JPS6322736Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Technical Field of the Invention] The present invention relates to a frequency modulation type demodulation device that counters the threshold phenomenon by using an oscillator that follows the instantaneous frequency of a frequency modulation signal. The present invention relates to a frequency modulation type demodulator that can improve the noise removal effect by increasing the ability to follow the instantaneous frequency. Here, tracking means that the oscillation frequency of the oscillator is pulled into the instantaneous frequency and oscillates in synchronization with that frequency.

[Technical background of the invention]

A general frequency modulation demodulator generally has a configuration as shown in the system diagram shown in FIG. That is, in FIG. 1, a frequency modulation (hereinafter referred to as FM) signal converted to an intermediate frequency at the front end of the tuner or an FM signal read from a record carrier is input to the input terminal 1.
The FM signal is subjected to an automatic gain control amplifier (hereinafter referred to as AGC amplifier) 2 for optimal amplitude adjustment to the amplitude limiter 3 at the next stage, and then is output to the amplitude limiter 3 at the next stage.
The amplitude changes of the FM signal are aligned, and the signal is further input to the next stage demodulation circuit (hereinafter referred to as DEMO circuit) 4 for frequency discrimination (or phase detection) to obtain a demodulated output signal at the output terminal 5.

However, recently, communication technology using microwave bands has come to be used in consumer devices.
In a simple FM demodulator with the configuration shown in Figure 1,
It has become impossible to obtain a demodulated output that is free from distortion and noise without being affected by the threshold phenomenon that is unique to the FM system.

Therefore, in order to deal with this phenomenon, a voltage controlled oscillator (hereinafter referred to as VCO) is controlled by the demodulated output, and the frequency of this VCO is locked to the frequency of the FM input signal, so that the control voltage of the VCO itself is demodulated. I had no choice but to adopt a method that provides output. This method is called the FM negative feedback method, and has traditionally been used in non-line-of-sight microwave communications and satellite communications.

The principle of the above method basically comes from the idea of a tracking filter. The idea is that since the threshold phenomenon generally occurs when the peak value of the noise entering the demodulator intermittently exceeds the peak value of the signal, only the noise component should be reduced at the peak of the noise.

The tracking filter as described above is a type of oscillator, and has a circuit configuration as shown in FIG. By using the tracking filter shown in FIG. 2 in place of the amplitude limiter 3 shown in FIG. 1, noise components are attenuated and only signal components are supplied to the DEMO circuit 4. That is, in FIG. 2, an attenuator 6, an amplifier 7, and an attenuator 8 are connected in series, and a feedback circuit 9 consisting of a coil and a capacitor is connected between the input and output of the amplifier 7 in parallel to the amplifier 7. ,
Both terminals 10 and 11 are connected to the AGC output path 2a and the input path 3a of the DEMO circuit 4 in FIG. 1, respectively.

The operation of the tracking filter as described above is such that the Q of the circuit is set so that the feedback circuit 9 resonates with the instantaneous frequency of the FM input signal, and the oscillator consisting of the amplifier 7 and the feedback circuit 9 is activated for only the signal component. Oscillates to obtain a demodulated output signal with less distortion and noise. By doing so, it is possible to suppress the threshold phenomenon that occurs when the peak value of noise exceeds the peak value of the signal when the received input power is small.

Here, to discuss the occupied bandwidth of the FM signal with reference to Figure 3, it is known that for the same carrier wave, the width of the energy distribution on the spectrum of the FM signal becomes wider as the modulation depth becomes deeper. There is. An equation that quantitatively expresses this relationship is an approximate equation called Carson's rule bandwidth. According to this, the occupied bandwidth A for transmitting an FM signal without distortion is , approximately 2 of the highest modulation frequency (the upper limit of the frequency range of the FM signal)
This is twice as necessary. Note that FIG. 3 is an explanatory diagram showing instantaneous changes in television signal frequency during a horizontal scanning period, with the horizontal axis representing time t and the vertical axis representing frequency.

Taking this into consideration, it seems contradictory at first glance that the tracking filter described above transmits only the neighborhood range D of the instantaneous frequency B of the FM input signal, but in a very short time the spectral distribution of the FM signal Since most of the noise is considered to be concentrated in the instantaneous frequency, the tracking filter can attenuate only the noise component instantaneously as described above.

