JP3198340B2 - Amplitude modulation circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an amplitude modulation (AM) circuit.

[0002]

2. Description of the Related Art In a conventional amplitude modulation circuit, as shown in FIG. 2, a modulated wave Sam is obtained by mixing a carrier wave Sc and a modulated wave Ss in a mixing circuit A.

That is, the carrier wave Sc and the modulated wave Ss are respectively expressed as Sc = Ec · sinωt (Es is amplitude, ω is angular velocity, t is time) (1) Ss = Es · sinpt (Ec is amplitude, p is angular velocity, t is (2), the modulated wave Sam is expressed as: Sam = Ec (1 + m · sinpt) · sinωt = Ec · [sinωt + (m / 2) · {cos (ω−p) t−cos (ω + p ) t}] (3) (where m is the modulation factor and m = Es / Ec).

When the modulation degree m is changed, the magnitude of the amplitude Es of the modulated wave Ss is changed by, for example, adjusting the gain of the amplifier B with respect to the modulated wave Ss.

[0005]

As described above, in the mixing circuit A, the non-linearity of an active element such as a transistor or a diode is used to generate the modulated wave Sam. When the amplitude Es of the modulated wave Ss is changed to change the modulation degree m, it is necessary to change the operating points of these active elements accordingly. However, when the operating point of the active element is changed as described above, the temperature characteristics are deteriorated, and the stability is lacking due to aging. In addition, there is a problem that the modulation distortion changes depending on the modulation degree m.

[0006]

DISCLOSURE OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and has been made in order to be able to adjust the degree of modulation without changing the amplitude of a modulated wave, and to have excellent temperature characteristics and aging. It is intended to obtain a stable amplitude modulation circuit which is less affected by the modulation degree and whose modulation distortion is not affected by the degree of modulation.

Therefore, in the amplitude modulation circuit of the present invention, a mixing circuit that inputs both a carrier wave and a modulation wave each having a constant amplitude and outputs a modulated wave having a certain degree of modulation, and an output from the mixing circuit. An adder circuit for adding the modulated wave and the carrier at a predetermined impedance ratio and outputting the result.

[0008]

In the above arrangement, since a carrier wave and a modulated wave having a constant amplitude are input to the mixing circuit, a modulated wave having a fixed modulation factor is output from the mixing circuit. In other words, since the degree of modulation is constant, there is no need to change the operating point of the active element forming the mixing circuit, and the linearity of the modulation is excellent. The adding circuit adds the modulated wave and the carrier output from the mixing circuit with a predetermined impedance ratio. Since the modulation factor of the signal output from the adding circuit is uniquely determined by the impedance ratio, a desired modulation factor can be obtained by adjusting the impedance ratio.

[0009]

FIG. 1 is a configuration diagram of an amplitude modulation circuit according to an embodiment of the present invention.

In FIG. 1, reference numeral 1 denotes an entire amplitude modulation circuit, and reference numeral 2 denotes a carrier having a constant modulation degree m by inputting both a carrier wave Sc having a constant amplitude Ec and a modulation wave Ss having a constant amplitude Es. A mixing circuit 3 for outputting the modulated wave Sam, and an adder circuit 3 for adding the modulated wave Sam and the carrier wave Sc output from the mixing circuit 2 with a predetermined impedance ratio and outputting the result.
In this example, two variable resistors 4 ₁ and 4 ₂ are connected in series, and a middle point O of both variable resistors 4 ₁ and 4 ₂ is a signal output terminal.

Next, the operation of the amplitude modulation circuit 1 having the above configuration will be described.

For the mixing circuit 2, each amplitude Ec,
Since both the carrier wave Sc and the modulated wave Ss having a constant Es are input, the modulated wave Sam having a constant modulation degree m (= Es / Ec) is output from the mixing circuit 2. As described above, since the modulation degree m in the mixing circuit 2 is constant, the operating point of the active element such as the transistor or the diode constituting the circuit 2 is fixed without having to change the operating point, and the modulation distortion is minimized. Can be adjusted.

The adder 3 adds the modulated wave Sam and the carrier Sc output from the mixer 2 with a predetermined impedance ratio. That is, the impedances of the variable resistors 4 ₁ and 4 ₂ of the addition circuit 3 are Z ₁ ,
And Z _2, when the output signal of the variable resistor 4 _1, 4 ₂ midpoint O and S, S = Sc + (Sam -Sc) · {Z 2 / (Z 1 + Z 2)} and comprising (4).

Here, Sc is calculated according to the above equation (1).
Since m is given by equation (3), equation (4) is given by: S = Ec · [sinωt + (m / 2) · {Z ₂ / (Z ₁ + Z ₂ )} · {cos (ω−p) t −cos (ω + p) t}]
(5), and the degree of modulation at this time is given by m · Z ₂ / (Z ₁ + Z ₂ ).

Therefore, the degree of modulation of the output signal S is
Since m is constant, the impedance ratio Z ₂
By adjusting / (Z ₁ + Z ₂ ), a desired degree of modulation can be obtained.

[0016]

According to the present invention, the modulation degree is changed by adjusting the impedance ratio in the adding circuit instead of changing the amplitude of the modulated wave as in the prior art. May be constant. Therefore, it is not necessary to use the nonlinearity of the active elements of the mixing circuit. Therefore, it is possible to obtain a stable amplitude modulation circuit having excellent temperature characteristics, little change over time, and without affecting the modulation distortion by the modulation factor.

In particular, in the present invention, it is necessary that both the amplitude of the carrier wave and the amplitude of the modulated wave input to the mixing circuit are both constant, so that they are preferably used in a signal generator or the like rather than a radio receiver or the like. be able to.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of an amplitude modulation circuit according to an embodiment of the present invention.

FIG. 2 is a configuration diagram of a conventional amplitude modulation circuit.

[Explanation of symbols]

 1. Amplitude modulation circuit, 2. Mixing circuit, 3. Addition circuit.

Claims (1)

(57) [Claims] 1. A carrier wave (Sc) and a modulated wave (Sc) each having a constant amplitude.
s) and a modulated wave (Sam) having a certain degree of modulation.
A mixing circuit (2) for outputting a modulated wave (Sm) output from the mixing circuit (2) and the carrier wave (Sc) with a predetermined impedance ratio and outputting the result. An amplitude modulation circuit comprising: