JPS6217907B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention is a method of AM stereo broadcasting.
This relates to AM stereo demodulation circuits using the Belar method and Magnavox method.

FIGS. 1 and 2 show a basic stereo transmission block diagram and a stereo demodulation block diagram of the Belar system. The transmitter will be explained with reference to FIG. A left signal (hereinafter referred to as Lch) and a right signal (hereinafter referred to as Rch) of a stereo signal are input to input terminals 5 and 6 of the matrix circuit 4, and output terminals 7 and 8 are respectively inputted by the matrix circuit 4. A difference signal (hereinafter referred to as LR) and a sum signal (hereinafter referred to as L+R) are output.
The (LR) signal is input to the frequency modulation circuit 2 via the pre-emphasis circuit 3, and frequency modulates the carrier signal generated by the carrier wave oscillator 1. The frequency-modulated carrier wave signal is then input to an amplitude modulation circuit 9, where it is amplitude-modulated by the (L+R) signal.

As mentioned above, in the Belar method, the carrier signal (L-
This is a method in which frequency modulation is performed using the R) signal and amplitude modulation is performed using the (L+R) signal.

Next, the reception and demodulation method will be explained with reference to FIG. The stereo transmission signal is transmitted through a high frequency amplification circuit 10,
The signal is converted to an intermediate frequency via an intermediate frequency amplification circuit 11. One side of the stereo signal converted to the intermediate frequency is (L+R) by the AM detection circuit 12.
demodulated into a signal. On the other hand, the amplitude limiting circuit 13 removes the AM component of the (L+R) signal, leaving only the FM signal modulated by the (L-R) signal, and this signal is
FM detection is performed by the FM detection circuit 14 (L-
R) Take out the signal. This (LR) signal is pre-emphasized at the time of transmission, and is output via the de-emphasis circuit 15 for correction. Next, each detected modulation signal (L-
R) and (L+R) signals are separated into L and R signals by a matrix circuit 16 and subjected to stereo demodulation.

Figures 3 and 4 show a stereo transmission block diagram and a stereo demodulation block diagram of the Magnavox system.
The difference from the Belar method is that the Belar method frequency modulates the (LR) signal, whereas the Magnavox method performs phase modulation (PM). Therefore, in transmission, the difference between the block diagrams in Figures 1 and 3 is that the frequency modulation circuit 2 in Figure 1 is replaced by the phase modulation circuit 4 in Figure 3.
4 and there is no pre-emphasis circuit 3.

The difference between the reception block diagrams in Figures 2 and 4 is as follows:
The FM detection circuit 14 in FIG. 2 becomes a PM detection circuit 45 in FIG. 4, and the de-emphasis circuit 15 is not provided. All other blocks are identical and the system operates as follows:
This is similar to the Belar method. Now, when Stereth demodulation is performed using both methods, the problem is the degree of separation between left and right. Generally, in the case of AM detection (detection of L+R signals), the output of the AM detection circuit 12 is connected to a rectifier circuit 19 as shown in FIGS. Since an AGC circuit that applies feedback to the frequency amplification circuit 11 is used, the input level vs. output level is 2 in Figure 7.
When the input level is small, the output level increases linearly, but when it reaches a certain value, the AGC operation is activated and the output level is saturated. However, the saturation level is not completely constant and gradually increases with the input level.

On the other hand, in the case of FM detection or PM detection (detection of L-R signals), in order to remove the AM component, the gain of the amplitude limiter is increased and when the input level is small, as shown at 21 in Figure 7. to keep the output level constant. As mentioned above, the input and output characteristics of AM detection and FM and PM detection are different, and there is a level difference between the detected signals (L+R) and (LR), respectively, so the matrix circuit 16 signal,
When separating into R signals, a problem arises in that the degree of separation deteriorates at weak input levels and at strong input levels.

SUMMARY OF THE INVENTION An object of the present invention is to provide a Belar type or Magnavox type Stereth demodulation circuit which alleviates the above-described drawback that the degree of separation deteriorates when the signal input level is weak or strong.

The key point of the present invention is that the AGC used for AM detection
The output of the circuit controls the gain of the FM and PM demodulators. In other words, a current-limiting differential amplifier circuit is used as the amplitude limiter, and its constant current amount is controlled. Furthermore, when a quadrature detector is used, the purpose is to similarly control the constant current amount of the differential circuit.

Embodiments of the present invention will be described below based on the drawings. FIG. 8 shows the Belar method showing an embodiment of the present invention.
This is an AM stereo demodulation circuit, and Figure 9 shows
This is a Magnavox type AM stereo demodulation circuit.
In Figures 8 and 9, the (L+R) signal is detected by AM detection, FM detection or
The operation of the system that extracts the (L-R) signal by PM detection and separates it into L and R signals in the matrix circuit 16 is shown in FIGS. 2, 4, and 5.
This is similar to that described in connection with the conventional circuit shown in FIG.
Only the control method will be explained here.

