1、BRITISH STANDARD BS 4478:1969 Methods of Measurement of intermodulation products in electronic tubes or valves intended for use in colour television transposers BS4478:1969 This British Standard, having been approved by the Telecommunication Industry Standards Committee, was published under the auth
2、ority of the Executive Board on 29 July 1969 BSI 12-1999 The following BSI references relate to the work on this standard: Committee reference TLE/5 Draft for comment 67/30389 ISBN 580 05870 0 Co-operating organizations The Telecommunication Industry Standards Committee, under whose supervision this
3、 British Standard was prepared, consists of representatives from the following Government departments and scientific and industrial organizations: The Government departments and scientific and industrial organizations marked with an asterisk in the above list, together with the following, were direc
4、tly represented on the committee entrusted with the preparation of this British Standard: British Broadcasting Corporation Electronic Valve and Semi-Conductor British Electrical and Allied Manufacturers Association* Manufacturers Association Institution of Electrical Engineers British Radio Equipmen
5、t Manufacturers Institution of Electronic and Radio Engineers* Association Institution of Production Engineers British Radio Valve Manufacturers Ministry of Defence Association* Ministry of Defence, Army Department British Railways Board Ministry of Defence, Navy Department* Cable and Wireless Ltd.
6、Ministry of Technology* Crown Agents for Oversea Governments Post Office* and Administrations Radio and Electronic Component Department of Employment and Productivity Manufacturers Federation (H.M. Factory Inspectorate) Relay Services Association of Great Electrical Contractors Association Britain (
7、Incorporated) Science Research Council-Radio f 1 t + A 2sin 2;f 2 t . + A msin 2;f m t) n . On expansion, these will give frequencies: P 1 f 1 P 2 f 2 P m f m where P 1+ P 2+ P m= NBS4478:1969 BSI 12-1999 3 The output signal at frequency f vis to be at X dB with respect to peak synchronization. The
8、output signal at frequency f sis to be at Y dB with respect to peak synchronization. The output signal at frequency f cis to be at Z dB with respect to peak synchronization. The maximum level of intermodulation product power within the transmitted band is to be W dB with respect to peak synchronizin
9、g power for any value of f cbetween f c1and f c2 . 4.3 Measurement technique. The tube or valve to be examined must be properly adjusted in accordance with the manufacturers instructions, to give the required output and bandwidth. Figure 2 illustrates the elements of the circuit for the three-freque
10、ncy method of measurement. Figure 1 The relationship between output power level and frequency for the three-frequency or three-tone methodBS4478:1969 4 BSI 12-1999 NOTE 1The generator group providing the three test signals includes: a) Generator V, at vision carrier frequency. b) Generator S, at sou
11、nd carrier frequency. c) Generator C, at colour sub-carrier frequency, variable. d) Level setting devices, v, s and c. e) The combining network and such components as may be necessary to prevent interaction between the three generators. f) A circulator to prevent interaction between the tube under m
12、easurement and the three generators. NOTE 2The circulators before the combining network are not essential but they will be needed in certain circumstances. NOTE 3This block contains such filters at input and/or output as may be specified by the tube or valve manufacturer as being essential for obtai
13、ning the required performance. NOTE 4The purpose of the attenuators (1 and 2) is to equalize the levels of the input and output samples at a level suitable to the measuring instrument. NOTE 5The measuring instrument may be either of two types: Spectrum analyser. A spectrum analyser is preferred beca
14、use all components of the complex output signal are displayed simultaneously. However, the following precautions must be observed. The r.f. mixer stages must have a dynamic range of70dB throughout which the sensitivity characteristic must not depart from linearity by more than 10%. The selectivity o
15、f the i.f. amplifier must be such that rejection of a signal1MHz away from the response central frequency is by more than60dB. The sweep rate must be so chosen that the display of small signals is not affected by the recovery time following a large amplitude signal. Selective voltmeter. A selective
16、voltmeter differs from a spectrum analyser only in that it is manually tuned instead of being continuously swept and the output is indicated on a meter. The same performance requirements apply as for the spectrum analyser. Figure 2 Block diagram for the three-frequency method of measurementBS4478:19
17、69 BSI 12-1999 5 4.4 Measurement procedure 4.4.1 With colour and sound carriers (Generators C and S) switched off, adjust the vision carrier (Generator V) to produce a c.w. 1)output from the amplifier tube or valve measured on the power meter at X dB down from the level required for peak synchronizi
18、ng performance. 4.4.2 Switch the measuring instrument to position SP2 and adjust attenuator 3 to provide at least (W + 10 X) dB of attenuation. Note the reading. Adjust the signal displayed to a convenient level by means of attenuator 2. This level will be taken as the reference level for the entire
19、 measurement, and thereafter2 must not be altered. 4.4.3 Reduce the attenuation of 3 by an amount (Y X) dB. Switch on the sound carrier (Generator S) and adjust the displayed output signal amplitude to the reference level by means of the generator attenuators. 4.4.4 Further reduce attenuation of 3 b
20、y an amount (Z X) dB. Switch on Generator C and adjust the displayed output signal amplitude to the reference level by means of the generator attenuator c. 4.4.5 Switch the measuring instrument to SP1 and use attenuator 1 to adjust the display so that the colour signal is at reference level. Measure
21、 the level of any spurious signal which appears in the channel whilst the signal at frequency f cis varied continuously between f c1and f c2 , using the attenuator 3. Ensure that the level of such a spurious signal, which may be due to interaction between the three generators or distortion in the mi
22、xer of the measuring instrument, is not greater than (W X + 10) dB with respect to vision carrier level. This makes certain that any spurious signal introduced at the amplifier tube or valve input will be, after amplification, at least 10dB with respect to the allowed intermodulation product level o
23、f W dB relative to peak synchronizing power. 4.4.6 Switch the measuring instrument to SP2 and examine the amplifier output spectrum. Measure the power level, with respect to the reference level, (i.e. vision signal level) of any spurious signal present in the channel whilst the signal at frequency f
24、 cis varied continuously between f c1and f c2 . For the measurement specified in4.2, the spurious signal power must not exceed W dB with respect to the output peak synchronizing power. 5 Precautions 5.1 Attention should be paid to any effect of variations of supply voltage on the tube and measuring
25、equipment performance whilst carrying out measurements. 5.2 Unless otherwise specified the lead for the tube should be well matched to the system so that the v.s.w.r. 2)measured at the output plane of the tube is less than 1.05:1. 5.3 The frequency response of the sampling couplers and the attenuato
26、rs must be within 0.1dB over the frequency band of interest. 5.4 The amplitude of the colour sub-carrier signal into the tube under measurement must be kept substantially constant (e.g. within 0.5dB) whilst the frequency is being varied. 5.5 It is most important to achieve a high degree of accuracy
27、in setting the initial power level (4.4.1) because the power level of an amplifier tube influences the level of intermodulation products generated. For example, the magnitude of the third order intermodulation product varies by 2 dB per 1 dB change in output power of the tube. 1) Continuous wave. 2) Voltage standing wave ratio.
