Telecommunications – Receiver or analog modulated signal frequency converter – Squelch
Reexamination Certificate
1997-11-07
2002-04-16
Hunter, Daniel (Department: 2684)
Telecommunications
Receiver or analog modulated signal frequency converter
Squelch
C455S212000, C455S214000, C455S219000, C455S222000
Reexamination Certificate
active
06374095
ABSTRACT:
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.
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patent: 3131354 (1964-04-01), Battin
patent: 3623106 (1971-11-01), Zerega, Jr.
patent: 3733554 (1973-05-01), Wycoff et al.
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patent: 4063033 (1977-12-01), Harbert et al.
patent: 4541118 (1985-09-01), Eastmond et al.
patent: 5481545 (1996-01-01), Maedjaja et al.
patent: 5617240 (1997-04-01), Hergault et al.
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Doyle Robert S.
Kolber Mark A.
Honeywell International , Inc.
Hunter Daniel
Zewdu Meless
LandOfFree
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