JPS5868349A - High-precision double balanced modulator - Google Patents

Abstract

PURPOSE:To perform monolithic IC-implementation easily without adjustment by utilizing two different modes wherein a carrier component superposed upon the output of a balanced modulator is in phase and out of phase, and averaging the carrier component. CONSTITUTION:A modulated input signal c(t) applied as digital data to an input terminal 1 is divided into two; one is inputted to a digital signal switching circuit 7, and the other is converted by a complement circuit 8 into data -c(t), which is inputted to the circuit 7. A carrier applied to an input terminal 2 is also branched; one is inputted to a carrier switching circuit 9, and the other is inverted by an inverter 10 and applied to the circuit 9. Those circuits 7 and 9 operates synchronizing with each other and are placed in two modes (Iand II) alternately. The data obtained by processing data from output terminals C and C' of the circuit 7 through D/A conversion 3 and a carrier obtained from the circuit are inputted to a switching circuit 4 for modulation, where balanced modulation is imposed, thereby sending the resulting output to a terminal 5 through a BPF11. Thus, the modesIand II are used alternately to average the output, obtaining an ideal wave of balanced modulation.

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.

Making circuit components monolithic is an effective way to downsize wireless devices, but ring modulators, which have traditionally been used as double-balanced modulators, use coils, so making them monolithic is difficult. there were. 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 modulated 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, the modulated input signal C is input to input terminal 1.
(t) is added as digital signal data, and a carrier wave cosω is input to the carrier wave input terminal 2. Add t.

It is assumed that the D/A converter 4 has a first analog output C and a complementary second analog output C as analog signal outputs, 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 the carrier wave obtained from the input terminal 2. As a result, the output terminal 5 has a C
A balanced modulated wave having a fundamental wave component of (t)·ωSωct is obtained.

However, in an actual circuit, a DC offset voltage occurs between the two analog outputs of the D/A converter 30, and therefore 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 and cause interference to adjacent channels, which is a problem.

In order to prevent this carrier wave component from being superimposed, it is necessary to make the offset voltage between the output terminals C1 and C of the D/A converter zero. Therefore, a high-precision D/A converter must be used, or the offset voltage can be reduced by adjusting the The disadvantage was that it had to be reduced to zero.

In order to improve these drawbacks, 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 (2) in which it is in negative 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 parts, one of which is input as is to the digital signal switching circuit 7, and the other is the two parts of the input data. After being converted into -c (t) data by the complement circuit 8 which outputs the complement, the data is input to the digital signal switching circuit 7. Also, a carrier wave cosω applied to input terminal 2. t is also branched, one is inputted as is to the carrier wave switching circuit 9, and the other 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 (n≧2) the highest frequency of the baseband signal is input to the clock input terminal 6, thereby controlling the digital signal switching circuit 7 and the carrier wave switching circuit 9. These two switching circuits operate synchronously and alternately switch between the two modes shown below every clock.

Mode I: Input terminals A and E are selected.

Mode ■: Input terminals A and E are 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
The signal is inputted to a conventional double-balanced modulator consisting of a D/A converter 3 and a switching circuit 4, subjected to balanced modulation, and then outputted to an output terminal 5 via a bandpass filter 11.

When the switching circuit is in mode (2), this circuit operates exactly like the circuit of FIG. At this time, D/A converter 3
If the offset voltage difference between the analog output terminal C and 0 of is δ, the output modulated wave vI(t) is V■(t) = C(t) ・cosωct+δ−co
sω. t-(+1.The second term in the above equation represents the carrier wave component.

On the other hand, the modulated wave VH(t) in mode ■ is VII(tl
= (-Qt)) ・(-ωsωc t ) ten δ(-
CO5(IJct)=12), 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 Mode I and Mode ■ are used alternately and the outputs are averaged, the carrier wave component is canceled and an ideal balanced modulated wave can 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 I and H switching frequency.

Since the carrier wave components are averaged and eliminated in this way, the accuracy of the D/A converter is improved (even if there is no adjustment, a high-precision double-balanced modulator suitable for monolithic ICs can be realized without adjustment). can do.

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 1
6 is a D/A converter, 4 and 17 are modulation switching circuits, 9 and 18 are carrier wave switching circuits, and 10 and 19 are inverters, among which 8 and 7 are given the same numbers as in FIG.
, 3 , 4 , 9 , 10 constitute a high-precision double-balanced modulator for the in-phase component, and the remaining 15 , 14 , 16 , 1
7, 18, and 19 constitute a high-precision double-balanced modulator for quadrature components. 11 is a band pass filter, and 21 is a modulated wave output terminal. The orthogonal 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 terminals 1 and 1 are required.
The modulated input signal data of cosφ(tL-sinφ(1)) is input to input terminals 2 and 2, respectively, and the carrier wave and clock similar to the circuit in FIG. 2 are input to input terminals 2 and 6. Digital signal data is input to input terminal A modulated input signal cosφI) applied as a modulated input signal cosφI) and an in-phase carrier wave cosω applied to input terminal 2. t is multiplied by a high-precision double-balanced modulator for the in-phase component and is input to input terminal 1 of the combiner. Similarly, the modulated input signal -5inφ(1) applied as digital signal data to the input terminal 12 and the orthogonal carrier wave s obtained by passing the carrier wave through the 90° phase shifter 13
inωQ1 is multiplied by a high-precision double-balanced modulator for orthogonal components and input to the input terminal Q of the combiner. The two signals applied to input terminals I and Q are superimposed by a synthesizer and then passed through a bandpass filter 11 to an output terminal 21.
Phase modulated wave cos (ωct+φ(t))
is obtained.

In this circuit, switching circuits 7, 14, 9.18 are connected to input terminal 6.
The following two modes are alternately switched in synchronization with the clock input to the carrier wave component, and the carrier wave components are averaged and canceled in the same manner as in the case of FIG.

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

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

Now, if the offset voltage between the output terminals C9C of the D/A converter 3 is δr, and the offset voltage between the output terminals S and S of the D/A converter 16 is δQ, then the amplitude of the modulated wave in mode ■ As shown in FIG. 4(a), the vector locus of 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 trajectory in the case of mode ■ is shown in Figure 4 (b), and when mode I and mode ■ are used alternately and the outputs are averaged, the transport and transport components cancel each other out, as shown by the broken line (C) in the figure. An ideal modulated wave can be obtained.

In this way, even if the accuracy of the D/A converter is not good, 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 adjustment, so when used in a quadrature modulator etc., it is possible to reduce the size of wireless equipment.・It can make a significant contribution to economicization.

[Brief explanation of drawings]

FIG. 1 is a configuration example of a conventional double-balanced modulator, FIG. 2 is a configuration example of a modulator according to a first embodiment of the present invention, and FIG. 3 is a configuration example of a modulator according to a second embodiment of the present invention. Configuration example, Figure 4 is the 3rd
This is an example of a vector locus in the circuit shown in the figure. 1 and J2...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 terminal, 7 and 14... Digital signal switching circuit, 8 and 15
...Complement circuit, 9 and 18...Carrier switching circuit, 10 and 19...Inverter, 11...Band pass filter, 13...900 phase shifter, 20...Synthesizer,
21...Modulated wave output terminal. Patent applicant Nippon Telegraph and Telephone Corporation

Claims (1)

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