JPH0325084B2 - - Google Patents

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to an improvement of a modulator used in a radio broadcasting device that transmits amplitude modulated (AM) waves, and aims to obtain high power efficiency and improve characteristics. It is a diagram.

(Prior art) Conventionally, class B modulators have often been used as modulators in amplitude modulation radio broadcasting equipment, but in recent years, PWM (pulse width modulation) modulation has also come into widespread use, resulting in more power-efficient broadcasting equipment. has also been put into practice. but a few
In order to create a high-voltage, high-power broadcasting machine that operates at 10kV, it is necessary to use vacuum tubes, which has the disadvantage of high maintenance costs due to the lifespan of the vacuum tubes. The present invention describes a method of realizing a modulator in a broadcasting machine where there is a restriction that a vacuum tube must be used as a modulator by using a semiconductor element such as a low-voltage FET (field effect transistor) without using a vacuum tube. A person can divide the input audio level from zero to the required maximum value into several ten steps (hereinafter referred to as steps) and digitize it, and construct a switching circuit of several tens of steps using relatively low voltage FETs. A modulator that outputs a sampled digital signal of each step and passes it through the primary side of the transformer for each step, then rectifies the secondary side output, connects them in series, and adds them to obtain a high voltage modulated output. is proposed separately. This method is very effective, but in order to improve the modulation characteristics in terms of faithfully transmitting the audio waveform, it is necessary to increase the number of division steps, and it requires switching circuits, transformers, and rectifiers for the number of steps, making it economical. The problem is that there is a severe conflict between gender and required characteristics.

(Specific Object of the Invention) When the modulated device of an AM modulation system broadcasting device is an anode modulation system such as a vacuum tube, the normally required high-voltage, high-power output modulator can be replaced with an active amplification device such as a high-voltage vacuum tube. The purpose is to realize the modulator by using a semiconductor element with a relatively low breakdown voltage without using an element, and to provide a modulator that is inexpensive and has good characteristics.

(Configuration of the Invention) FIG. 1 is a diagram showing an example of the configuration of a highly efficient and high performance amplitude modulator (DSB modulator) embodying the present invention. In the figure, _EV on the audio input side is a DC reference voltage source that supplies a sufficiently stable voltage to determine the carrier wave level of the modulator, and determines the carrier wave level of the modulator output.
C _C is a DC blocking capacitor that prevents _EV from affecting the audio input side. Symbol 1 is the input (a
This is an A/D converter that digitally converts the voltage at point), and in this example, it has a 5-bit output, so the
It has a resolution of (This operation may be described as sampling the input voltage at a constant clock frequency and performing linear quantization.)
2 is a decoder, which outputs the digitized binary code of A/D converter 1 according to its value.
1 to C31 change, for example, if there is no output for each bit of output b1 to b5 of 1, C
There is no output in 1 to C31, and if there is an output in all b1 to b5, the output will appear in all C1 to C31.As a result, the level change range at point a is decomposed into 32 steps including zero. The output will appear.

3 (G1 to G31) are normal AND gates, 4
is the driver that drives FETQ1 to Q31,
T1 to T31 are transformers whose primary and secondary coils are sufficiently insulated in terms of direct current. D1 to D31 form full-wave rectifiers using diodes, and L ₀ and C ₀ form low-pass filters (LPF) to remove components higher than audio frequencies.

7 is a differential amplifier that divides the audio input signal and the waveform of the voltage divided by the series-parallel synthesis circuit of C1 to Cn and R1 to Rn and the parallel circuit of C _V and R _V in the output of this modulator. Make it work dynamically. 6 is a triangular wave generator of approximately 100 kHz, and 5 is a pulse width modulator whose output pulse width changes depending on the level of the difference component between the output of 7, that is, the audio input and the output of a modulator having a corresponding size. Cf is a capacitor that cuts out the DC component of the voltage generated at C _V and R _V and has low reactance even to low audio frequency components.

(Operation of the invention) First, the basic operations shown in FIG. 1 will be explained using (1) and (2).

(1) When there is no audio signal input AF as shown in Figure 1.

