JPH06237281A - Carrier leak adjusting circuit - Google Patents

Abstract

PURPOSE:To provide the carrier leak adjusting circuit which performs adjustment without inputting actual data to an quadrature modulator by inputting digital data, whose value is closest to the center value of input data, to a D/A converter. CONSTITUTION:ROMs 1 and 2 of FFs are provided in preceding stages of D/A converters 11 and 21, and input digital data D1 and D2 are outputted to converters 11 and 21 as they are by an external switching signal C at the time of normalcy. At the time of maintenance, output data to converters 11 and 21 are as follows independently of input data D1 and D2; 8-bit data of the center value in a prescribed range of converters 11 and 21 is always outputted or 8-bit data '10000000' of value 128 or '011111111' of value 127 closest to center value 127.5 is always outputted if the input range is 0 to 255 of 8 bits. As the result, the simple carrier leak adjusting circuit is constituted which performs adjustment without inputting actual data to the quadrature modulator.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of constructing a carrier leak adjusting circuit for reducing so-called carrier leak during maintenance of a quadrature modulator of ASK type or QAM type which is a kind of amplitude modulation of digital multiplex radio.

[0002]

2. Description of the Related Art FIG. 5 shows a circuit configuration of a conventional QAM (quadrature amplitude modulation) quadrature modulator. Input I channel
D / A converter that converts two series of Q-channel baseband digital data into positive and negative analog signals, respectively
1,21 and the baseband amplifiers 12 and 22 of the operational amplifier for amplifying each converted analog signal to a predetermined level,
High-cut roll-off filters 13 and 23 for limiting the occupied band of each output, a local oscillator 3 for generating an unmodulated carrier signal, a carrier signal at the output of the local oscillator, and the roll-off filter. Mixers 14 and 24 for amplitude modulation by multiplying the outputs of 13 and 23 and outputs of the mixers 14 and 24 are combined in mutually orthogonal phases (90 ° phase difference) to obtain a QAM modulated wave output. It consists of a hybrid 4 and.
The root cause of the so-called carrier leak that causes deviation of the orthogonality (90 °) between the I channel component and the Q channel component of the QAM modulated wave output is the DC component of each analog output of the D / A converters 11 and 21. Is. If the degree of the DC component of the carrier leak is large, the pull-in characteristic of the PLL of the carrier regenerating circuit of the QAM demodulator on the receiving side may be deteriorated, which may adversely affect the modulation / demodulation characteristic of the digital multiplex radio. . Conventionally, in order to reduce carrier leakage during maintenance of the quadrature modulator, by adjusting the so-called offset voltage on the negative input side of each operational amplifier of the baseband amplifiers 12 and 22, the D / A converters 11 and 21 are adjusted. DC of each output of
It absorbed the amount and reduced the carrier leak.

[0003]

However, in the above-mentioned conventional carrier leak mitigation method, the baseband amplifier is used.
Adjustment of each operational amplifier of 12,22 becomes a fairly fine adjustment,
The carrier leak cannot be reduced easily. Further, even if the carrier leak is adjusted to the minimum during the test adjustment of each modulator, when the modulator is actually mounted and used in the transmitter, the carrier leak becomes large due to the temperature characteristics of the components. Sometimes Therefore, the conventional adjustment of carrier leak is to input the actual digital data into the quadrature modulator, modulate it, output the modulated wave, and observe the modulated wave output using a so-called spectrum analyzer, etc. It was a method of adjusting each 12,22 operational amplifier. That is, the output of the modulated wave was swept in time and observed in a band shape.
It is an object of the present invention to realize a simple carrier leak adjusting circuit which can adjust carrier leak without inputting actual digital data into the quadrature modulator during maintenance.

