JPH0355922A - Heterodyne type fm receiver circuit - Google Patents

Abstract

PURPOSE:To simplify the circuit and to reduce the power consumption by shifting the frequency of a local oscillation circuit by a prescribed value when the carrier detection operation mode is used, and outputting a carrier detection signal in response to the DC component of the output voltage of an FM detection circuit. CONSTITUTION:A frequency of a local oscillator 3 is shifted by DELTAf[kHz] with a control signal 13 to a carrier detection control terminal from a microcomputer or the like at the carrier detection operation mode. In this case, when a carrier signal is inputted to an antenna 1, an IF signal shifted by DELTAf[kHz] from the normal value is outputted from a mixer 4 and the output DC level of the FM detection circuit is shifted by a level corresponding to the DELTAf. The voltage is applied to a low pass filter, 8 where an audio signal is eliminated, the result is compared with a reference voltage 10 at a comparator 9 and a carrier detection signal representing the presence of carrier is outputted from a discrimination signal output terminal. When no carrier exists, since the output DC level of the FM detection circuit is not shifted, the detection signal represents the absence of carrier.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a heterodyne FM receiver, and particularly to a power-saving heterodyne FM receiver used in cordless phones, etc.
Regarding the receiver.

[Conventional technology]

In a cordless phone, in a standby state for battery saving on the receiver side, a receiver that operates intermittently temporarily detects the presence or absence of a carrier from a base unit. When a carrier is detected, each circuit on the receiver side is turned on, and the tuner itself is controlled by a microcomputer or the like so that it is in a talking state and the tuner itself is switched to continuous operation.

FIG. 3 is a block diagram of a heterolog FM receiver including a conventional carrier detection circuit. As shown in the figure, a high frequency signal received by an antenna 1 is amplified by an RF amplifier 2, then mixed with a local signal from a local oscillator 3 by a mixer 4, and converted into an intermediate frequency signal. The frequency of the intermediate frequency signal is selected by a bandpass filter 5, amplified by an IF amplifier 6 until the amplitude is saturated, and demodulated into a baseband signal by an FM detection circuit 7. For carrier detection, an AM detection circuit 14 is supplied with a signal branched from the IF amplifier 6.
This is done by extracting only the DC component from the output voltage using the low-pass filter 8 and comparing it with a predetermined value using the comparator 9.

[Problem to be solved by the invention]

In the conventional method described above, although the original signal is FM modulated, the carrier is detected only by the amplitude, so an AM detection circuit is required, which has the disadvantage of increasing the circuit scale and reducing power consumption. There are some flaws that make it unsuitable. An object of the present invention is to provide a heterolog-type FM receiving circuit that can detect carriers without using an AM detection circuit.

[Means to solve the problem]

The FM receiver of the present invention includes means for shifting the frequency of the local oscillation circuit by a predetermined value in the carrier detection operation mode, and means for shifting the frequency of the local oscillation circuit by a predetermined value in the carrier detection operation mode, and a means for shifting the frequency of the local oscillation circuit by a predetermined value in the carrier detection operation mode. It is characterized by comprising means for outputting a carrier detection signal in response to a certain amount of direct current.

〔Example〕

FIG. 1 is a block diagram for explaining one embodiment of the present invention. In the carrier detection operation mode, that is, in the standby state, the frequency of the local oscillator 3 is changed to Δf[:K] by the control signal l3 from the microphone etc. to the carrier detection control terminal.
Hzl shifted. At this time, when a carrier signal is input to antenna 1, mixer 4 outputs △f from the normal value.
(KHz) The shifted IP signal is output, (△f is set within the passband of the bandpass filter 5) FM
The output DC potential of the detection circuit is shifted by a voltage corresponding to Δf.

The sound signal is removed from this W pressure by a low-pass filter 8, and the converter 9 compares and determines with a reference voltage JEIO, and a carrier detection signal indicating the presence of a carrier is output from the determination signal output terminal. When there is no carrier, the output DC potential of the FM detection circuit does not shift, so the detection signal indicates the absence of carrier.

When this type of FM receiving circuit is used in a cordless phone, carrier detection is performed only in standby mode, so the local frequency shift is canceled during signal demodulation during a call, so it does not affect signal demodulation.

FIG. 3 shows an embodiment of the local oscillator circuit used in the present invention, in which one end of C1 is connected to the C of Q2 depending on the presence or absence of a control signal to the control input terminal C.
The oscillation frequency is shifted depending on whether or not it is grounded through the impedance between E and E.

〔Effect of the invention〕

As explained above, the present invention provides a simple frequency shift function to the local oscillator, thereby eliminating the need for an AM detection circuit for carrier detection, and has the effect of simplifying the circuit and reducing power consumption.

[Brief explanation of drawings]

FIG. 1 is a block diagram for explaining an embodiment of the present invention, FIG. 2 is a circuit diagram for explaining an example of a local oscillation circuit used in the present invention, and FIG. 3 is a block diagram for explaining a conventional example. It is a block diagram. 1... Antenna, 2... RF amplifier,
3...Local oscillation circuit, 4...Mixer 5...Band pass filter, 6...
IF amplifier, 7...FM detection circuit 18...
・Low pass filter, 9... Converter, 1
0... Reference voltage source, 11... Judgment signal output end, 12... Audio signal output end, 13
...Signal detection control end, 14...
・Amplitude detection circuit, 15...Power control end, 16...Power control circuit, Rl, R2e R
s, R4...Resistance, C,, C2, C3, C4
...Capacitor s Ql# Q2...
Transistor, L1...Reactor, XL...
···Crystal oscillator.

Claims (1)

[Claims] In a heterodyne FM receiving circuit that detects input of a carrier signal, means for shifting the frequency of a local oscillation circuit by a predetermined value in a carrier detection operation mode;
1. A heterodyne FM receiving circuit comprising: means for outputting a carrier detection signal in response to a DC component of an output voltage of an FM detection circuit.