The voltage at the input a of the A/D converter 1 is only the reference voltage E _V for determining the modulator output voltage when there is only a carrier wave. Since the output of the A/D converter is a 5-bit binary code, the resolution of the decoder is 1/32. To make it easier to understand, if you set it to 15 when only the carrier wave is used, the A/
The output of D converter 1 is only b1 to b4 and b5
There is no output. When this 5-bit binary code is converted into 32 steps (including zero) by the decoder 2, outputs appear at C1 to C15. Each step output of decoder 2 is AND
are given to gates G1 to G31, respectively,
All of these AND gates are switched with a rectangular wave (output of pulse width modulator 5) with a sampling frequency of approximately 100 kHz, so a rectangular wave signal appears on the output side of the AND gate where the output of decoder 2 exists, and pulse driver 4 through a switching transistor (FET in this example)
Q ₁ to Q ₁₅ repeat on-off.

Now, if we pay attention to the switching of _Q1 only,
A switching current due to _Q1 flows in the primary coil of the transformer T1, and a voltage proportional to the transformation ratio appears in the secondary coil. The outputs of the diode circuits D1 to D31 that full-wave rectify the secondary coil output are all connected in series, so the output voltage rectified by D1 passes through D2 to D31.
It appears at the output e after passing through the LPF consisting of L ₀ and C ₀ .
In the case of carrier wave only, Q1 to Q1 as described above.
5 is repeating on-off, so D1 to D1
A stack of voltages rectified by 5 appears at the output e.

(2) When an audio signal is input.

When the combined voltage of the audio signal and _EV is applied to the A/D converter 1, its outputs b ₁ to b ₅ change in accordance with the audio signal. Note that the A/D converter 1 and decoder 2 perform data conversion and data latching using a sufficiently high clock frequency of several MHz, but since these are well known, they are omitted.

When b1 to b5 change according to the input audio level, the outputs of C1 to C31 of the decoder 2 also change. If all of C1-C31 have signal outputs, all of Q1-Q31 are switched and all of the transformers T1-T31 are switched and this condition produces the maximum output for this modulator. Also, as the audio signal changes, a
When the combined voltage at the point becomes zero, all output side signals of C1 to C31 disappear and all switching of Q1 to Q31 is stopped. This state is the lowest output of this modulator, which is zero volts. In this way, when audio is input around the carrier wave level, the output of the modulator changes according to the audio level, and the modulated device can be driven. Further Q1~
Since the Q31 performs switching operation (class D operation), the power efficiency is approximately 90%, which is extremely high.However, if the number of steps is increased to improve resolution, faithful waveform output can be obtained as described above, but there are economic constraints. There is.

Next, the means of the present invention for solving this problem without increasing the number of steps by the functions of 5, 6, and 7 will be explained.

(3) Operation of pulse width modulator 5, triangular wave generator 6 and differential amplifier 7.

The triangular wave generator 6 has a sampling frequency of approx.
Generates a 100kHz triangular wave. This triangular wave and the output of the differential amplifier 7 are input to the pulse width modulator 5. There is a comparator inside this pulse width modulator 5, which compares the triangular wave input and the output level of 7 and determines that the input level of 7 is higher.
The output pulse width to the AND gate becomes wider, and 7
When the input level is lower, the output pulse width becomes narrower. Next, differential amplifier 7
The audio input signal AF is input to the + input side of the modulator, and a part of the output voltage of the output e of the modulator is input to the - input side without phase rotation. This input will be called a feedback signal. here
When the AF and feedback signals are amplified by a differential amplifier, if there is distortion in the modulator output, a difference signal will be generated. The output level of the differential amplifier 7 increases when the level of the - input is lower than the level of the + input, and conversely, the output level decreases when the level of the - input is higher. That is, if there is a difference between the audio waveform of the modulator output e and the audio input AF, an operation is performed to eliminate this difference component, and this operation will be explained in detail next.