[Problems with background technology]

In the above operation, since the feedback circuit 9 is a resonant circuit, the traceability of the filter characteristics, that is, the traceability of the oscillator described in the [Technical field of invention] is influenced by the Q of the circuit. It is.
In other words, increasing the Q of the circuit increases the noise removal effect, but has the disadvantage of weakening the ability to follow FM signals with large frequency shifts (wideband). In order to compensate for this followability, it is necessary to damp the Q of the circuit and increase the input level of the FM signal. However, increasing this input level has the advantage of using a tracking filter, that is, it prevents the threshold phenomenon from occurring even at an input level lower than that which causes the threshold phenomenon in a normal demodulator, so that the noise removal performance does not deteriorate. It becomes impossible to do something like this.

Thus, in the past, the Q of the tracking filter circuit had to be set to a compromise size, and the level of the input signal had to be increased by the amount that the Q was lowered.

Furthermore, since the tracking filter oscillates at a free-running oscillation frequency when there is no FM input signal, it is preferable to keep this free-running oscillation frequency equal to the carrier frequency. However, if the Q of the circuit is lowered as described above, there is a problem that the free oscillation frequency becomes unstable due to the nature of the oscillation circuit, and that adjustment to make it equal to the carrier wave frequency becomes difficult.

[Purpose of invention]

The present invention was developed in view of the above circumstances, and is an FM demodulator that adopts the concept of a tracking filter to improve the threshold.The tracking filter has good tracking performance even for FM signals with large frequency deviations. To provide a frequency modulation type demodulator which can obtain sufficient noise removal performance without increasing the input level of an FM signal, that is, an effect without reducing the degree of threshold improvement. With the goal.

[Summary of the idea]

In order to achieve the above object, the present invention controls the oscillation frequency of the tracking filter using a demodulated output signal.
The oscillation frequency is made to sufficiently follow the FM input signal frequency.

[Example of idea]

An embodiment of the present invention will be described below with reference to FIGS. 4 and 5. Here, FIG. 4 shows a basic system diagram of the present invention, and FIG. 5 shows its specific circuit diagram.

First, the configuration of FIG. 4 will be explained, and the same reference numerals are used for the same elements as those in FIG. 1.
The FM input signal that has entered the input terminal 1 is applied via the AGC amplifier 2 to a synchronous oscillator 12 that oscillates in synchronization with the instantaneous frequency of the FM input signal. This synchronous oscillator 12 is a type of VCO in which the Q of the resonant circuit is selected to be large, and is composed of discrete components or an integrated circuit. Thereby, the oscillation frequency is controlled to always be near the instantaneous frequency by a control signal to be described later. Furthermore, this synchronous oscillator 12 is set to free run at the frequency of the carrier wave when there is no FM input signal.

The signal component of the FM input signal selected by oscillating the synchronous oscillator 12 in this way is DEMO
It is designed to be led out to an output terminal 5 via a circuit 4. Here, a feature of the present invention is that the demodulated output signal derived from the output terminal 5 is applied to the frequency control terminal 12a of the synchronous oscillator 12. This demodulated output signal corresponds to the control signal described above, and the demodulated output signal itself causes the synchronous oscillator 1
Since the oscillation frequency of 2 is controlled, the synchronous oscillator 12
The oscillation frequency of the FM input signal is locked to the instantaneous frequency of the FM input signal.

According to such a configuration, the oscillation frequency of the synchronous oscillator 12 is controlled by the output of the DEMO circuit 4,
The oscillation frequency will change to track the instantaneous frequency. At this time, the Q of the resonant circuit of the synchronous oscillator 12 is selected to be large, so the holding power (tracking ability) to be synchronized with the instantaneous frequency is much higher than before, and it is effective against signals with large frequency deviations. There is no risk of it coming off. Therefore, only frequency components near the instantaneous frequency (see Figure 3D) are transmitted, and a band containing noise components below that frequency (Figure 3A) is transmitted.
is suppressed. As a result, even if the input level of the FM signal is reduced, noise components are not transmitted, the level at which the threshold phenomenon occurs can be reduced, and a demodulated output with a low signal-to-noise ratio can be obtained. be.

FIG. 5 shows a specific example of FIG.

In this Fig. 4, the parts surrounded by three dotted lines are the AGC amplifier 2, the synchronous oscillator 12,
Corresponds to DEMO circuit 4. The AGC amplifier 2 is
The FM input signal received from the input terminal 1 is passed through the bandpass filter 13 to the controlled amplifier 14, buffer amplifier 15, detection diode 16, and DC amplifier 17.
The output is applied to a gain control loop consisting of
is supplied to.

