JPH01319309A - Amplitude modulation circuit - Google Patents

Abstract

PURPOSE:To eliminate the saturation of discharge phenomenon and to avoid waveform distortion by using a constant current source for discharging a capacitor. CONSTITUTION:A series circuit comprising a protection diode D2 and a constant current source 7 is connected between a circuit earth and a power voltage (-Vc). Then a connecting point between the protection diode D2 and the constant current source 7 is connected to a capacitor C1 and the constant current source 7 consists of, e.g., transistor(TR) Q1 and a resistor R1. That is, the capacitor C1 is discharged by the constant current source 7 and the voltage across the capacitor C1 decreases linearly and the saturation of the capacitor voltage by a time constant of C.R is avoided. Then the waveform of voltage across the capacitor C1 is nearly an envelope of a carrier subject to amplitude modulation and the waveform distortion is eliminated.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to improvement of modulation distortion in an amplitude modulation circuit.

[Conventional technology]

In the field of measuring instruments, various high frequency signals (e.g.
There is a signal generator that generates an FM modulated signal, etc.). The high frequency signal obtained from this signal generator is
For example, it is used as a reference (standard) signal for various tests, and high accuracy is required for the signal waveform.

Figure 4 shows a conventional FI used in such a signal generator.
The amplitude modulation circuit of FIG. 4, which represents a width modulation circuit, takes the output amplitude modulated signal into a feedback loop and derives the envelope from it. This envelope waveform is then compared with the reference modulated wave (usually an audio signal), and feedback is applied to eliminate the error between the two signals, so that the output modulated waveform is determined as the modulated wave (audio signal). This is to make them match.

In the figure, numeral 1 denotes a modulator that amplitude-modulates the introduced carrier wave S^ with an output signal 8B of an ALC amplifier 3, which will be described later. Note that the modulator 1 receives the carrier wave S^ by the signal 8B.
The output of this modulator 1, which may have any configuration as long as it has the function of controlling the amplitude of the signal, is amplified by an output amplifier 2, and an amplitude-modulated signal is taken out from a terminal P1.

Further, the peak value of the signal at the terminal P1 is applied to the capacitor C1 via the detection diode 01.

A resistor "1" is connected in parallel to the capacitor C1, and the envelope (detected waveform) of the amplitude-modulated waveform is obtained from the capacitor C1 by the time constant of this C8 circuit.The charging voltage waveform of the capacitor C1 (Detected waveform) is shown in FIG. 5, in which (a) is an amplitude-modulated carrier wave, and (b) is a charging voltage waveform of capacitor C1.

This voltage waveform of capacitor C1 is applied to the negative input terminal of ^tC amplifier 3. On the other hand, a modulated wave from the signal generator 5 is applied to the positive input terminal. In addition, AL
A DC signal is applied from an output level regulator 4 to the negative input terminal of the C amplifier 3, and the DC level VB of the detected waveform shown in FIG. 4 is adjusted.

The signal generator 5 introduces a digital signal DA indicating a generation frequency from a processor (not shown), and this signal 0
Outputs a modulated wave with a frequency indicated by ^. Normally, this modulated wave is a sine wave, and in this case, the signal generator 5 includes a Wien bridge oscillator, a bridged T oscillator, etc. (not shown), for example, The frequency of the output sine wave is controlled by switching the values of a resistor R and a capacitor C (not shown) that constitute these oscillators using a digital signal DA.

ALC (＾automatic Level Control
ol) The amplifier 3 introduces the charging voltage signal of the capacitor C1 and the modulation signal of the signal generator 5 and applies this error signal to the modulator 1. By providing such a feedback loop, the circuit in FIG. 4 operates so that the detected waveform from the capacitor C1 and the modulated waveform from the signal generator 5 are the same, and amplitude modulation distortion is reduced. It's in control.

Note that by changing the DC signal applied from the output level adjuster 4, the output level (c) shown in FIG. 5 can be adjusted. That is, the output level regulator 4 determines the output level in a steady state (in the case of no modulation).

[Problem to be solved by the invention]

The conventional amplitude modulation circuit as described above has a problem in that as the degree of modulation increases (for example, the degree of modulation approaches 100%), the width modulation type increases. This will be explained using FIGS. 3 and 6.

FIG. 3 shows the charging voltage waveform (solid line waveform) of the capacitor C1 obtained by the circuit of FIG. 4 and the envelope (dotted line waveform) of the amplitude-modulated waveform.

As shown in this figure, the charging voltage waveform (solid line waveform) of the capacitor C1 does not form an envelope of the dotted line waveform in the (d) part, but is distorted. Since the distorted solid waveform shown in FIG. 3 is applied to the ALC amplifier 3, the amplitude-modulated waveform obtained from the output terminal P1 is also distorted. To explain, the third
A portion (d) in the figure is a period in which the potential of the capacitor C1 is higher than the potential of the output terminal P1, and the diode D1 is off.

When diode 01 is off, the feedback loop of FIG. 4 is in an "open" state, and since no feedback is applied, the distorted waveform of capacitor C1 directly affects the amplitude modulated waveform of output terminal P1.