[0004]

The basic structure of the present invention for achieving this object is based on the following concept. Modulation is performed by always supplying constant data of the center value of the positive and negative boundaries as digital data of the inputs of the D / A converters 11 and 21 that output positive and negative analog signals without inputting actual changing digital data to the modulator. When observing the wave output, only one wave of the carrier signal output from the local oscillator 3 is observed. Here, the central value of the inputs of the D / A converters 11 and 21 is the central value 127 of the decimal number 0 to 255 if the input is, for example, 8-bit digital data.
Note that it is either 10000000, which is the value closest to 5, or 01111111, which is 127. Therefore, in front of the D / A converters 11 and 21, the external switching signal C
Depending on the input data D _1, D ₂ of the specified number of bits as it is or data closest to the center value of the input data D _1, D ₂ is selected and output Sometimes, the digital data D _1, D _{2 of the} input from the ROM _{1, 2}
Is output as it is, and at the time of maintenance, the digital data closest to the center value of the input digital data D _{1 and} D ₂ is output.

[0005]

According to the present invention, the input of the D / A converters 11 and 21 has an 8-bit 100-value which is the closest to the center value 127.5 of the center value of the input data, for example, 0-255 if the input data is 8-bit.
By inputting the digital data of 00000 or 01111111, the D / A converters 11 and 21 output the analog signal of only the minimum DC component, and the output of this DC component is observed as the modulator output. . That is, since the modulator output at this time becomes a carrier leak as it is, conversely, as the DC component is observed as the modulator output, the baseband amplifiers 12, 2
By adjusting the carrier leak by adjusting each operational amplifier in (2), the carrier leak of the quadrature modulator is minimized.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The principle diagram of FIG. 1 shows the circuit configuration of the first embodiment of the present invention as it is. Compared to the conventional example of FIG. 4, this embodiment is provided with ROM 1 and ROM 2 of a flip-flop FF in front of each D / A converter 11 and 21 of I channel and Q channel. Each ROM 1 and ROM 2 is changed by the switching signal C from the outside.
The output can be switched individually during normal operation and during maintenance.In normal times, the input digital data D _{1 and} D ₂ are output to the D / A converters 11 and 21 as they are, and during maintenance, each input data D 1 is output. D / A converter 11,21 regardless of _{1, 2}
The output data to the D / A converters 11 and 21 is the center value of the predetermined range, and if the input range is 0 to 255 of 8 bits, the value 128 is 10000000 which is the closest value to the center value 127.5. Or 127
Each 8bit data of 01111111 is always output. The actual carrier leak adjustment method is to adjust the offset voltage of each operational amplifier of the baseband amplifiers 12 and 22 while measuring the modulated wave output at this time with, for example, a power meter during maintenance with the switching signal C from the outside. Then, the power meter adjusts the offset voltage value to the minimum value. With this simple method, it is possible to adjust the carrier leak at the modulator output to the minimum. However, in the method of the first embodiment shown in FIG. 2, the DC signals are included in the analog signals output from the D / A converters 11 and 21. Input data
If D _{1 and} D ₂ are 8 bits, for example, the input range is 0 to 255.
The true median of 127.5 is 128 of 10000000 and 127 of 0
Since it is in the middle of 1111111, it is impossible to eliminate the DC component corresponding to 128−127.5 = 127.5 −127 = 0.5 from the output of D / A converters 11 and 21. Therefore, in order to eliminate the DC component of the output of the D / A converters 11 and 21 in a pseudo manner, as in the method of the second embodiment of FIG. 21 to 12
Two 8-bit data, 10000000 of 8 and 01111111 of 127, are alternately input periodically. Then, the output signal of each D / A converter 11 and 21 becomes a pseudo sine wave, and the DC component is not observed. When the carrier leak is adjusted by the method of the second embodiment, it can be adjusted more accurately than the method of the first embodiment. In addition, as a third embodiment, in the configuration of the first embodiment or the second embodiment,
By controlling the gains of the baseband amplifiers 12 and 22 at the time of turning on the power or at the time of maintenance by the switching signal C, it is possible to automatically perform the adjustment to minimize the carrier leak of the output of the quadrature modulator.