The output of the AM detection circuit 12 is input to the matrix circuit 16 and also to the rectifier circuit 19. The rectifier circuit 19 generates a DC voltage according to the level of the detected output. The rectifier circuit 1
The output of 9 is fed back to the high frequency amplifier circuit 10 and the intermediate frequency amplifier circuit 11, forming a gain control circuit (AGC circuit) that keeps the output substantially constant when the input signal level increases. The output of the rectifier circuit 19 is connected to the control terminal 48 of the amplitude limiting circuit 13, and controls the gain and amplitude of the amplitude limiting circuit 13 in proportion to the ANT input level.

An example of the configuration of the amplitude limiting circuit 13 is shown in the 10th embodiment.
Illustrated in the diagram. In FIG. 10, the transistor 22,
23 constitutes a differential pair, resistors 24 and 25 are respective load resistances, and constant current source transistor 26
The collector of the differential pair transistors 22 and 23
The base is connected to the power supply 27. Further, the emitter is connected to the drain of the field effect transistor, the source is grounded, and the gate is connected to the output of the rectifier circuit 19. The gain and output amplitude of such a differential amplifier circuit configuration depend on the amount of constant current of the differential amplifier circuit.

To vary the amount of constant current, the transistor 26
The field effect transistor 28 connected to the emitter of is used in the variable resistance region. At the gate
Since the output voltage of the rectifier circuit 19 that produces a rectified output voltage proportional to the ANT input level is applied,
The resistance value between the drain and the source, that is, the emitter resistance of the transistor 26 can be varied. Since the base of the transistor 26 has a constant voltage, when the emitter resistance changes, the amount of current changes, and the gain and amplitude of the differential amplifier constituting the amplitude limiting circuit 13 can be controlled.

FIG. 11 shows the above-mentioned relationship diagram and will be explained. 1st
In Figure 1, the horizontal axis represents the ANT input level, and the vertical axis represents the demodulator detection output level. FM detection,
As is well known, the input-output characteristics of PM detection increase linearly when the input level is small, but when the input level exceeds a certain level (when it reaches the limiter level of the amplitude limiter), the detection output level becomes constant. The characteristics are shown by broken lines 49, 50, 51, and 52 in the figure. Here we will explain the differences in each characteristic. As described above, the amplitude limiting circuit 13 uses a differential amplifier. The limiter level mentioned above is determined by the amount of constant current of this differential amplifier. In other words, the larger the amount of constant current, the larger the gain and output amplitude, the more the limiter is applied starting from the smaller input level, and the larger the amplitude, the larger the output level. This will be explained using the block diagram of FIG.

When the input level is 100 dBμ, the output voltage of the AM detection circuit 12 is large, so the rectifier circuit 1
The DC output voltage of No. 9 also increases. Since this DC output voltage is applied to the control terminal 48 of the amplitude limiting circuit 13, the resistance value between the drain and source of the field effect transistor 28 becomes small, and the emitter resistance of the transistor 26, which determines the amount of constant current, becomes small. Therefore, the amount of constant current increases. Therefore, the gain and amplitude of the amplitude limiting circuit 13 become large, resulting in the input-output characteristics shown by the broken line 49. Similarly, the input level
When it becomes smaller to 70 dBμ, 30 dBμ, and 23 dBμ, the DC output voltage of the rectifier circuit 19 decreases in proportion to each, the constant current amount of the amplitude limiting circuit 13 decreases, and the gain and amplitude also decrease. Therefore, the input level to which the limiter is applied increases. Furthermore, the detection output becomes smaller. Input-output characteristics of broken lines 50, 51, and 52 indicate this state.

However, the above input-output characteristics 49 to 52 are input-output characteristics when the input level is assumed to be constant and the amplitude limiting circuit 13 has a gain when a corresponding constant current flows. In reality, since the input level is also changing, the amount of constant current and gain are also changing. In other words, when the input level is 100 dBμ, the actual output level is the intersection between the broken line 49 and the intersection 53 when the input level is 100 dBμ, and when the input level is 70 dBμ, the intersection between the broken line 50 and the intersection 54 when the input level is 70 dBμ is the actual output level.
5 and 56 are the output levels when the input level is 30 dBμ and 23 dBμ. The solid line connecting these intersection points corresponds to
These are the input-output characteristics of the FM detection circuit 14 and the PM detection circuit 45 controlled by the output voltage of the AM detector 12.

Using the above control method, at a certain reference input level, for example 70 dBμ, the AM detection output level and FM
Alternatively, the constant current amount of the amplitude limiting circuit 13 is adjusted so that the PM detection output level becomes the same. That is, the voltage of the rectifier circuit 19 applied to the gate of the field effect transistor 28 which determines the amount of constant current is adjusted.
If you make the above adjustments, it will depend on the AM detection output level.
FM detection output level or PM detection output level
It can be changed in proportion to the AM detection output level, and even if the input level changes, the sum signal (L + R), which is the AM detection output, and the difference signal (L - R), which is the FM and PM detection output, are almost equal. can,
It is possible to reduce the deterioration of the degree of separation due to fluctuations in the input level as described in the conventional circuit.

