JPS58186232A - Signal detecting circuit of superheterodyne receiver - Google Patents

Abstract

PURPOSE:To realize a signal detecting circuit preventing malfunction at a strong input, by providing the 2nd intermediate frequency near the normal intermediate frequency and discriminating random noise and a tuning signal, in a superheterodyne receiver. CONSTITUTION:Electromagnetic waves sent from an antenna 1 are led to the 2nd intermediate frequency selection circuit 20 producing a frequency different from the 1st intermediate frequency by DELTAf, and to the 1st intermediate frequency selection circuit 4 via an amplifier 2 and a mixer 3, and when a strong input signal other than the tuning point of the receiver enters an antenna 1 and random noise appears at the output of the mixer 3, the spectrum of an output of the amplifier 5 shows a peak symmetrically, the output of a detector 6 is canceled for positive and negative components to be zero and inputted to a gate 24 via an inverter 23. Since the spectrum 18 of the input of the detector 22 is symmetrical due to the characteristics of random noise, the output is zero and inputted to the gate 24, thus preventing the generation of malfunction to drive a control section 9, a phase comparator 10 and a local oscillation section 11 at the strong input.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a superheterodyne receiver, and particularly to a signal detection circuit suitable for muting and auto-search operations.

First, the prior art will be explained using FIG. 1.

The radio waves supplemented by the antenna 1 are amplified by the high frequency amplification section 2, and the beat is taken by the output of the local oscillator 11 and the mixer 5 to obtain an intermediate frequency. The desired signal selected by the selection element 4 and the intermediate frequency amplifier 5 enters the FM detector 6 to obtain a demodulated signal, which is output via the stereo demodulator 7 from the output terminal 8 as an audio signal.

At the same time, the FM detector 6 uses a microprocessor, etc. when the amplitude of the intermediate frequency signal exceeds a set threshold, and the signal is within a predetermined frequency band of the frequency discrimination circuit (S curve). It notifies the control unit 9 that the signal has been received and stops auto search or cancels muting. The output of the local oscillator 11 is put into a phase comparator 10 with a programmable frequency divider to create a tuning voltage and control the local oscillator frequency.
configuring Kudlube).

If the input level of antenna 1 or 6 is about 100 dBμ below the mute threshold level, normally muting operation or auto search operation will not produce 1 operation, but if a strong input that adds up to 100 dBμ is applied to antenna 1, high frequency amplification stage 2 The mixer 6 becomes saturated and generates random noise having a fairly broad spectrum and at a level sufficient to cause muting and autosearch to malfunction. Observing this situation with the output of the mixer, we get
As shown in Figure 2, #1 is placed at the center near the intermediate frequency.
A constant level of noise 16 that is approximately bilaterally symmetrical is generated. Furthermore, when tuned to a signal, one spectrum 12 is generated at the center of the intermediate frequency. On the other hand, after the random noise goes back through the selection single element 4 and the intermediate frequency amplifier 5, it becomes a chevron-shaped spectrum 15 in FIG. 3 centered on the intermediate frequency, with the selectivity characteristic of the selection element unchanged. Further, when tuned to the signal, only one spectrum 14 in FIG. 6 exists above the intermediate frequency mantissa. When random noise enters the FM detector 6, the S
The integral values of the positive and negative regions of curve 17 in FIG. 6 cancel out and become almost zero, making it impossible to distinguish from the zero output of the S curve, 16 in FIG. 3, when tuned to the signal.

As a result, in the conventional technology, when a strong input signal of 100 dBμ or more enters the vicinity of the reception band frequency of a superheterodyne receiver, muting is canceled at a frequency unrelated to the frequency of the signal, or auto-sweep is performed. This had the drawback of causing malfunctions that caused the system to stop working.

SUMMARY OF THE INVENTION An object of the present invention is to provide a signal detection circuit for a superheterodyne receiver that eliminates the drawbacks of the prior art described above and prevents malfunctions during strong input.

Therefore, the present invention focuses on the spectrum characteristics of random noise over a wide range, and attempts to distinguish between random noise and No. 1148 by providing a second intermediate frequency near the regular intermediate frequency.

