JPH118658A - Digital microwave transmitter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a control loop where a spurious wave level due to intermodulation distortion of a digital transmitter is checked without a measurement device and the spurious wave level is made constant. SOLUTION: Changeover devices 7, 8 that switch an input signal of an orthogonal modulator into a test signal, a microwave branching filter 35 that extracts a transmission output signal, a frequency converter 33 that restores the branched signal into an IF signal, detectors 39, 40 that detect a level of a signal wave in the IF signal and an IM wave, and a comparator 41 compares outputs of the detectors are added to the digital transmitter. The transmitter is provided with an IM test control circuit 25 that controls a gain of an AGC gain variable circuit 27 to receive the result of comparison and to set the IM level to be a specified level in order to control the IM level to be a constant level and incorporates a function that holds the best value just before the end of the test.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a transmitting apparatus for wirelessly transmitting an image signal by digital modulation / demodulation.

[0002]

2. Description of the Related Art QPSK, QAM, OFDM, and the like have been adopted as modulation and demodulation systems for wireless transmission of digital images. In these modulation and demodulation systems, if the circuit is non-linear, the signal is not transmitted well, but if the output level increases by about several decibels, the error correction circuit operates and it is not possible to judge the operation. On the other hand, when the output level increases by about several decibels, the level of interference with adjacent channels rapidly increases due to intermodulation distortion (IM) generated between signals. Here, when the adjacent channel is an analog modulated wave, it is particularly susceptible to interference from the digital modulated wave, and the S / N of the received demodulated signal is significantly reduced.
For this reason, in the case of the digital system, it is necessary to operate after confirming that the existing analog line is not affected. FIG. 3 shows the configuration of the prior art. Here, signal processing such as error correction and interleaving is omitted. The transmitter is
It comprises a transmission control modulation section 1 and a transmission high frequency section 3, which are connected by a coaxial cable 2. Control and status monitoring are performed between the housings via a telephone line.

[0003] Digital data from an encoder (not shown) is input to an input terminal 4 and serial / parallel converted by a serial / parallel converter 5.
The mapping is performed by the mapping circuit 6 for various modulation schemes such as QAM and OFDM. This data is modulated by quadrature modulators (9 to 13) and amplified by an amplifier 14,
Output as a 130 MHz intermediate frequency IF. This output is superimposed on the communication line signal via the duplexer 15 and transmitted to the transmission high frequency unit 3 via the coaxial cable 2. In the transmission high-frequency unit 3, even if the amount of attenuation due to the cable length changes,
A variable gain circuit 17 and an AGC control circuit 26 for controlling the variable gain circuit are provided to keep the level constant, and the output level is controlled to be constant. Further, a frequency converter 30 (FIG. 2 shows a case where two frequency conversions are performed to obtain a predetermined frequency) and a power amplifier 34 for frequency mixing with the local oscillators 28 and 29 so that this output has a specified frequency. And amplifies and outputs power. This output wave is connected to the antenna.

[0004]

Since the prior art does not detect the spectrum level to the adjacent channel,
Even if the amount of interference to the adjacent area changes, there is a risk that the signal will be output as it is. Microwave band power amplifiers have large gain changes due to changes in temperature and circuit matching conditions.
Further, since the matching condition changes depending on the channel (frequency), there is a problem that the amount of interference changes. In order to eliminate these disadvantages, the present invention provides an IM test function for a digital microwave transmitter to check an unnecessary wave level without a measuring instrument, and to provide a control loop for keeping the unnecessary wave level constant. It is intended to be assembled.

[0005]

According to the present invention, an IM test function is added to a digital microwave transmitter to achieve the above object. In the transmission control modulator,
When turned on the IM test switch, the test signal (oscillation frequency f _T is 5 MHz) comprises a switching means for switching input to the quadrature modulator, a display for displaying the IM test result, the power amplifier output in the transmission frequency portion And a filter 37, 38 for detecting a signal level and an IM level again by extracting an IF frequency by an IM monitor frequency converter for extracting a difference frequency of a local oscillator. It has detectors 39 and 40, determines whether the IM level is normal or not by comparing the detector output voltages, and displays the result on a transmission control modulator via a communication line. Also, AG
It has a means for controlling the C circuit, and has a function of holding the good set value, thereby controlling the IM value.

