JPH051130Y2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Field of Application] This invention relates to an amplitude modulation circuit, and more particularly, to an amplitude modulation circuit useful when performing amplitude modulation in radio equipment, measuring instruments, etc.

[Conventional technology]

Conventionally, a circuit shown in FIG. 8 or 9 is known as an amplitude modulation circuit.

The amplitude modulation circuit shown in FIG. 8 is composed of an input matching circuit A, a high frequency power amplification transistor B, an output matching circuit C, and a low frequency amplifier D. The gain of the power amplification transistor is changed according to the modulation signal to output an amplitude modulated wave.

On the other hand, the one shown in Fig. 9 consists of a balanced mixer E, a linear amplifier F, and a circuit G that inputs a bias voltage according to the modulation signal, and utilizes the nonlinearity of the rectifying action of the balanced mixer E. It is mainly used for frequency conversion, generation of double sideband signals, etc.

[Problems with conventional technology]

However, in the circuit shown in FIG.
The output power of the low-frequency amplifier requires approximately half the input power of transistor B, so in the case of a high-power transmitter, a high-power low-frequency amplifier is required, making it large and heavy, and reducing power consumption. It has the disadvantage that it becomes large.

Also, even if the collector voltage of transistor B becomes 0V, the base. Since the carrier wave leaks to the output side through the coupling capacitance between the collectors, as shown in FIG. 11, in this type of amplitude modulation circuit, modulation is applied in multiple stages to obtain a deep modulation degree.
In this case, it is necessary to adjust the gain of transistor B in each stage in order to obtain a deep modulation depth with low distortion.
The gain of the high-frequency power amplification transistor B has a large variation depending on the element, and the gain also changes when the frequency of the carrier wave changes, so that adjustment is extremely difficult.

In addition, since the high frequency power amplification transistor B superimposes a modulation signal on the collector voltage, it must be operated at half the rated voltage, and this amount (approximately -
6dB) The gain of the transistor was reduced, and the number of stages of the modulated amplifier circuit had to be increased, resulting in a larger size.

Further, it has the disadvantage that it is unsuitable for use as a broadband amplitude modulator in which the frequency of the carrier wave varies over a wide range, since the modulation characteristics also vary as the gain of the high-frequency power amplification transistor changes.

Next, in the circuit shown in Fig. 9, the balanced mixer E is composed of rectifiers H ₁ and H ₂ and high-frequency transformers T ₁ and T ₂ as shown in Fig. 11, and is modulated using the nonlinear characteristics of the rectifier. Due to the non-linear characteristics of _the rectifiers _{H 1 and H 2} , high frequencies are often generated _. Deep modulation depth cannot be obtained due to insufficient insulation between the L terminals.Furthermore, the insulation between the R and L terminals varies depending on the frequency of the carrier wave, so when used as a broadband or amplitude modulator, the output and modulation depth will change. However, there were various drawbacks.

[Problems solved by invention]

In order to eliminate the above-mentioned drawbacks, this invention provides an amplitude modulation circuit that prevents the generation of unnecessary waves, does not change the degree of modulation even if the carrier frequency changes over a wide range, and can be made compact. It was done for the purpose of

[Technology to solve problems]

That is, the amplitude modulation circuit of this invention includes a variable attenuator whose attenuation changes in proportion to a control voltage, a high frequency power linear amplifier that amplifies the output of the variable attenuator, and a directional coupler that detects the output state of the amplifier. and an envelope detector that converts the output from the directional coupler into a voltage proportional to its envelope; detecting a DC component from the directional coupler, the envelope detector, and the envelope detector; an automatic output control circuit consisting of a comparison amplifier that compares it with a reference voltage; a rectification feedback consisting of the directional coupler, an envelope detector, and a comparison amplifier that compares the output from the envelope detector with a modulation signal and extracts an alternating current component; The automatic output control circuit and the rectification feedback circuit are fed back to the variable attenuator, and the variable attenuator is used as a carrier wave input section and the directional coupler is used as an output section. .

〔Example〕

Next, this invention will be explained using examples.

FIG. 1 is a circuit diagram of this invention.

The amplitude modulation circuit of this invention includes a variable attenuator 1 whose attenuation changes in proportion to the control voltage Ec as shown in FIG. A linear amplifier 2 and a directional coupler 3 that detects the output state of this amplifier 2
, an envelope detector 4 that converts the output from the directional coupler 3 into a voltage Ee proportional to its envelope, the directional coupler 3, the envelope detector 4, and the envelope detector 4. An output control circuit 6 consisting of a comparator amplifier 5 that detects the DC component of and compares it with a reference voltage Eo, a directional coupler 3, an envelope detector 4, and an output from the envelope detector 4 as a modulation signal em
The output control circuit 6 and the rectification feedback circuit 8 are fed back to the variable attenuator 1, and the variable attenuator 1 is a carrier wave input section, a directivity A coupler 3 is configured as an output.