Embodiments of the present invention will be explained below using FIGS. 6 and 4. In Figure 4, the radio waves captured by antenna 1 are
The signal is led through the high-peripheral amplification section 2 and the mixer 3 to a first intermediate frequency selection circuit 4 and a second intermediate frequency selection circuit 20 whose center frequency differs from the first intermediate frequency by the amount f. Audio 0! Similar to the prior art, the signal system for the hook enters the FM stereo training section 7 via the first intermediate frequency amplification section 5 and the first FM detector 6, and outputs it as left and right audio signals from the output terminal q'. . Second
The intermediate frequency of l'i! is selected by the second intermediate frequency selection circuit 20, the second intermediate frequency l'i! '1 frequency amplifier 21 to the second FM detector 2
2 and the meeting output (outputs high when the signal is good, outputs low when out of synchronization) A N l) Gate 2
4 into one end of the input, and the other end input is the inverter 26.
The meeting output (outputs high when the signal is trained and low when out of tune) of the first FM detector in front GB is manually inputted through the FM detector.

Here, when a strong chi-g sequence other than the pivot point of the receiver is added to the antenna 1 and random noise is mixed in the output of the mixer 3, the spectrum of the input signal of the first FM detector 6 is as shown in FIG. As shown in Fig. 15, it becomes a symmetrical mountain-shaped waveform centered on the first intermediate frequency, and the first S curve 17 becomes zero as the positive region and negative region cancel each other out, and the first S curve 17 becomes zero, as in the case of signal tuning. The output of the FM detector 6 is low. The dough follows the inverter 23 and is converted to a high signal, AND
It enters one end of the input of gate 24. On the other hand, the spectrum of the input signal of the second FM detector 22 has a mountain shape centered at a point △f apart from the first intermediate frequency, as shown at 18 in FIG. If it is set at a distance of △f from the first intermediate frequency, the output of the second S-curve approaches zero and the output of the second FM detector becomes low because it is random noise with a symmetrical spectrum. ,AN
It is added to the other end input terminal of the D gate 24. Therefore, in this state calendar, the AND gate 24 low is output to the control circuit 9, and it is not assumed that the signal has not been received, thereby preventing malfunctions at the time of strong input. According to this embodiment, only the device tuned to the signal with the correct frequency receives the high output from the AND gate 24.
This signal is output to the expansion 1 circuit 9, cancels muting and stops auto search, and has the effect of avoiding malfunctions caused by strong input signals.

In addition, according to the present invention, in conventional superheterodyne receivers, it is possible to cancel muting at a single point of non-uniformity, which is a serious problem in areas with strong electric fields where the input level is 100 dnμ or more, and to cancel automatic muting in digital tuners, etc. This has the effect of being able to solve the phenomenon that interferes with search reception and stops at a frequency other than the signal.

[Brief explanation of drawings]

Fig. 1 is a conventional block diagram, Fig. 2 is a spectrum of medium r[71haj wave number output from the mixer during strong input and signal tuning, and 6th [nu1] is the input of the first FM detector. Spectrum and S-curve Based on the hourglass shape and the present invention (input spectrum and S-curve waveform diagram of the second FIVI detector,
FIG. 4 is a block diagram 1 of an embodiment according to the present invention. 1... Antenna, 2... Shoan wave amplification section, 6
...Mixer, 4...Intermediate frequency selection circuit, 5
...Intermediate frequency amplifier, 6...FM detector, 7...
F1Vi stereo demodulator, 8...output terminal, 9...
Control unit, 10... Programmable frequency divider and phase comparator, 11... Local oscillator, 14... First when receiving a signal
15... Spectrum of the first FM detector input at the time of strong input non-recurrence 1, 16... S curve output of the first FM detector at the time of signal reception ( 17... S-curve output of the first FM detector when strong input is out of tune, 18... Spectrum of second FM detector input when strong input is out of alignment, 19... Strong input S-curve output of the second FM detector during non-simultaneous training, 20... Second intermediate frequency selection circuit, 21... His intermediate cylinder e amplification circuit, 22... Second FM detector , 26...inverter, 24...AND gate. Figure 1 2nd port

Claims (1)

[Claims] t The mixer, the intermediate frequency selection amplifier circuit of U1, the amplitude above the threshold level of the intermediate frequency signal, and the frequency discriminator to cancel muting or obtain a stop signal for automatic tuning at a frequency within a certain set range. In a superhetero gain receiver consisting of one detector, a second intermediate frequency selection amplifier circuit slightly detuned from the first intermediate frequency, and a second intermediate frequency selection amplifier circuit having the same function as the first detector; has a detector, and only when the first detector determines that it is tuned to the signal and the second detector determines that it is not tuned to the signal,
A superheterodyne receiver (i! signal detection circuit) that determines that it is truly tuned to a signal and cancels muting and stops automatic tuning.