[0007]

As described above, according to the present invention, the degree of carrier leakage generated in the quadrature modulator can be easily minimized. In addition, the effect of automatically adjusting the carrier leak can be obtained at the time of turning on the power or performing maintenance.

[Brief description of drawings]

FIG. 1 is a principle diagram showing a basic configuration of a carrier leak adjusting circuit according to claim 1 of the present invention.

FIG. 2 is a diagram showing correspondence between 8-bit digital data input to the D / A converter and a decimal number for explaining the operation of the first embodiment of the present invention.

FIG. 3 is a circuit configuration diagram of an embodiment corresponding to claim 2 of the present invention.

FIG. 4 is a circuit configuration diagram of an embodiment corresponding to claim 3 of the present invention.

FIG. 5 is a block diagram of a conventional QAM quadrature modulator.

[Explanation of symbols]

1, 2 are flip-flops FF that store input data D _{1 and} D ₂ in a certain range with a predetermined number of bits of the I channel and the Q channel.
A memory ROM consisting by switching signal C from the outside, usually at the time of the data D ₁ of the _input, D ₂ as it _is, the maintenance time digital data value closest to the central value of the data D _1, D ₂ Output by switching between. 11,21 is ROM
1, the output data D ₁ of the from ROM _2, D ₂ positive and negative D / A converter for converting the analog signals, 12 and 22 baseband amplifier, 13 and 23 roll-off filter, 14, 24 the amplitude A multiplier for modulation, 3 is a local oscillator, and 4 is a hybrid that synthesizes in a quadrature phase to obtain a quadrature modulated wave output.

Claims (3)

[Claims] 1. A predetermined number of bits of digital data (D _1,
D ₂₎ the, D / A converter for converting the respective positive and negative analog signals (1
1,21) and a baseband amplifier (12,22) of an operational amplifier that amplifies each converted analog signal to a predetermined level.
, A high-frequency cut roll-off filter (13, 23) for limiting the occupied band of each amplified output, a local oscillator (3) for generating a radio frequency unmodulated carrier signal, and the local oscillator The output carrier signal and the roll-off filter (1
Mixer (14,24) that multiplies each output of (3,23) and performs amplitude modulation
In the carrier leak adjusting circuit for adjusting the carrier leak of the quadrature modulator to a minimum, which is composed of a hybrid (4) for synthesizing the outputs of the mixers 14 and 24 in mutually orthogonal phases to obtain a quadrature modulated wave output. , In front of the D / A converter (11, 21), by a switching signal (C) from the outside, the digital data (D _1, D ₂ ) of a predetermined number of input bits as it is or the digital signal of the input It has a memory ROM (1, 2) that selects and outputs the digital data closest to the central value of the data (D _1, D ₂ ). Normally, each digital data of the input from the ROM (1, 2) (D _1, D ₂ ) as it is is output to the D / A converter (11, 21), and at the time of maintenance, the digital data closest to the center value of the input digital data (D _1, D ₂ ) is output. D
Carrier leak adjustment circuit characterized by outputting to the / A converter. 2. When the quadrature modulator defined by the switching signal (C) from the outside is maintained, the memory ROM (1,
From 2), the two digital data closest to the center value of the input digital data (D _1, D ₂ ) are alternately and periodically replaced by the D / A.
2. The carrier leak adjusting circuit according to claim 1, wherein a DC component of the output of the quadrature modulated wave is eliminated by outputting to the converter (11, 21). 3. The baseband amplifier (12, 22) is rectified by a power supply voltage only when maintenance or power-on of the quadrature modulator by the switching signal (C) from the outside is performed.
The carrier leak adjusting circuit according to claim 1, wherein the carrier leak adjusting circuit automatically starts the carrier leak adjusting circuit by controlling the gain of the carrier leak adjusting circuit.