JPS6333825B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high-precision double-balanced modulator that can be easily fabricated into a monolithic IC.

Double-balanced modulators are widely used in wireless communications to perform operations such as signal multiplication and frequency conversion. Monolithic ICs are an effective way to downsize wireless devices, but
Ring modulators, which have traditionally been used as double-balanced modulators, use coils, so they are not monolithic.
Conversion to IC was difficult. An example of a conventional configuration of a double-balanced modulator using digital circuit elements that has improved this point is shown in FIG. In FIG. 1, 1 is a modulation signal input terminal, 2 is a carrier wave input terminal, 3 is a D/A converter,
4 is a modulation switching circuit, and 5 is a modulated wave output terminal.

To operate this circuit, a modulated input signal C(t) is applied to the input terminal 1 as digital signal data, and a carrier wave cosω _c t is applied to the carrier wave input terminal 2. The D/A converter 4 outputs an analog signal.
It is assumed that the circuit has a first analog output C and a second analog output complementary thereto, and outputs C(t) and -C(t), respectively. These two analog outputs are alternately selected and output by a modulation switching circuit 4 controlled by a carrier wave obtained from the input terminal 2. As a result, the output terminal 5 has C(t)・cosω _c
A balanced modulated wave with t as the fundamental wave component is obtained.

However, in the actual circuit, the D/A converter 3
A DC offset voltage is generated between the two analog outputs of the modulated wave, so that a carrier wave component is superimposed on the modulated wave. In particular, when performing angle modulation with a constant envelope using a quadrature modulator, when a carrier component is superimposed on a modulated wave, envelope fluctuation occurs. If this signal is passed through a saturation type amplifier, it will generate an out-of-band spectrum, causing interference to adjacent channels, which poses a problem.

In order to prevent this carrier wave component from being superimposed, the D/A
It is necessary to reduce the offset voltage between the output terminals C and C of the converter to zero, which requires the use of a high-precision D/A converter or adjustment to reduce the offset voltage to zero. Ta.

In order to improve this drawback, the present invention provides a mode in which the carrier wave component superimposed on the output of the balanced modulator is in positive phase and a mode in which it is in reverse phase, and by using these two modes alternately. The carrier wave components are averaged and canceled, and the purpose is to provide a double-balanced modulator that requires no adjustment and can be easily fabricated into a monolithic IC.

FIG. 2 shows an embodiment of a double-balanced modulator according to the invention. This circuit is obtained by adding a clock input terminal 6, a digital signal switching circuit 7, a complement circuit 8, a carrier switching circuit 9, and an inverter 10 to the circuit shown in FIG.

In this circuit, the modulated input signal C(t) applied to the input terminal 1 as digital signal data is branched into two, one of which is inputted as is to the digital signal switching circuit 7, and the other is the two parts of the input data. -C by the complement circuit 8 which outputs the complement.
After being converted into data (t), it is input to the digital signal switching circuit 7. Further, the carrier wave cosω _c t applied to the input terminal 2 is also branched, and one side is inputted to the carrier wave switching circuit 9 as is, and the other side is inputted to the switching circuit 9 after its phase is inverted by the inverter 10 . On the other hand, a clock having a frequency n times the highest frequency of the baseband signal (n≧2) is input to the clock input terminal 6, thereby controlling the digital signal switching circuit 7 and the carrier switching circuit 9. These two switching circuits operate synchronously,
The following two modes are alternately switched every clock.

Mode: Input terminals A and E are selected.

Mode: Input terminal is selected.

The data obtained from the output terminal C of the digital signal switching circuit 7 and the carrier wave obtained from the output terminal G of the carrier switching circuit 9 are input to a conventional double-balanced modulator consisting of a D/A converter 3 and a switching circuit 4. After being subjected to balanced modulation, the signal is outputted to the output terminal 5 via the bandpass filter 11.

When the switching circuit is in mode, this circuit
It operates exactly the same as the circuit shown in the figure. At this time, if the offset voltage difference between the D/A converter 3 and the analog output terminal C is δ, the output modulated wave is
V _I (t) becomes V _I (t)=C(t)・cosω _c t+δ・cosω _c t (1). The second term in the above equation represents the carrier component. The modulated wave V〓(t) in one mode is V〓(t)=(-C(t))・(-cosω _c t)+δ(-c
osω _c
t) ...(2), and the balanced modulation wave component is in phase with respect to equation (1), but the carrier wave component is in opposite phase. Therefore, if the modes are used alternately and the outputs are averaged, the carrier component will be canceled and an ideal balanced modulated wave will be obtained. The bandpass filter 11 has the function of averaging the output, and its center frequency must be equal to the carrier wave frequency, and its bandwidth must be less than twice the mode switching frequency.

In this way, the carrier wave components are averaged and canceled, so even if the accuracy of the D/A converter is not good, a high-precision double-balanced modulator suitable for monolithic IC implementation can be realized with bright adjustment. be able to.