The output pulse of the pulse width modulator 5 is the voltage between R _V , which is the divided voltage of the modulator output e, and the ground, which is the audio signal.
When the pulse width is lower than the AF level, the pulse width becomes wider and is applied to G1 to G31. On the other hand C1-C
The signal No. 31 holds the instantaneous level of the audio frequency, and since the frequency of the audio signal is very low compared to the sampling frequency of about 100 kHz, the hold time is long. This held C
1 to C31 are switched by pulse width modulated sampling pulses of about 100 kHz in G1 to G31. When the width of the output pulse of the pulse width modulator 5 becomes wider, the ON time of Q1 to Q31 becomes longer, and T1 to T3 from the DC power supply becomes longer.
1 will take longer. Since the voltage of the DC power supply is constant, the power supplied to the transformer increases at this time, and the power obtained by rectifying the output of the transformer also increases. This indicates that since the impedance of the modulated device is approximately constant, the modulator output voltage increases. Conversely, when the voltage between R _V and ground is higher than the level of the audio signal AF, the width of the output pulse of the pulse width modulator 5 becomes narrower, the power supplied to T1 to T31 becomes lower, and the modulator output voltage decreases. do. By repeating this operation, the waveform of the modulator output approaches the audio signal input waveform. (See Figures 2 and 3) (4) Explanation of an example of the effect Figures 2 and 3 show an example of the effect of the present invention in which the feedback circuit is configured to change the switching width using pulse width modulated pulses. This will be explained using figures. FIG. 2 shows waveforms at various parts in FIG. 1 when pulse width modulation feedback is not applied. In these figures, i is the current waveform flowing from the DC power source to each transformer, d is the output waveform of D31, e is the modulator output waveform, and AF is the audio signal. Furthermore, these waveforms show the case where the voltage of AF increases by one step between time t ₀ and t ₁ in the output of the decoder. In Fig. 2, for a smooth audio signal like AF shown by a thin line, the output of the decoder changes by one step as i (sampling pulse width is constant), and the rectified output d also changes as shown in the figure. Because it increases by one step like
The modulator output waveform e that has passed through the LPF changes as shown by the thick line. However, this figure is shown as an extreme example to make it easier to understand. It is possible to improve the waveform distortion to some extent by lowering the cutoff frequency of the LPF.

FIG. 3 is a waveform diagram of each part of the circuit of the present invention, and it is clear that by applying pulse width modulation feedback, a characteristic faithful to the smooth input audio AF as shown by the curve e in the figure can be obtained.

(Effect of the invention) As shown in Fig. 1, the output of each switching circuit using the sampling pulse of the output which is digitized and divided into several tenths is sent to a transformer, and the DC-insulated secondary By performing the above-mentioned pulse width modulation feedback to the modulator which obtains a stepped waveform by cascading the rectified outputs on the side, a modulated output faithful to the audio signal input can be obtained. At this time, the degree of division of the digitized signal is set to a fixed number, for example, 32 (2 ⁵ ), and is characterized in that sufficiently good characteristics can be obtained without increasing the number of steps beyond that.

[Brief explanation of drawings]

Figure 1 is a configuration example diagram of a high-efficiency amplitude modulator according to the present invention, Figure 2 is a waveform diagram of each part of Figure 1 when pulse width modulation feedback is not applied, and Figure 3 is a diagram when pulse width modulation feedback is applied. FIG. 1 is a waveform diagram of each part of FIG. 1...A/D converter, 2...decoder, 3
...AND gate, 4...Pulse driver, 5...
...Pulse width modulator, 6...Triangular wave generator, 7...
Differential amplifier.

Claims (1)

[Claims] 1. An input audio signal is input to an A/D converter and a decoder circuit together with a carrier wave level setting reference voltage, and is digitized step by step. A digital signal is taken out from the switching gate circuit of each stage switched by a sampling pulse from a pulse generator and amplified, and the switching semiconductor element of each stage whose load is the primary winding of the transformer is generated. A voltage obtained by adding rectified voltages obtained from the secondary winding of the transformer, which is DC-insulated from the primary winding, in the order of the steps, is passed through a low-pass filter. An amplitude modulator that uses the input audio signal as an output, and a differential amplifier that uses the input audio signal as one input and a voltage obtained by dividing the modulator output as the other input; and an amplitude modulator that uses the output from the pulse generator as the difference. a pulse width modulator that performs pulse width modulation using the output from the dynamic amplifier to produce the sampling pulse; A high-efficiency amplitude modulator characterized in that the amplitude can be changed wider or narrower depending on whether the voltage is higher or lower than the divided voltage.