In the example shown in FIGS. 8 and 9, the constant current amount of the amplitude limiting circuit is controlled by a variable resistance element, but in FIGS. 12 and 13, a quadrature detection circuit is used as the detection circuit. Give an example. 12th
The detection method and control signal extraction in FIGS. 8 and 13 are the same as those described in FIGS. 8 and 9.

FIG. 14 shows a quadrature detection circuit with variable gain and amplitude. The differential pair transistors 29 and 30 constitute a first switch circuit 41, and the collector of the transistor 29 has a differential pair transistor 31,
A common emitter of a second switch circuit 42 consisting of 32 is connected. transistor 31,
Resistors 33 and 34 are connected between the collector of 32 and a power source 43, serving as a load resistance. The collector of the transistor 30 is connected to a power source 43. Further, the common emitters of the differential pair transistors 29 and 30 are connected to the collector of the transistor 26. The base of the transistor 26 is connected to the power supply 27
The emitter is connected to field effect transistor 2.
It is connected to the drain of 8. The source of the field effect transistor 28 is grounded, and the gate is connected to the output of the rectifier circuit 19. Input terminal 3
A frequency-modulated or phase-modulated input signal is applied to 6, which is input to the first switch circuit 41 and simultaneously input to the phase shift circuit 38, so that the phase is set to 90.
The phase-shifted signal is input to the second switch circuit 42. A multiplication circuit composed of the first switch circuit 41 and the second switch circuit 42 performs FM detection or PM detection.

At this time, the field effect transistor 28 connected to the emitter of the transistor 26, which determines the gain and output amplitude of the detection circuit and determines the amount of constant current, is used in a variable resistance region. The field effect transistor 2
The gate of 8 is connected to the output of the rectifier circuit 19, and the resistance value between the drain and source changes depending on the input level, so the amount of current flowing through the first and second switch circuits 41 and 42 also changes. The gain and output amplitude of the variable detection circuit can be controlled according to the input level, the characteristics shown in FIG. 11 can be obtained, and deterioration of the degree of separation can be eliminated.

As described above in the embodiments, the present invention allows the gain of the FM and PM detection circuits to be adjusted based on the output level of AM detection.
By controlling the amplitude, the amplitude can be obtained by AM detection (L
+R) signal, (L) obtained by FM and PM detection
-R) Signal level differences can be eliminated, and deterioration of separation can be prevented.

[Brief explanation of the drawing]

Figures 1 and 2 are transmission and reception block diagrams of the Belar system, Figures 3 and 4 are transmission and reception block diagrams of the Magnavox system, and Figure 5 is a conventional Belar system circuit.
Figure 6 shows the conventional Magnavox circuit, Figure 7 shows the input/output characteristics of both conventional circuits, Figure 8 shows the first embodiment using a Belar type variable output limiter, and Figure 9 shows the first embodiment using a variable output limiter using the Belar method.
The figure shows the first embodiment using a variable output limiter using the Magnavox method, FIG. 10 shows the variable gain amplitude limiting circuit in the first embodiment using both types, and FIG.
12 and 13 show a second embodiment using a quadrature detection circuit using the Belar method and Magnavox method, respectively, and FIG. 14 shows a variable gain quadrature detection circuit in the second embodiment. show. 12...AM detection circuit, 13...limiter circuit, 14...FM detection circuit, 19...rectifier circuit.

Claims (1)

[Claims] 1. In an AM/FM multiplex receiver, at least AM
It is equipped with a detection circuit, a rectification circuit, an amplitude limiting circuit, and an FM detection circuit, and the input of the AM detection circuit and the input of the amplitude limiting circuit have a common input terminal, and the output of the amplitude limiting circuit is connected to the FM detection circuit. and the output of the AM detection circuit is connected to the control terminal of the amplitude limiting circuit via the rectifying circuit, and the gain and amplitude of the amplitude limiting circuit are controlled according to the output level of the rectifying circuit. It was
AM stereo demodulation circuit. 2 In an AM/PM multiplex receiver, at least
It is equipped with a detection circuit, a rectifier circuit, an amplitude limiting circuit, and a PM detection circuit, and the input of the AM detection circuit and the input of the amplitude limiting circuit have a common input terminal,
The output of the amplitude limiting circuit is connected to the PM detection circuit, the output of the AM detection circuit is connected to the control terminal of the amplitude limiting circuit via the rectifier circuit, and the amplitude is adjusted according to the output level of the rectifier circuit. characterized by controlling the gain and amplitude of the limiting circuit.
AM stereo demodulation circuit. 3. In claims 1 and 2, the rectified output is connected to the control terminal of the FM and PM detection circuit, and the gain and amplitude of the FM and PM detection circuit are controlled according to the rectified output level. This is an AM stereo demodulation circuit characterized by: