate  48  produces an output signal, switch  46  is enabled and the audio signal from filter  44  is passed through to a final audio amplifier  49  for producing an audio output to speakers or head phones (not shown) as desired. This output signal from OR gate  48  is produced when the signal level is above the carrier squelch threshold or when the signal has sufficient quieting to be below the noise squelch threshold. 
     The output of audio amplifier  42  is also presented to a summing circuit  50  which also receives an automatic gain control threshold signal on a line  51 . The difference is sent to an integrator  52  to produce the AGC signal for use in controlling the gain of IF amplifier  36 , and, through break point amplifiers  54  and  56 , controlling the gain of IF amplifier  26  and RF amplifier  16 , respectively, in order to get a constant level from the envelope detector  40  that does not depend on the signal level from the antenna  12 . 
     A carrier squelch comparator  60  also receives the AGC signal from integrator  52  on a line  61  and compares it with a predetermined carrier squelch threshold signal on a line  62 . Since the AGC voltage for a given receiver gain is an estimation of the signal level, it may be used to determine if the signal at the antenna  12  is above or below the predetermined threshold. If the AGC signal is above the threshold signal, a signal is presented by carrier switch comparator  60  on a line  62  to OR gate  48  which then enables audio switch  46  thus turning the audio output on. 
     The second IF signal from IF amplifier  36  (or the first IF signal from IF amplifier  26 ) may be used to provide a noise squelch function as follows: the output from IF amplifier  36  on a line  64  (or the output from IF amplifier  26  on a line shown as dashed line  66 ) is presented to an FM discriminator  70  which performs an FM demodulation on the IF signal. When there is no signal on line  64  (or line  66 ), there is a lot of noise from the FM discriminator  70  on a line  71 . Similarly, when there is a signal on line  64  (or line  66 ) then the noise on line  71  decreases. The amount of noise on the output of FM discriminator  70  is thus an indication of the signal strength for the noise squelch circuit. A high pass filter  72  receives this noise signal and filters away the speech frequencies and leaves only the noise at its output  73  which is presented to a noise rectifier  74  which converts it to a signal on line  75  that is proportional to the noise voltage. A noise squelch comparator  76  receives the signal on line  75  and compares it to a predetermined noise squelch threshold signal on a line  77 . If the noise voltage on line  75  is less than the threshold value on line  76 , a signal on a line  78  is presented to OR gate  48  and the audio switch  46  is enabled and the audio output is turned on. Thus the audio output is turned on either when the signal received by the antenna is strong or the noise level is low. Stated differently, the OR gate  48  enables the audio when the signal power, as measured by the AGC loop, is above a threshold or when the FM noise is below a threshold. 
     This circuit works well with channel spacing of 25 kilohertz because only 8 kHz is required for transmission of audio information in speech. The remaining bandwidth, above the speech frequencies but below the channel limits, has been used by the noise squelch. Unfortunately, new requirements for airborne very high frequency communications have produced much narrower bandwidth channels which has resulted in splitting each of the 25 kHz channels into three 8.33 kHz channels. While this is sufficient for spoken communications, the band of frequencies used by the noise squelch has been eliminated. 
     SUMMARY OF THE INVENTION 
     The present invention uses a coded signal, for example, a low frequency (sub-audible) FM tone, to modulate one of the local oscillators and an FM demodulator at the output of the receiver is fed to a narrow bandwidth filter that is tuned to the sub-audible tone frequency. When a desired signal is present, an FM modulated tone will be detected by the FM demodulator. When a desired signal is not present, the attempt to modulate the noise with the sub-audible tone will only produce more noise and consequently, only noise will be detected by the FM demodulator. The presence of the tone, indicative of there being a desired signal, is compared to a threshold value and used to enable the receiver. The absence of a tone, indicative of there being no desired signal, will disable the receiver. The normal communication path of the radio contains a high pass filter that strips off any of the tone were it to manifest itself in the AM detector. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic drawing of a prior art radio with squelch capabilities; and, 
     FIG. 2 is a schematic drawing of an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The operation of FIG. 1 has been set forth in the Background Of The Invention section above. In FIG. 2, most of the components operate as they did in FIG.  1  and are shown with the same reference numerals. The different structure is shown at the left of FIG. 2 where a coded signal generator shown in the preferred embodiment as a sub-audible tone oscillator  100 , generates an audio tone that is below the frequency of spoken language. The frequency is also selected so that it may easily be filtered out before it reaches the audio amplifier  49  so that it will not be heard. The output of oscillator  100  is shown on a line  101  connected to an FM modulator  102  which is placed between the first local oscillator  22  and the first mixer  20  so as to modulate the first local oscillator  22  slightly in frequency. FM modulator  102  could, instead, be placed between the second local oscillator  32  and the second mixer  30 , if desired. Also, phase modulation PM, may be used since FM is just a time derivative of PM which for the sinusoidal tone is the same. Furthermore, as mentioned above, a single conversion amplifier or a triple conversion amplifier may be used. In any event, when a desired signal is present at the antenna  12 , the FM tone is converted to the first IF frequency (or the second IF frequency). When there is no desired signal at the antenna  12 , the tone is not converted to the first (or second) IF frequency. 
     The output of the second IF amplifier  36  (or amplifier  26  if a single conversion amplifier is used), is connected to an FM discriminator  104  which demodulates the sub-audio tone and produces an output on a line  105  to a sub-audio tone detector  106  which detects if the sub-audio tone is present and, if so, creates a signal on a line  107  which is proportional to the amount of sub-audio tone present. The output of the sub-audio tone oscillator  100  may also be connected by a line  108  to the sub-audio tone detector  106  so as to provide the frequency information thereto for operation at the same frequency. 
     A squelch comparator  110  receives the signal on line  107  and compares it to a preselected tone squelch threshold signal on a line  111 . If the magnitude of the sub-audio tone present on line  107  is greater than the threshold on line  111 , a signal from comparator  110  on line  115  to OR gate  48  causes the enabling of audio switch  46  and the turning on of the audio output. In other words, OR gate  48  enables the audio when the signal power, as measured by the AGC loop, is above a threshold or when the sub-audio tone is above a threshold. 
     Thus is seen that we have provided a squelch circuit which operates with the present day VHF narrow band AM receivers. The system is substantially immune from common transmitter and receiver deficiencies such as undesired FM or PM modulation that some AM transmitters may generate and which might disrupt the squelch systems and is immune from hum since the sub audible frequencies are far away from power line frequencies. The system is also compatible with existing channel allocations as well as new allocations and has the same advantages regarding jamming and interference as the prior art system had. Many changes and modifications will occur to those having skill in the art. For example, as mentioned, the sub-audible tone oscillator and the FM modulator  102  may be placed between the second local oscillator  32  and the second mixer  30 . The second mixer  30  and the second local oscillator  32  may be omitted if a single conversion amplifier is used. PM may be employed rather than FM to modulate the sub-audible tone. Also, many of the individual components may be interchanged with others having similar functions and, the system, while designed primarily for AM radios, may also be used on narrow band FM systems as well. For instance, the received signal may be modulated by the sub-audible tone in many places as it passes through the receiver. Also, the sub-audible tone may be replaced with other forms of coded signal that can be detected on a desired signal but that are not detected on noise such as a predetermined digital sequence. Accordingly, we do not wish to be limited to the specific structures used in connection with the description of the preferred embodiment.