GB/T 3789.28-1991 Test method for electrical performance of transmitting tubes - Test method for cross modulation

This standard specifies the test method for cross-modulation of transmitting tubes. This standard applies to transmitting tubes used in television transmitters. GB/T 3789.28-1991 Transmitter electrical performance test method Cross-modulation test method GB/T3789.28-1991 Standard download decompression password: www.bzxz.net

National Standard of the People's Republic of China
Test method of transmitting electrical properties
Test method of cross modulation
Measurements of the electrical properties of transmitting tuhesMethod of measuring the cross modulationSubject content and scope of application
This standard specifies the test method of cross modulation of transmitting tubes. This standard applies to transmitting tubes used in television transmitters. 2 Referenced standards
GB/T3789.1 Test method of transmitting tube electrical properties General GB6277 Measurement method of television transmitters
3 Terminology
Cross modulation of cro
GB/T 3789.28--91
Cross modulation of the audio carrier frequency is the unwanted amplitude modulation of the audio carrier frequency caused by the modulation of the image carrier frequency. It is usually expressed as a percentage of the peak-to-peak value of the unwanted amplitude modulation to the maximum value of the audio carrier frequency after cross modulation. 4 Test block diagram
The test block diagram is shown in Figure 1. The frequency selection detection device can be composed in different ways. Method A: Use a spectrometer. Method B: Use an audio bandpass filter and an image carrier notch filter, as well as a linear detector and a DC oscilloscope. The passband unevenness of the audio bandpass filter should be less than 1 dB within the range of the audio carrier frequency f1 ± 300 kHz. The attenuation of the filter chain at f2 ± 1 MHz should be greater than 20 dB. The attenuation at the image carrier frequency f2 should be greater than 40 dB. When the demodulation device itself has a good enough selectivity, the filter can be used less or not. When using method B, special attention should be paid to the fact that the filter must be very flat near the audio carrier frequency so as not to produce additional FM AM conversion.
Approved by the State Bureau of Technical Supervision on August 15, 1991 and implemented on April 1, 1992
Test signal
Generator
Test equipment and test rules
GB/T 3789. 28-91
Combined type
Exciter
Test device
Test tube
Test device
Sampling device
Spectrum analyzer
Figure 1 Block diagram of inter-modulation test
Auxiliary frequency meter or
Test load
Accompanying sound． Bandpass
Filter
Image trap
Direct current detector
DC oscilloscope
Test equipment and test rules should comply with the provisions of GB/T3789.1 and Chapter 4 of GB6277. 6 Test steps
6.1 Apply voltages to each pole according to the specification.
6.2 Adjust the amplifier of the tube under test to the specified working state. 6.3 The test signal generator sends the test signal A shown in Figure 2, and the accompanying sound signal is not modulated. ny
GB/T 3789- 2891
Line Synchronous Pulse
Figure 2 Test Signal A (Field Square Wave)
6.4 When the frequency selection detection device is in mode A, the spectrometer is tuned to the audio carrier frequency, switched to the linear display position and the zero sweep frequency position, and the intermediate frequency bandwidth is set to 300kHz. The display scanning frequency is set to an appropriate frequency so that the field frequency waveform can be clearly seen. 6.5 The maximum value a and the peak-to-peak value b of the useless modulation are read from the display screen, as shown in Figure 3. Figure 3bZxz.net
7 Data Processing
a. Use the a and values ​​read in 6.5 to calculate the cross modulation CM using the following formula (1): × 100%
. When the frequency selection detection device is in mode B, read α and 5 from the DC oscilloscope display screen and calculate according to formula (1). 1
Considering the difference in actual effects (auditory effects) between the field frequency square wave component and the synchronization signal component in the cross modulation waveform, it is allowed to test and calculate the cross modulation caused by them separately. The field frequency square wave part in the cross modulation:
The synchronization signal part in the cross modulation:
Total alternating modulation:
×100%
×100%
Additional instructions:
GB/T 3789. 28-91
cM = cso + cM.
This standard is proposed by the Ministry of Machinery and Electronics Industry of the People's Republic of China. This standard is drafted by the Electronic Standardization Institute of the Ministry of Machinery and Electronics Industry and Factory 779. (4
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