The reason for the waveform shown in FIG. 3 will be explained using FIG. 6. FIG. 6 is an enlarged drawing of the detected waveform (solid line waveform) portion of FIG. 3. In FIG. 6, (a) is an amplitude-modulated carrier wave (same as (a) in FIG. 5), and (b) is a charging voltage waveform (detected waveform) of capacitor C1 (5th waveform).
The potential of the capacitor C1 decreases with the time constant C-R (C is the capacitance of the capacitor C1, R is the resistance value of the resistor "1"), the peak of the carrier wave (a) is set to Ql, Q
2. ..., at the time of Q6, capacitor C1
The voltage at the output terminal P1 is higher than the voltage at the output terminal P1 (the height of Q6).

Therefore, after Q6, the voltage of capacitor C1 becomes higher as shown in Fig. 6, and as a result, as shown in Fig. 3 (d).
Distortion as shown in the image below appears.

An object of the present invention is to provide an amplitude modulation circuit in which distortion (d) shown in FIG. 3 is reduced.

[Means to solve the problem]

In order to solve the above-mentioned problems, the present invention provides a modulator (1) that modulates the amplitude of a carrier wave using an output signal of an ALC #4 width filter (described later), and a capacitor that extracts the peak voltage of the amplitude modulation signal based on the output of this modulator. The detection diode (D1) that charges the (C1) and the digital signal DA
A signal generator (5) that introduces a signal generator (5) and outputs a modulation signal of a frequency corresponding to the signal, and an AIC that introduces a charging voltage signal of the capacitor (C1) and a modulation signal of the signal generator (5) and outputs an error signal. An amplitude modulation circuit comprising: an amplifier (3); a DA converter (8) that introduces a signal based on the digital signal DA and converts it into an analog signal; and an output current that is applied to the capacitor (C1); , a constant current source (7) whose output current level is controlled by the output signal of the DA converter.

[Effect]

In the present invention, since a constant current source is used for discharging the capacitor C1, the discharge phenomenon does not become saturated as in the case of discharging with the C-H time constant.

Therefore, waveform distortion can be eliminated.

〔Example〕

Hereinafter, the present invention will be explained in detail using the drawings.

FIG. 1 is a diagram showing an embodiment of the amplitude modulation circuit according to the present invention, and FIG. 2 is a diagram showing a charging voltage waveform of the capacitor C1 obtained by the circuit of FIG. 1.

In FIG. 1, the block portion 10 is different from FIG. 4, and other components are the same as in FIG. 1. Therefore, the same constituent elements will be given the same constituent numbers and their explanation will be omitted.

In block 10, 02 is a protection diode, 7 is a constant current source, 8 is a digital-to-analog converter (hereinafter referred to as DAC), and 9 is a digital multiplier.

Protective diode D between circuit ground and power supply voltage (Vc)
2 and a constant current source 7 are connected in series.

The connection point between the protection diode D2 and the constant current source 7 is connected to the capacitor C1. The constant current source 7 can be composed of, for example, a transistor Q1 and a resistor R1. Then, the base potential of transistor Q1 is changed to DAC8.
When controlled by , a constant current corresponding to the voltage level of this DAC8 flows through the transistor Q1.For example, if the output voltage of the DAC8 is v, the current Ic flowing through the resistor R1 is Ic = (VD-Vc Vs E ) / R1(
1), and the constant current (VsE:) transistor Q1
base-emitter voltage).

The DAC 8 introduces the digital signal O^ via the multiplier 9 and converts it into an analog signal.

The operation of the circuit shown in FIG. 1 will be explained below. First, a detailed explanation of the present invention will be given.0 In the present invention, since the discharge of the capacitor C1 is performed by the constant current source 7, the voltage of the capacitor C1 decreases linearly (as in the conventional case, with the time constant of C-H). Therefore, the waveform of the capacitor C1 becomes as shown by the dashed line in FIG. 6, which is essentially the envelope of the amplitude-modulated carrier wave. Note that FIG. 6 is a theoretically drawn waveform, and the actual waveform of capacitor C1 is a complicated waveform shown by a dashed line in FIG.
This becomes a sine wave shown by the solid line in the figure and is applied to the ALC amplifier 3.

This will be explained in detail below. The digital signal 0^ is a signal that commands the frequency, and when DA=01, the signal generator 5 outputs a sine wave of frequency = IKH2, for example, and 0^=2.
When it is 01, outputs frequency=2KH2. On the other hand, DAC
8 introduces k·DA through a multiplier 9 and applies a voltage of, for example, lv, 2v to the constant current source 7. Here, the multiplier of the multiplier 9 corresponds to k in equation (6), which will be described later.

The functions and effects of protection diode 02 will be described. If a signal is not generated at the output terminal P1 for some reason - the capacitor C1 is not supplied with charge from the detection diode D1, and is pulled by the constant current source 7 to (Vc). In this way, the power supply voltage ( If a signal is output from the output amplifier 2 while the potential of the capacitor C1 has fallen to Vc), it takes time for the potential of the capacitor C1 to rise to its original normal level, resulting in poor response. Furthermore, there is a possibility that the output transistor of the output amplifier 2 will be adversely affected. On the other hand, if the protection diode 02 is provided, the potential of the capacitor C1 is clamped to -0.6V (the forward saturation voltage of the diode), and the above-mentioned disadvantage does not occur.