FIG. 3 shows an embodiment in which the double-balanced modulator according to the present invention is applied to a quadrature modulator. In the figure, 1 and 12 are modulation signal input terminals, 2 is a carrier wave input terminal, 6 is a clock input terminal, and 13 is a 90° phase shifter. 7 and 14 are digital signal switching circuits;
8 and 15 are complement circuits, 3 and 16 are D/A converters, 4 and 17 are modulation switching circuits, 9 and 18 are carrier wave switching circuits, and 10 and 19 are inverters, among which the same numbers as in Fig. 2 are used. 8, 7, with
3, 4, 9, and 10 constitute a high-precision double-balanced modulator for the in-phase component, and the remaining 15, 14, 16, and 1
7, 18, and 19 constitute a high-precision double-balanced modulator for orthogonal components. 20 is a combiner that superimposes the in-phase component and the orthogonal component, 11 is a band pass filter, and 21 is a modulated wave output terminal. The quadrature modulator is generally effective for angle modulation systems, but in order to specifically describe the problems and features of the present invention, a case will be described below in which a phase modulated wave with a constant envelope is generated.
In order to operate the circuit shown in Figure 3, input terminal 1
Modulated input signal data cosφ(t) and −sinφ(t) are input to input terminals 2 and 6, respectively.
A carrier wave and a clock similar to those in the circuit of FIG. 2 are respectively input to the circuits. Modulated input signal cosφ(t) added to the input terminal as digital signal data
and the in-phase carrier wave cosω _c t applied to input terminal 2
is multiplied by a high-precision double-balanced modulator for the in-phase component and input to the input terminal 1 of the synthesizer 20. Similarly, a modulated input signal −sinφ(t) applied to the input terminal 12 as digital signal data,
The orthogonal carrier wave sinω _c t obtained by passing the carrier wave through the 90° phase shifter 13 is multiplied by a high-precision double-balanced modulator for the orthogonal component, and is then applied to the input terminal Q of the combiner 20.
is input. The two signals applied to the input terminals I and Q are superimposed by the combiner 20 and then output to the output terminal 21 via the bandpass filter 11, and a phase modulated wave cos(ω _c t + φ(t)) is obtained. It will be done.

In this circuit, the switching circuits 7, 14, 9, and 18 alternately switch between the following two modes in synchronization with the clock input to the input terminal 6, and average the carrier wave components in the same manner as in the case of Fig. 2. It has been cancelled.

Mode: Input terminals A, B, E, F are selected.

Mode: Input terminal, , is selected.

Now, if the offset voltage between the output terminal C of the D/A converter 3 is δ _I and the offset voltage between the output terminal S of the D/A converter 16 is δ _Q , then the modulated wave in the mode is As shown in FIG. 4A, the vector locus of amplitude and phase becomes a circle whose center is shifted due to the carrier wave component, and envelope fluctuation occurs. On the other hand, the vector locus in the case of mode is shown in Figure 4b, and if the modes are used alternately and the output is averaged, the carrier components cancel each other out, and an ideal modulated wave shown by the dotted line c in the figure is obtained. .

In this way, even if the accuracy of the D/A converter is poor, a highly accurate quadrature modulator can be realized without adjustment.

As explained above, according to the present invention, it is possible to realize a high-precision double-balanced modulator that can be easily fabricated into a monolithic IC without any adjustment.・It can greatly contribute to economicization.

[Brief explanation of the drawing]

Figure 1 shows an example of the configuration of a conventional double-balanced modulator;
The figure shows an example of the configuration of a modulator according to the first embodiment of the present invention, FIG. 3 shows an example of the configuration of a modulator according to the second embodiment of the present invention, and FIG. 4 shows an example of a vector locus in the circuit of FIG. 3. It is. 1 and 12...Modulation signal input terminal, 2...Carrier wave input terminal, 3 and 16...D/A converter, 4 and 17...Modulation switching circuit, 5...Modulation wave output terminal, 6...Clock Input terminals, 7 and 14...Digital signal switching circuit, 8 and 15...Complement circuit, 9 and 18...Carrier switching circuit, 10 and 1
9...Inverter, 11...Band pass filter,
13...90° phase shifter, 20... combiner, 21...
Modulated wave output terminal.

Claims (1)

[Claims] 1. A modulation signal input terminal that receives a digital signal that is a modulation input signal, a digital signal switching circuit that alternately selects the modulation input signal and its two's complement, and a digital signal switching circuit that converts the output of the switching circuit into an analog signal and mutually a digital-to-analog converter that generates a set of analog outputs that are complementary to each other; a carrier wave input terminal that receives a carrier wave; and a carrier wave switching circuit that alternately selects the carrier wave and a waveform that is in an inverted phase relationship with the carrier wave. , one of the digital-to-analog converters
a modulation switching circuit that alternately switches the output of the pair every half cycle according to the output of the carrier switching circuit to output a balanced modulated wave; a filter that is connected to the output of the circuit and removes unnecessary waves outside the band; and its output. a first mode comprising a modulated wave output terminal connected to the digital signal switching circuit and a clock signal means for controlling switching of the digital signal switching circuit and the carrier switching circuit, and performing modulation using the modulation input signal and the input carrier wave; A high-precision double-balanced modulator, characterized in that a second mode in which modulation is performed using a two's complement of a modulated input signal and a negative phase wave of an input carrier wave is periodically and alternately switched by the clock signal means.