Squelch detecting circuit with squelch start determining means
First Claim
1. A squelch detecting circut for detecting a carrier signal level of a predetermined frequency carrier in an input signal thereto to produce a squelch alarm signal when said carrier level signal is equal to, or lower than, a predetermined threshold level, said squelch detecting circuit comprising:
- narrow bandpass filter means for eliminating all but those frequency components close around said predetermined frequency component in said input signal, said narrow bandpass filter means producing, as a filtered signal, the frequency signal close around said predetermined frequency;
detector means for detecting an envelope of said filtered signal to produce, as an envelope signal, a voltage signal representative of the envelope of said filtered signal;
first comparator means for comparing a voltage level of said envelope signal with a first reference voltage, said first reference voltage being corresponding to said predetermined threshold level, said first voltage comparator means producing a first signal of a first voltage level when said envelope signal voltage level is equal to, or lower than, said first reference voltage, said first voltage comparator means producing a second signal of a second voltage level when said envelope signal voltage level is higher than said first reference voltage; and
squelch start determining means responsive to said first signal for producing said squelch alarm signal continuously after said first signal is continuously received for a predetermined time period, said squelch start determining means stopping to produce said squelch alarm signal when said second signal is received.
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Accused Products
Abstract
A squelch detecting circuit comprises a narrow bandpass filter (16) for obtaining, as a filtered signal, a desired IF carrier frequency signal. The filtered signal is envelope-detected at a detector (17) and the detected signal is compared with a predetermined squelch level at a voltage comparator (21). The voltage comparator produces a first signal when the detected signal is below the squelch level and produces a second signal when the detected signal is above the squelch level. A squelch start determining circuit (22) produces a squelch alarm signal after the first signal is continued for a predetermined time period, and the squelch alarm signal is maintained until the second signal is inputted to the squelch start determining circuit (22), so that the transmitter and receiver characteristic test using the carrier frequency sweeping can be performed without any squelch alarm signal produced.
34 Citations
3 Claims
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1. A squelch detecting circut for detecting a carrier signal level of a predetermined frequency carrier in an input signal thereto to produce a squelch alarm signal when said carrier level signal is equal to, or lower than, a predetermined threshold level, said squelch detecting circuit comprising:
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narrow bandpass filter means for eliminating all but those frequency components close around said predetermined frequency component in said input signal, said narrow bandpass filter means producing, as a filtered signal, the frequency signal close around said predetermined frequency; detector means for detecting an envelope of said filtered signal to produce, as an envelope signal, a voltage signal representative of the envelope of said filtered signal; first comparator means for comparing a voltage level of said envelope signal with a first reference voltage, said first reference voltage being corresponding to said predetermined threshold level, said first voltage comparator means producing a first signal of a first voltage level when said envelope signal voltage level is equal to, or lower than, said first reference voltage, said first voltage comparator means producing a second signal of a second voltage level when said envelope signal voltage level is higher than said first reference voltage; and squelch start determining means responsive to said first signal for producing said squelch alarm signal continuously after said first signal is continuously received for a predetermined time period, said squelch start determining means stopping to produce said squelch alarm signal when said second signal is received. - View Dependent Claims (2, 3)
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Specification