The variable attenuator 1 of the above embodiment can be applied to any circuit as long as it is a variable attenuator whose attenuation changes in proportion to the control voltage, but as shown in FIG. It is most desirable to use diodes CR1 and CR2.

PIN diodes have pure resistance characteristics with very good linearity above a certain frequency, and the capacitance between terminals is extremely small, so they have good insulation up to high frequencies and good frequency characteristics, and can be used at carrier frequencies over a wide range. This is because it becomes possible to attenuate the

In addition, since PIN diodes are small and lightweight,
This is also desirable from the standpoint of downsizing the device.

Further, in the above embodiment, the automatic output control circuit 6 and the rectification feedback circuit 8 are each conventionally known circuits, but in the automatic output control circuit 6, the time constant determined by the resistor RI and the capacitance _C1 is set to the lowest value of the modulation signal. If it is set sufficiently longer than the frequency period, it becomes possible to detect the DC component of Ee.

In addition, in the above embodiment, the control voltage Ec of the variable attenuator 1 is calculated by adding the feedback system (9 in the figure).
However, as shown in FIG. 3, the respective feedback system signals Em and Ed may be input to variable attenuators 1A and 1B arranged in series.

Further, in this case, the inputs of Em and Ed may be arranged in the reverse order as shown in the figure.

[Effect]

Next, the operation of the embodiment of this invention will be explained.

In FIG. 1, the carrier wave e _c is given an attenuation amount proportional to the control voltage (e _c ') by a voltage-controlled variable attenuator 1, amplified by a high-frequency power linear amplifier 2, and outputted via a directional coupler 3. Ru.

Next, the operation of the output control circuit 6 will be explained.

To simplify the explanation, we will explain the case where there is no modulation signal e _n . A part of the output of the high frequency power linear amplifier 2 is detected by the directional coupler 3, and detected by the envelope detector 4 in proportion to the envelope of the output. It is converted into a voltage signal Ee.

In the comparator amplifier 5, the DC component of Ee becomes the reference voltage Eo
Generate a control voltage Ed that is equal to .

The variable attenuator 1 gives the carrier wave e _c an amount of attenuation proportional to the control voltage E c based on this Ed.

The output is therefore proportional to the reference voltage Eo. That is,
If the reference voltage is constant, the output will remain constant even if the gain of the high frequency power linear amplifier changes.

Further, if the output of the directional coupler 3 is constant regardless of the frequency, the output will be kept constant even if the frequency of the carrier wave changes.

Next, the operation of the rectifier feedback circuit 8 will be explained.

In Fig. 1, the capacitor _C2 is for extracting the AC component of the output Ee of the envelope detector 4.
It is compared with the modulation signal e _n in a comparator amplifier 7 to generate a control signal Em.

Therefore, the variable attenuator 1 is controlled by a control signal based on this Em, and the carrier wave e _c is subjected to amplitude modulation.

Also, at this time, since the comparator amplifier 7 compares the output envelope and the modulation signal e _n , the variable attenuator 1,
Distortion caused by the high frequency power linear amplifier 2 and the like is improved.

Next, the operation of the voltage controlled variable attenuator 1 will be explained.

As mentioned above, the variable attenuator 1 receives the control voltage signal Ec
Amplitude modulation is performed based on

In Fig. 4, C is the amplitude when no modulation (carrier level), and a is the maximum amplitude when modulated at 100%, then a=2c 20 log c/a=-6dB Therefore, as shown in Fig. 5, The operating point of the voltage-controlled variable attenuator 1 must be a point A where the control voltage is less than or equal to Vo.

The amplitude maximum point at the maximum modulation degree of the amplitude modulation signal occurs when the voltage-controlled variable attenuator has the minimum attenuation amount, and the amplitude minimum point occurs when the voltage-controlled variable attenuator has the maximum attenuation amount.

That is, the degree of modulation is determined by the amount of attenuation of the voltage controlled variable attenuator 1.

In this case, if the amplitude values of the amplitude modulation signal are a and b in FIG. 4, the degree of modulation (m) is m=a-b/a+b×100(%)...

