JP2927164B2 - Phase modulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a phase modulator, and more particularly to a phase modulator used for digital satellite communications and mobile communications.

[0002]

2. Description of the Related Art FIG. 5 shows a conventional four-phase modulator used in digital satellite communications and digital mobile communications. The digital data to be communicated is converted from serial data into two parallel signals of Ich and Qch by a serial / parallel converter 1 and then to desired characteristics such as Nyquist band limitation by table look-up filters (TLF) 2a and 2b. Bandwidth is limited. Thereafter, digital-to-analog conversion is performed by the D / A converters 4a and 4b, and based on this signal, the local signals (L
O) is modulated. Here, the local signal applied to the Ich mixer 5a and the local signal applied to the Qch mixer 5b have a 90 ° phase difference due to the 90 ° hybrid 6. The modulated signal is synthesized by the hybrid 7 and output as a phase modulated wave such as QPSK, π / 4 shifted QPSK, or the like.

Next, a description will be given of a phase modulator shown in FIG. 6 (JP-A-62-194766) as an example of an improvement of the phase modulator of FIG. The modulated signal synthesized by the hybrid 7 is amplified by the amplifier 16 and output through the variable attenuator 17, the amplifier 18, and the attenuator low-pass filter 19. The output level of a part of the output of the amplifier 18 is converted to a DC voltage through a detection circuit 20,
1 is input. The output of the voltage comparator / integrator 21 is fed back to the variable attenuator 17 so as to suppress the fluctuation of the output level.

[0004]

In the conventional phase modulator shown in FIG. 5, when the amplitude characteristics and the phase characteristics of the mixer and the 90 ° hybrid fluctuate due to temperature fluctuations and local frequency fluctuations, the modulation is performed as it is. There is a disadvantage that the signal is output as a signal. Since digital satellite communication is required to operate at a low C / N, there is a very strict requirement that an amplitude error of Ich and Qch be ± 0.1 dB as an amplitude characteristic and 90 ° ± 2 ° as a phase characteristic. Further, mobile communication is required to operate in a very wide temperature range from -25 ° C to + 60 ° C for use outdoors. Furthermore, in recent years, direct modulation in the microwave band has been increasing due to miniaturization, and the characteristics required for the phase modulator have become more and more severe. However, the conventional example shown in FIG. 5 did not satisfy this requirement.

If the mixer characteristics are not linear, distortion occurs. If the output does not become zero when there is zero data (there is an offset), clock leakage occurs and the modulation characteristics deteriorate in either case.

The method of FIG. 6 is improved over the method of FIG. 5 in that the output level is suppressed from fluctuating.
There is a disadvantage that the amplitude error and the phase error of ch and Qch, and the problems of non-linearity and cross leak all remain.

[0007]

The phase modulator according to the present invention comprises:
An output signal of the table lookup filter, temperature information, and frequency information are input as address information between the conventional table lookup filter and the D / A converter, and output to the D / A converter. Data table ROM storing corrected data
A temperature detecting circuit comprising a thermistor operational amplifier, and an A / D converter for converting the output from analog to digital and supplying it as address information.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing a phase modulator according to one embodiment of the present invention. The digital signal is a serial-parallel converter 1
Are converted into two-channel 8-bit signals of Ich and Qch, and input to the table look-up filters (TLFs) 2a and 2b, respectively. In the table look-up filters 2a and 2b, the Nyquist band is limited and the output is stored in the data table ROMs 3a and 3b.
Is input to Data table ROM is Ich, Q
and 8 bits of data and 4 bits of temperature information as input signals, and a total of 16 bits of 4 bits of frequency.
It has 8 bits and outputs 8-bit data as output data.

FIG. 2 shows the structure of the data table ROM. As shown in the figure, the ROM is a 16-bit address bus and an 8-bit data bus. FIG. 3 shows this data table R
OM characteristics are shown. FIG. 3 shows a function of input data and output data when the temperature information bit is changed. In this embodiment, the temperature is in increments of 5 ° C. and 16 kinds (4 bi
The data of t) is stored. FIG. 3 shows a part of them. Mixer 5 when temperature changes
a, 5b and the characteristics of the 90 ° hybrid 6 change, and the gain and phase change.

FIG. 4 is a diagram showing the state of modulation before data correction. At 35 ° C., the gain is reduced and the offset (remaining modulation error) is reduced at 25 ° C. as compared with the case of 25 ° C. It has occurred. As shown in FIG. 3, the data table ROM stores data for correcting the characteristics at 35 ° C., so that the overall characteristics are such that the characteristics always remain constant even when the temperature changes. The effect of the temperature is cancelled.

[0011] Similarly, frequency data is input from the bits of the frequency information, and the modulation characteristics when the local frequency changes are corrected. The data corrected in this way is converted from digital to analog by the D / A converters 4a and 4b, and the local signals are modulated by the mixers 5a and 5b.
The signals of the channels are combined to form a modulated output. The local signal from the synthesizer 15 is input to the mixers 5a and 5b with a 90 ° phase difference by the 90 ° hybrid 6.

The temperature information bits for the data tables ROM 3a and 3b are used to amplify the fluctuation of the resistance value of the thermistor 10 with the operational amplifier 9 and to use the A / D converter 8 to set data when the analog / digital processor 14 sets the frequency of the synthesizer 15. We are using.

The data table ROMs 3a and 3b and the table look-up filters 2a and 2b are shown separately in FIG. 1 for the sake of explanation. However, since the table look-up filters 2a and 2b normally use ROMs, It is also possible to realize the same ROM as the table ROMs 3a and 3b.

[0014]

As described above, according to the present invention, by providing the data table ROM, the influence of the clock rook due to the amplitude error and phase error of Ich and Qch, the distortion due to the non-linearity and the offset due to the temperature fluctuation and the frequency fluctuation. It is possible to cancel all. Further, a high-precision phase modulator can be configured with a smaller number of parts, and is very effective for portable or on-vehicle satellite communication equipment and mobile communication equipment that require miniaturization.

[Brief description of the drawings]

FIG. 1 is a diagram of one embodiment of the present invention.

FIG. 2 is a diagram for a configuration of a data table ROM.

FIG. 3 is a diagram showing characteristics of a data table ROM.

FIG. 4 is a diagram for explaining that characteristics change with temperature when no correction is performed.

FIG. 5 is a diagram of a conventional phase modulator.

FIG. 6 is a diagram of a conventional example in which the conventional phase modulator of FIG. 5 is improved.

[Explanation of symbols]

1 serial / parallel converter 2a, 2b table lookup filter (T
LF) 3a, 3b Data table ROM 4a, 4b D / A converter 5a, 5b Mixer 6 90 ° hybrid 7 Hybrid 8 A / D converter 9 Operational amplifier 10 Thermistor 11, 12, 13 Resistor 14 Microprocessor 15 Synthesizer 16, 18 Amplifier 17 variable attenuator 19 attenuator low-pass filter 20 detection circuit 21 voltage comparison / integrator

Claims (1)

(57) [Claims] 1. A table look-up filter for limiting a band of a modulated digital signal;
A phase modulator comprising: a D / A converter for digital-to-analog conversion of an output of a look-up filter; and a mixer for receiving a local signal and performing modulation by an output of the D / A converter. The phase provided between the filter and the D / A converter and stored in advance based on the output of the table lookup filter and the temperature information.
Information on the temperature fluctuation characteristics and frequency fluctuation characteristics of the modulator itself
Table RO for outputting correction data based on information
M and a data table R in the D / A converter.
A phase modulator to which an output of an OM is input, wherein the data table ROM further receives frequency information as address information.