This synchronous oscillator 12 is provided with attenuators 18 and 19 on the input and output sides, and a Colpitts type oscillation circuit 21 made up of a transistor 20 is interposed between these. This Colpitts type oscillator circuit 21 has a high Q feedback circuit consisting of a capacitor C and a coil L connected between the collector and base of the transistor 20, and a variable capacitance diode 22 provided between the collector and ground. A control signal supplied from the DEMO circuit 4 via a conductor 23 is applied to the diode 22. With this configuration, the Colpitts type oscillation circuit 21
serves as a VCO whose oscillation frequency can be controlled by the control signal, and supplies its output to the DEMO circuit 4 via the attenuator 19. Here,
The attenuators 18 and 19 are used to adjust the signal level in order to improve the differential gain characteristics of the DEMO circuit 4 in the subsequent stage.

Next, the DEMO circuit 4 combines the input ends of the first and second amplifiers 24 and 25, branches the output sides, and connects a capacitor C ₁ and a coil L ₁ to each output side.
A tuned detection circuit section consisting of a detection diode 26, a capacitor C ₂ , a coil L ₂ and a detection diode 27 is provided to extract signal components at frequencies above and below the carrier wave, and the difference between these signal components is transferred to a resistor 2.
The structure is such that it is derived from the connection intersections of 8 and 29. Further, this output is applied to the first output amplifier 31 via the resistor 30, and the second
It is also applied to the output amplifier 32. While the output of this second output amplifier 32 is derived to the output terminal 5, the first output amplifier 31 is configured so that a frequency setting circuit 33 can select the free oscillation frequency of the Colpitts type oscillation circuit. There is.
Then, the output of this first output amplifier 31 is connected to the conductor 2.
3 to the variable capacitance diode 22 of the synchronous oscillator 12.

In the above configuration, when there is no FM input signal, the first output amplifier 31 causes the Colpitts type oscillation circuit to oscillate at the free oscillation frequency, that is, the carrier frequency. Then, when the FM input signal is input,
A control signal for causing oscillation in synchronization with the frequency is applied from the first output amplifier 31 to the variable capacitance diode 22. The output of the first output amplifier 31 changes according to the frequency deviation of the FM input signal, and acts to always cause the oscillation frequency of the Colpitts oscillator to follow the input frequency.

In this way, the present invention controls the oscillation frequency of the synchronous oscillator 12 by feeding back the demodulated output.
The meaning of this feedback is different from that of a demodulator using a conventional feedback method, and the synchronous oscillator 12 can be considered as a characteristic variable filter, whose characteristics are also controlled by the demodulated output.

[Effect of idea]

As explained above, according to the present invention, an oscillator that oscillates in synchronization with the frequency of the FM signal and acts as a filter only on frequency signals near the oscillation frequency is connected to the demodulator, front end section, etc. In the FM demodulator installed in between, the oscillation frequency of the oscillator is controlled by the output of the demodulator, so the ability to follow the frequency of the FM input signal is extremely high, making it possible to handle signals with large frequency deviations. There is no possibility that the synchronization will be lost. Therefore, the signal-to-noise ratio of the demodulator is improved, and even FM signals with low input levels can be
This has the excellent effect of not producing noise peculiar to a demodulator.

[Brief explanation of drawings]

Fig. 1 is a system diagram showing the circuit system of a general frequency modulation type demodulator, Fig. 2 is a system diagram showing the circuit system of a synchronous oscillator inserted in the circuit of Fig. 1, and Fig. 3 is a system diagram showing the circuit system of a synchronous oscillator inserted in the circuit of Fig. FIG. 4 is a system diagram showing a circuit system of a frequency modulation type demodulator according to the present invention, and FIG. 5 is a circuit diagram showing a circuit embodying FIG. 4. 4...Demodulator, 12...Synchronous oscillator, 18,1
9... Attenuator, 20... Transistor, C... Capacitor, L... Coil, 22... Variable capacitance diode, 23... Signal conductor.

Claims (1)

[Claims for Utility Model Registration] A signal that transmits a frequency-modulated signal to a demodulator by a synchronous oscillator that oscillates in synchronization with a frequency substantially equal to the frequency of an input signal and passes an instantaneous occupied band component of the signal. A frequency modulation type demodulation device, characterized in that the oscillation frequency of the synchronous oscillator is controlled by the output of the demodulator, in a frequency modulation type demodulator interposed in a path.