ITU-R REPORT SM 2055-2006 Radio noise measurements《无线电噪声测量》.pdf

1、 Rep. ITU-R SM.2055 1 REPORT ITU-R SM.2055 Radio noise measurements (Question ITU-R 231/1) (2006) 1 Summary This Report describes a new, general, measuring technique for determining radio noise in practical radio applications. This proposed technique is not equipment-specific; critical elements of t

2、he method will be described in detail. The proposed method is mainly useful for measurements where no statistical processing of raw sampled data is necessary. To cope with the fact that the method should work with different measurement receivers and analysers, a post processing method is developed t

3、hat works with both scanning and single frequency devices. This method, which will be described in 4, should yield comparable results for both types of devices. Although some examples in the Report are based on HF (3 MHz) 10 0 2nd order intercept (dBm) 60 (3 MHz) 50 Preselection Set of suboctave ban

4、d filters or tracking filter Tracking or fixed filter Low pass/high pass filter Noise figure 15 dB (2 MHz) Sensitivity (500 Hz bandwidth) 10 dBV 7 dBV 7 dBV LO-phase noise 120 dBc/Hz in 10 kHz offset 100 dBc/Hz in 10 kHz offset 100 dBc/Hz in 10 kHz offset IF rejection (dB) 80 90 100 Image rejection

5、(dB) 80 90 100 Automatic gain control Measurement outputs should have no agc applied Electromagnetic compatibility of the measurement setup including computers and interface All interference produced and received by the setup should be 10 dB below the average noise to be measured VSWR: voltage stand

6、ing wave ratio. The intermediate frequency (IF) selectivity between 60 and 6 dB should be accurately known to calculate the equivalent noise bandwidth when measurements with different IF filters have to be compared. 5.2 r.m.s. or average detector In 4.1 is concluded that a measurement using an r.m.s

7、. detector is needed. A simultaneous operating average detector would be a useful addition. One of the advantages of the r.m.s. detector in correlation work is that for broadband noise the output obtained from it will be proportional to the square root of the bandwidth, i.e. the noise power is direc

8、tly proportional to the bandwidth. This feature makes the r.m.s. detector particularly desirable and is one of the main reasons for adopting the r.m.s. detector to measure noise. Another advantage is that the r.m.s. detector makes a correct addition of the noise power produced by different sources,

9、for example impulsive noise and random noise, thus for instance allowing a high degree of background noise. r.m.s. detector The r.m.s. detector calculates the true power over each measurement interval. The result corresponds to the signal power within the chosen bandwidth. For the r.m.s. calculation

10、, the samples of the envelope are required on a linear level scale. The following applies: =NiismrNV12.1(3) Rep. ITU-R SM.2055 13 where: Vr.m.s.: r.m.s. value of voltage (V) N: number of samples allocated to the pixel concerned Vi: samples of envelope (V). Average detector An average detector is des

11、igned to read the mean value of the envelope of the signal passed through the pre-detector stages. The average detector calculates the linear average over each measurement interval. For this calculation the samples of the envelope are required on a linear level scale. The following applies: = NiiAVG

12、NV11(4) where: VAVG: average voltage (V) N: number of samples allocated to the pixel concerned Vi: samples of the envelope (V). Depending on the type of input signal, the different detectors partly provide different measurement results. 5.3 Sensitivity The measured uncorrected noise signals (no corr

13、ection factors applied) should be at least 10 dB above the equipment noise floor to guarantee sufficient measurement accuracy. When the measured values are less then 10 dB below the equipment noise floor the r.m.s. detector requires a custom calibration. Other methods like raw data sampling method h

14、ave to use a sample detector because the processing includes r.m.s. calculations that are done afterwards. 5.4 Input impedance The typical input return loss of a receiver is RX 13 dB with the internal attenuator switched off. The receiver will be used with an external attenuator of at least 3 dB, th