Here, assuming that the IM test signal oscillation frequency f _T is 5 MHz, the transmission spectrum of the power amplifier is:
As shown in FIG. 2, for the center frequency f ₀ , f ₀ ± 5M
2f ₂ −f ₁ and 2f ₁ −f ₂ are generated by the spectrum of Hz (referred to as f ₁ and f ₂ ) and the IM distortion. This spectrum is again converted by the frequency converter 30 into an IF
Signals, f ₁ + f ₂ or f ₁ or f ₂ , and (2f
₂₋ f ₁ ) + (2f ₁ −f ₂ ) or 2f ₁ −f ₂ or 2f ₂
-F ₁ to demold the filter, detection, it is possible to know the IM level by comparison.

[0007]

FIG. 1 is a block diagram showing an embodiment of a digital microwave transmitter according to the present invention. When the IM test control switch 16 is turned on, the test signal oscillator 20 is oscillated, and the switches 7 and 8 are switched to the test signal oscillator 2.
Push to the 0 side. As a result, the output of the quadrature modulator has a spectrum of frequencies f ₁ and f ₂ as shown in FIG.

Here, since distortion is likely to occur in the power amplifier 34 having the maximum output level, a microwave distributor 35 is provided at the output of the power amplifier 34. The degree of coupling of the distributor 35 must be -20 to -30 dB, and the input of the frequency converter 30 needs to be at a sufficiently low level so that IM does not occur in the converter 30. IM monitor frequency converter 33
The main levels and IM levels of the frequencies f ₁ and f ₂ are extracted from the outputs of
Detect at 40. This detection voltage is compared by a detection voltage comparator 41, and if the IM level is equal to or less than the reference value, it is determined that the voltage is good.
The display 17 is displayed via the communication lines 24 and 19.
The IM distortion is caused by the gain variable circuit 27, the frequency converter 30
Alternatively, if the power level is generated by the power amplifier 34, the output level of the AGC control circuit 26 is set to be lower, and
Distortion can be reduced to a specified value or less.

Since the IM test result is substantially equal to the adjacent interference level in the actual modulated wave, the interference level can be monitored by performing the above test before operation. With respect to the OFDM modulation method, if a configuration is adopted in which a majority carrier wave is finally modulated by a quadrature modulator, a test can be performed with the same test circuit configuration. Also, by replacing the microwave demultiplexer 35 with a switching device and normally connecting it to the output 36, and connecting it to the IM monitor frequency converter 33 at the time of testing, it is also possible to transmit after the test is completed.
In this case, it is necessary to arrange an attenuator for reducing the input level between the switching device and the converter so that the IM monitor frequency converter 33 does not generate IM.

[0010]

As described above, the present invention has the following effects. 1. It is possible to check the interference wave level to other channels. 2. The interference wave level can be controlled to a specified value or less. 3. Even after the test is completed, the transmission output level can be displayed by the main level detection output. 4. Even after the test is completed, I
With the detection output of the M level, it is possible to continue to check whether the IM level is extremely increased. 5. If the set value of the IM control for each channel is controlled for each channel, the IM level can be adjusted to the specified value.

[Brief description of the drawings]

FIG. 1 is a block diagram showing the overall configuration of the present invention.

FIG. 2 is a spectrum of a power amplifier output when an IM test is on.

FIG. 3 is a block diagram showing the entire configuration of a conventional technique.

[Explanation of symbols]

2: coaxial cable, 4: digital signal input, 7, 8:
Switch, 9, 10: Mixer, 11: Adder, 31, 3
2: distributor, 35: microwave demultiplexer (directional coupler), 37, 38: bandpass filters.

Claims (2)

[Claims] In a digital radio transmitter, a switching means for switching and inputting a test signal to a quadrature modulator, a part of a transmission output wave is taken out, converted to an IF frequency again, and generated by a signal level and intermodulation distortion. A digital microwave transmitter comprising: means for detecting each level; a comparator for comparing the two levels; and an indicator for displaying whether or not the distortion level is equal to or less than a specified value based on a result of the comparison. 2. The digital microwave transmitter according to claim 1, further comprising gain control means for controlling a gain according to an output of said comparator.