Here, a design example for specifically determining the output current value of the constant current source 7 is shown. As shown in FIG. 2, the point with the largest slope in the sine wave is at the point P^ (phase angle σ). This point P
The slope of the tangent at ^ is given by the following formula.

, 1) (3i n ω t ) = [ω cos ω
tlt''o = ω...Q) Therefore, if the slope of the dashed-dotted line in Fig. 6 (the slope of the discharge) is determined so that the slope is ω, then for each period of the carrier wave,
The terminal voltage of capacitor C1 is at the peak point Ql in FIG.
Q2. Q3. Since the voltage becomes lower than ... and peak detection is possible, the detected waveform will not be distorted.

Therefore, the constant current value IC at which the slope of discharge of the capacitor C1 becomes ω is determined. In FIG. 2, the tangent at point PA is extended to find the point of intersection with voltage axis E. Here ω=
There is a relationship of 2πI. Also, set the value of point PA on the time axis to 1
1, point PA is 1i4 of modulated wave SC of frequency I
Since it is a period, tl=1/4/. Therefore, E=ω・t1=(2πI)・(1/4/)=π/2
(3) For easy explanation, if the amplitude of the modulated wave SC is 1, and the value of the point where a straight line drawn from the apex of this modulated wave SC with an inclination ω intersects the time axis t is 12, then t2 is , is expressed by the following equation.

t2=(1/4/)・(2/π) =1/2π/(4) On the other hand, since the capacitor C1 has the following relationship: Q=CV=Ic-t (Ic: current of constant current source) , Ic=C・
(V/1) holds. Therefore, the current Ic of the constant current source is calculated by the following formula: % Now, frequency of modulated wave/=1x10" (Hz) Capacity C of capacitor C1 = IX104 (F) Modulated wave SC amplitude V
If we assume that =1 (V), then i, =fi, 2aμA.

Furthermore, as can be seen from equation (6), the current value I of the constant current source 7
c is in a relationship where the frequency of the modulated wave SC is multiplied by . Therefore, the multiplier 9 in FIG. 1 multiplies the digital signal DA corresponding to the frequency l and adds it to the DAC 8, so that equation (6) holds true.

Note that when the frequency I of the modulated wave SC is changed, the slope ω of the tangent at the point P^ explained in FIG. 2 also changes, so
In order to perform the above operation, it is necessary to also change the current value Ic of the constant current source 7 according to the frequency I.9 In the present invention, the digital signal DA indicating this frequency I is input to the DAC.
8 converts it into an analog signal and controls the constant current value Ic, so the constant current source 7 automatically changes as the frequency I changes.
The current value 1c also changes so as to satisfy the above-mentioned conditions, thereby preventing the occurrence of amplitude modulation distortion.

Furthermore, although the discharge gradient of the capacitor C1 has been described above as the gradient of the tangent at point PA in FIG. 2, the present invention is not limited to this (gradient ω).

That is, in FIG. 6, it is clear that even when the slope of the discharge is larger than ω (waveform indicated by a dashed-dot line), the distortion of the waveform can be significantly improved compared to the conventional method (waveform indicated by a dotted line).

[Effects of the present invention]

As described above, according to the present invention, the distortion of the detected waveform applied to the ALC amplifier 3 can be improved compared to the conventional one, so the distortion of the amplitude-modulated waveform (the waveform of the output terminal 21) can also be improved.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an embodiment of an amplitude modulation circuit according to the present invention,
FIG. 2 is a diagram showing the charging voltage waveform of capacitor C1 in the circuit shown in FIG. 1, FIG. 3 is a diagram showing how the detected waveform is distorted in the conventional circuit, FIG. 4 is a diagram showing the conventional example, and FIG.
The figure shows a modulated carrier wave and capacitor voltage in a conventional circuit, and FIG. 6 is a diagram comparing the capacitor charging voltage waveforms of the conventional example and the present invention. DESCRIPTION OF SYMBOLS 1... Modulator, 3... ALC amplifier, 5... Signal generator, 7... Constant current source, 8... DAC. Figure 2 A (Yu 5 M month π 3 diagram

Claims (1)

[Claims] A modulator (1) that modulates the amplitude of a carrier wave using an output signal of an ALC amplifier (described later), and a detection diode that extracts the peak voltage of the amplitude modulation signal based on the output of this modulator and charges a capacitor (C1). (D1), a signal generator (5) that introduces the digital signal DA and outputs a modulated signal of a frequency corresponding to the digital signal DA, and a charging voltage signal of the capacitor (C1) and a modulated signal of the signal generator (5). An amplitude modulation circuit comprising: an ALC amplifier (3) that introduces a signal based on the digital signal DA and outputs an error signal; a DA converter (8) that introduces a signal based on the digital signal DA and converts it into an analog signal; An amplitude modulation circuit comprising: a constant current source (7) that applies an output current to C1) and whose output current level is controlled by the output signal of the DA converter.