The relationship between each amplitude and the attenuation amount of the voltage-controlled variable attenuator 1 is as follows: a: amplitude at the minimum attenuation b: amplitude at the maximum attenuation, the attenuation at the minimum is 1/a ₀ the amplitude at the maximum 1/Ka ₀ , the formula is m=1/a ₀ -1/Ka ₀ /1/a ₀ +1/Ka ₀ ×100=100 (1
-2/K+1).

In this case, as shown in Fig. 6, if the attenuation amount of the voltage-controlled variable attenuator 1 is α, the output is αVc cosω _c t...Also, since α is proportional to the control voltage, if the proportionality constant is k, then α= k(Vo+Vmcosω _s t) ... Substitute KVc(Vo+Vm cosω _s t)cosω _c t =k VcVocosω _c t +(kVcVm/2)×cos(ω _c +ω _s )t +(kVcVm/2) [ cos(ω _c −ω _s )t]... The first term in the equation is a carrier wave component, and the second and third terms are upper and lower sideband waves.

Therefore, no unnecessary waves are generated during modulation.

By the way, the nonlinear element 10 as shown in FIG.
Analyzing ^the _conventional amplitude modulation _{according to} a ₂ is a proportionality constant), and as shown in Figure 7, the input voltage Vi
is expressed as Vi=Vccosω _c t+Vscosω _s t .

Substituting into the equation, Vo=a ₁ Vscosω _c t (1+2a ₂ Vscosω _s t/a ₁ ) +a ₁ Vscosω _s t +a ₂ Vc ² cos ² ω _s t +a ₂ Vs ² cos ² ω _s t =a ₁ Vccosω _c t+a ₂ VcVscos (ω _c +ω _s )t +a ₂ VcVscos (ω _c −ω _s )t+a ₁ Vscosω _s t +a ₂ Vs ² /2+(a ₂ Vs ² cos2 ω _s t)/2 +a ₂ Vc ² cosω _c t/2...

In the equation, the first term is the carrier wave component, and the second and third terms are the upper and lower sidebands, which are the intended amplitude modulated waves, but in addition to this, there are up to 8 terms, and especially the 8th term is This becomes the second harmonic of the carrier wave, that is, an unnecessary wave.

In this example, for simplicity, we stopped at the Vi ² term as shown in the equation, but in reality, there are higher-order terms, so even more unnecessary waves are generated.

As is clear from the above, in the circuit of the present invention, almost no unnecessary waves are generated.

〔effect〕

This invention is constructed and operates as described above, and has the following effects.

(1) The circuit of the present invention uses a variable attenuator, which is a linear element, and changes the amplitude of the carrier wave with a modulation signal, so that unnecessary waves are not generated in the modulation circuit.

(2) Since the modulation degree is determined by the amount of attenuation of the variable attenuator, not only does the modulation degree not change over a wide range of changes in the carrier frequency, but also the modulation degree does not change even if the transmission output is varied. That is, it has excellent broadband characteristics.

(3) Since the circuit of this invention is composed of a feedback system, it automatically compensates for changes in amplification gain and conversion gain due to temperature changes, power supply voltage fluctuations, and secular changes, so adjustment is very simple and requires only output adjustment. It becomes easy.

(4) The output can be changed continuously, and unnecessary waves are not generated even when the output is changed.

[Brief explanation of the drawing]

Figure 1 is a circuit diagram of this invention, Figure 2 is a partial circuit diagram of a variable attenuator, Figure 3 is a circuit diagram of another embodiment,
Figures 4 and 5 are explanatory diagrams of the operation of the variable attenuator, and Figure 6
Fig. 7 is an explanatory diagram of the operation of the conventional example, and Figs. 8 to 1
FIG. 1 shows a circuit diagram of a conventional example.

Claims (1)

[Scope of utility model registration request] a variable attenuator whose attenuation changes in proportion to the control voltage; a high-frequency power linear amplifier that amplifies the output of the variable attenuator; a directional coupler that detects the output state of the amplifier; An envelope detector that converts the output into a voltage proportional to its envelope, and a comparison amplifier that detects the DC component from the directional coupler, the envelope detector, and the envelope detector, and compares it with a reference voltage. An automatic output control circuit consisting of;
a rectifier feedback circuit comprising the directional coupler, an envelope detector, and a comparator amplifier that compares the output from the envelope detector with a modulation signal and extracts an alternating current component; the automatic output control circuit and the rectifier feedback circuit are connected to the variable 1. An amplitude modulation circuit, wherein feedback is provided to an attenuator, the variable attenuator being a carrier wave input section, and the directional coupler being an output section.