Telecommunications – Receiver or analog modulated signal frequency converter – Squelch
Patent
1979-12-05
1983-09-06
Chin, Tommy P.
Telecommunications
Receiver or analog modulated signal frequency converter
Squelch
455154, 455205, H04B 110, H04B 126
Patent
active
044033494
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention concerns an intermediate frequency detection circuit of a radio receiver which is ensured to generate an output indicative of the reception for broadcasting waves with no erroneous operations caused by suprious signals or the like.
2. Description of the Prior Art
Electronic tuning type radio receivers capable of automatic search channel selection or the like recognize the tuning to a broadcasting wave when an intermediate frequency stage generates an output. The frequency sweeping is then interrupted and a receiving state is entered. The intermediate frequency stage generates its output by the reception not only of broadcasting waves but also of harmonic waves (sprious signals) thereof to interrupt the channel selection operation. A circuit for taking out the intermediate frequency output for channel selection (hereinafter referred to as IF detection circuit) is, desirably, designed so that it produces no output upon reception of the harmonic waves or noises. Moreover, while the IF detection circuit is adapted to monitor the output from the intermediate frequency stage at a certain threshold level and generates an output when the intermediate frequency output exceeds this level, this system results in a problem that a frequency width exceeding the threshold level changes depending on the level of the input signals. If the frequency width becomes broader exact tuning is of course made difficult. It is, accordingly, desired for the IF detection circuit to output a narrow width pulse at an exact tuning point.
SUMMARY OF THE INVENTION
It is, accordingly, an object of this invention to provide an F detection circuit capable of satisfying the aforesaid requirements with a simple circuit structure. According to this invention there is provided a first circuit for producing a signal indicating the output level of an intermediate frequency stage of a superheterodyne type radio receiver, a second circuit for producing a signal indicating the output level of a high frequency stage of the above receiver and a logic circuit to which the outputs from the above first and second circuits are inputted. The logic circuit generates a signal indicating the reception of a broadcasting wave when the intermediate frequency output is high and the high frequency output is low. According to this system, erroneous operation does not result from noises or spurious signals and thus search channel selection is not interrupted. Also this system provides an advantage of easy exact tuning since a detection signal with an approximately constant pulse width is obtainable.
The signal indicating the output level of the intermediate frequency stage is, simply, obtained from an AGC (Automatic Gain Control) circuit. Since a time constant in the AGC circuit is relatively large, particularly, in an AM receiver, a problem arises that the signal indicating the output level of the intermediate frequency can not be obtained rapidly. In view of the above, a switching circuit is provided by this invention, which decreases the time constant of the AGC circuit during channel selection.
Other objects and advantages of this invention will become apparent from the following explanations referring to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing a basic embodiment of this invention,
FIG. 2 is a block diagram for explaining the states of voltages in each of the sections,
FIG. 3-FIG. 8 are graphs for explaining the characteristics or waveforms of signals in each of the sections,
FIG. 9 is a schematic circuit diagram showing a specific emodiment of this invention, and
FIG. 10 is a schematic circuit diagram showing a modification for a part of the circuit in FIG. 9.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, there is shown an antenna 10, a tuning circuit 12, a high frequency amplifier 14, a frequency mixer 16, a local oscillator 18, an intermediate frequency amplifier 20, an intermediate frequency transformer
REFERENCES:
patent: 3569840 (1971-03-01), Tanaka
patent: 3995220 (1976-11-01), Hansen
patent: 4322857 (1982-03-01), Grohmann
Chin Tommy P.
Fujitsu Ten Limited
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