JPS5818376Y2 - Eizou Soushinkino Ishiyu Henchyou Zatsuon Sokuteiki - Google Patents

Description

[Detailed explanation of the invention] Video transmitters for television broadcasting use an amplitude modulation method, but in reality, phase modulation accompanying the amplitude modulation of the signal also occurs (hereinafter referred to as heterogeneous modulation noise). ).

This heterogeneous modulation noise generated in the video transmitter not only deteriorates video transmission characteristics such as differential phase characteristics, but also causes buzz to the audio signal and interferes with the multiplexed signal during multiplexed broadcasting.

Conventionally, there has been little interest in this heterogeneous modulation noise, and the only thing that has been done is to statically measure changes in the phase relationship of output signals with respect to changes in input voltage in klystrons, traveling wave tubes, etc. using a phase meter.

However, recently, due to trends such as higher performance of transmitters and multiplexing of television broadcasting, the problem of heterogeneous modulation noise can no longer be ignored.

The purpose of the present invention is to measure the heterogeneous modulation noise of this video transmitter quantitatively and in accordance with the actual operating conditions including the modulator.

First, we will explain the measurement principle.

As shown in Fig. 1, the modulated wave M(t) when amplitude modulated with a signal having a constant luminance and a level d is as follows when accompanied by phase modulation, except for the synchronizing signal part. It can be written as

M(t)−(D −d) sin (ωe −φ
(d) ) - (1) Here, D sin ωct is the carrier wave, and φ (d) is the magnitude and angle of phase modulation when the brightness level is d.

When this signal is phase-detected using a synchronous carrier wave called Ecos (ωct-θ) and the low frequency component v(t) is extracted, V(t
)=2 E (D d ) 81 n (θ=φ(d
))-...-(2) Here, if you adjust the phase θ of the synchronous carrier so that ■(t) becomes zero, θ-φ(d)
By examining θ, we can find out the size of φ(d).

Therefore, if the luminance level d is continuously changed from the black level to the white level 1 and the magnitude of φ(d) is known for each, the phase transformation with respect to the signal amplitude can be calculated for the entire range of amplitude changes used. You can see the size.

As shown in Figure 2, if the modulation is done with a 5:1 staircase wave (a test signal for video equipment in which a staircase wave and a signal with constant brightness are combined at a ratio of 5:1), Measurement becomes easier, and measurement can be performed while changing the average gradation level.

The present invention is based on the measurement principle described above.

That is, the output signal of the transmitter under test is amplitude-modulated with a staircase wave signal as shown in Fig. 2, and this received signal is synchronized with a carrier wave, that is, a carrier wave with a constant amplitude that has the same frequency and phase as the carrier wave of the received signal. The signal is applied to a phase detector through a phase shifter, and the output waveform is observed with a DC-coupled oscilloscope. This is a heterogeneous modulation noise measuring device for video transmitters that can measure the magnitude of phase modulation for each amplitude of a staircase wave by knowing the amount of phase shift.

FIG. 3 shows an embodiment according to the present invention.

Reference numeral 1 denotes a video transmitter used for measurement, which is composed of a carrier wave oscillation section 4, a modulation section 5, and a power amplification section 6.

The video transmitter 1 has a 5:1 signal generated by the staircase wave signal generator 3.
A staircase wave signal is amplitude modulated to a specified modulation degree.

The modulation waveform at this time is as shown in FIG.

2 is the receiving terminal 7 which is the main body of the heterogeneous modulation noise measuring instrument.
The output signal of the transmitter under test, amplitude modulated with a staircase wave signal, is applied via a directional coupler or the like.

In this case, since the unmodulated carrier wave of the transmitter under test 1 is used as the synchronous carrier wave, a part of the output of the carrier wave oscillation section 4 is branched and applied to the carrier wave input terminal 8.

After adjusting the received signal to an appropriate amplitude with the variable attenuator 9,
It is applied to one input terminal of the phase detector 15.

The synchronous carrier wave applied to the carrier wave input terminal 8 passes through a synchronous carrier adjustment section consisting of a variable attenuator 10, an amplifier 11, a sub variable phase shifter 12, and a main variable phase shifter 13, and is then sent to the phase detector 15. added to the other input terminal.

It is desirable to avoid using an amplifier as much as possible on the received signal side, which is the signal to be measured.

This is because heterogeneous modulation noise generated by the amplifier itself is included in the measurement results.

The sub variable phase shifter 12 has a phase shift amount of +1180 degrees with respect to the synchronous carrier wave, and is used for setting a reference point.

The main variable phase shifter 13 has a phase shift amount of ±30 degrees, and has a phase shift amount of 0.1
The amount of phase shift can be read with an accuracy of about 100 degrees.

These variable phase shifters are required to have good VSWR and have no change in attenuation due to change in phase shift amount, and in the embodiment, they are mainly composed of variable length coaxial tubes.

The level meter 14 is used to check the amplitude of the received signal applied to the phase detector 15 and the synchronized carrier wave, and the measurement point is switched by a switch 16.

In this device, the selection of the amplitude values of these two signals is extremely important, and although it depends on the phase detector used, in this example, the received signal is 2.0 Vp-p' synchronous carrier is 1,
The best value was 5Vp-p.

However, when checking this amplitude value with the level meter 14,
Regarding the received signal, since the fluctuation differs depending on the average gradation level, it is
There are 3 types (0 and 3 types), and it is necessary to calibrate it in advance.

The output of the phase detector 15 is led to an output terminal 17, and its waveform is observed with a DC-coupled oscilloscope 18.

FIG. 4 is a sketch of an example of the waveform at this time.

The actual measurement procedure is as follows.

(1) The transmitter under test 1 is modulated with a staircase wave signal and adjusted to a specified degree of modulation.

(2) The phase detector 15 is detected by the level meter 14,
Check the amplitude values of both signals and adjust with variable attenuators 9 and 10.

(3) The scale of the main variable phase shifter 13 is set to zero, and the sub variable phase shifter 12 is adjusted so that the potential of the synchronous peak value portion becomes zero (FIG. 4 shows this state).

) (4) Adjust the phase shifter 13 so that the potential becomes zero at each amplitude of the staircase wave, and read the amount of phase shift to determine the magnitude of phase modulation corresponding to each amplitude of the staircase wave, i.e. It is possible to know the magnitude of heterogeneous modulation noise.

FIG. 5 shows the results of measuring the heterogeneous modulation noise of a 50KW video transmitter according to the present invention, in which (a) is a broadcaster using a high-power stage modulation method, and (b) is a broadcaster using an intermediate frequency stage modulation method.

The horizontal axis in Figure 5 is the amplitude ratio with the synchronous peak amplitude as 1.
0.125 corresponds to the white level, and 0.75 corresponds to the black level.

In addition, for the measurement of 载), a frequency converter was separately prepared to mix the intermediate frequency and the local oscillation frequency to obtain a synchronous carrier wave.

If it is difficult to obtain a synchronous carrier wave from the transmitter under test, a synchronous carrier generation circuit is required on the measuring instrument side.

An automatic phase adjustment circuit is used for this purpose, but the reference signal must be configured to sample and use a part whose amplitude is always constant, such as the synchronization signal part of the received signal. Must be.

FIG. 6 shows an example thereof. That is, the input terminal 19
The received signal that has entered is divided into a sampling circuit 23 and an envelope detection circuit 20, and the output signal of the envelope detection circuit 20 is then passed through a synchronous separation circuit 21 and a pulse shaping circuit 22 to generate sampling pulses.

Using this sampling pulse, the synchronization signal part in the received signal is taken out, and using this as a reference signal, a synchronization carrier wave is generated by an automatic phase adjustment circuit composed of a sub-phase detector 24 and a variable reactance oscillator 25, and a synchronization carrier wave is generated at an output terminal 26.
It is taken out.

In terms of actual commercialization, after converting the received signal from the transmitter under test to an intermediate frequency, a synchronous carrier wave is generated by an automatic phase adjustment circuit as shown in Figure 6, and amplitude adjustment and phase detection are also performed. It is advantageous to perform all measurements at an intermediate frequency because many parts can be shared regardless of the frequency used by the transmitter under test.

However, in this case, sufficient care must be taken to avoid generation of heterogeneous modulation noise in the frequency conversion section and intermediate frequency amplification section.

In fact, even when we measured the heterogeneous modulation noise of a 50KW video transmitter using this method, we obtained results that were the same as those shown in FIG.

[Brief explanation of the drawing]

FIG. 1 shows the waveform of a modulated wave when amplitude modulated with a video signal of constant brightness. FIG. 2 shows the waveform of a modulated wave when amplitude modulated with a staircase wave signal, and the dotted line shows the state when the average gradation level is changed. FIG. 3 shows an embodiment of the present invention. FIG. 4 is an example of the output waveform of the phase detector observed in the example. Figure 5 shows the measurement results of different modulation noises actually measured using the present invention for a 50KW video transmitter. FIG. 6 shows an example of an automatic phase adjustment circuit when generating a synchronous carrier wave on the measuring instrument side. DESCRIPTION OF SYMBOLS 1...Video transmitter to be measured, 2...Main body of heterogeneous modulation noise measuring device, 3...Staircase wave signal generator, 4...Carrier wave Oscillation section, 5...Modulation section, 6...Power amplification section, 7-0-Reception terminal,
8... Carrier wave input terminal, 9 and 10...
・Variable attenuator, 11...Amplifier, 12...
...Sub variable phase shifter, 13...Main variable phase shifter, 1
4...Level meter, 15...Phase detector, 16...Switcher, 17...Output terminal, 18...Oscilloscope, 19...
Input terminal, 20... Envelope detector, 21...
... Synchronization separation circuit, 22 ... Pulse shaping circuit, 23 ... Sampling circuit, 24 ...
- Sub-phase detector, 25...variable reactance oscillator, 26...synchronous carrier wave output terminal.

Claims (1)

[Claims for Utility Model Registration] A variable attenuator 9 to which the output signal of the transmitter under test is added, an envelope detector 20, a synchronous separation circuit 21, a pulse shaping circuit 22, a sampling circuit 23, and a sub-phase detector 24. an automatic phase adjustment circuit composed of a variable reactance oscillator 25; a variable attenuator 10 and an amplifier 1 to which the output of the variable reactance oscillator 25 or a non-modulated carrier wave of the transmitter under test is added;
1, a sub variable phase shifter 12, and a main variable phase shifter 13; and a main phase detector 15 to which the output of the variable attenuator 9 and the output of the main variable phase shifter 13 are added. An oscilloscope to which the output of the main phase detector 15 is applied, and a different modulation noise measuring device for a video transmitter.