15、e lower the attenuation the better. 5.5 Spurious radiation Spurious radiations from the receiver are components at any frequency, radiated by the equipment/cabinet. This radiation must be as low as possible. Computers used in automated measurement setups not specifically designed for measurements ra

16、diate wideband noise. The computer itself and the switching power supply can be placed in an RF shielded enclosure. The radiation of cabling and interfaces should be measured and adequately shielded cables should be chosen. 5.6 LNA and preselectors These devices are different, but serve overlapping

17、functions so are considered together. An LNA is a wideband RF amplifier that is positioned between the receiver and the antenna. In some cases, the LNA is mounted at the antenna and is used to overcome transmission line losses. In other cases, it is positioned right at the receiver antenna input. A

18、preselector is a tuned circuit that passes the desired frequencies. As the name implies, a preselector preselects the RF signals that will be applied to the 14 Rep. ITU-R SM.2055 receiver input. The typical installation of a preselector is right at the antenna terminals of the receiver, or by means

19、of a short piece of coaxial cable to permit operator access. Some preselectors are manually tunable and need proper calibration before use. Preselectors should be used when overloading of the receiver/analyser front-end is suspected and there is no other possibilities to prevent the overload. These

20、preselectors have a negative effect on measurement accuracy unless they are included in the receiver/analyser and calibrated as part of the total receiver calibration. LNAs introduce noise and should be avoided unless needed to overcome cable loss or compensate for relatively small antenna structure

21、s. An LNA should be used when the received noise is less than 10 dB above the equipment noise floor. The requirements for such an amplifier are given in Table 2 which does not describe a new set of measurement receiver or LNA specifications but only points out the additional or specific requirements

22、 necessary for an LNA used for noise measurements. TABLE 2 LNA recommendations Function Frequency range Frequency range 20-50 MHz 50-500 MHz 0.5-3 GHz Input (antenna input) VSWR 50 , nominal 1.5 Gain (dB) 18 25 25 Gain stability 0.1 dB at 10-30 C Noise figure (dB) 2 Gain flatness over the frequency

23、range of interest (dB) 0.1 0.2 0.5 Care should be taken not to overload the receiver when using an LNA. An external band filter can be applied to prevent overloading. 5.7 Cables, cable routers and connectors It is recommended that wherever possible, solid, semi-rigid or double-screened cables should

24、 be used for all RF connections. This is to ensure maximum screening between adjacent cables and feeders and to reduce coupling between equipment. Solid outer conductor cables have superior passive intermodulation performance than braided screen cables but double shielded braided cables like RG223 a

25、nd RG214 can be used without problems. Single screened cable (e.g. UR67, UR43, RG58) should not be used. The direct and shortest route is always the best for minimum radiation and minimum insertion loss. It is often convenient to break down very large feeder cables to a more convenient size and ther

26、e is a tendency to place connectors just inside the equipment room when reducing the main incoming feeder to a more manageable size. It is best, however, to take the main feeder as close as possible to the equipment to which it is to be connected before interrupting its outer conductor. The only exc

27、eption to this rule is to provide an Earth connection for lightning conductor purposes. This should be carried out by means of an external clamp on the outer copper conductor and this should be taken via the most direct route. Incoming feeders must not be interconnected by a “patch panel”, as used i

28、n conventional setups. The “patch panel” is a source of Earth current coupling and intermodulation and should be avoided. Rep. ITU-R SM.2055 15 It is also recommended that high quality connectors be used and a minimum standard would be type N. Only stainless steel or at least nickel plated connector

29、s should be used, silver plated connectors should be avoided since the material scraped off causes connector contamination. All connectors must be fitted in conformity with manufacturers instructions to ensure proper sealing and electrical uniformity and should be tightened to the manufacturers reco

30、mmended torque settings to guarantee measurement reproducibility. 5.8 Feeder identification, terminations and grounding Feeder cables should be uniquely and permanently identified at each end and at the point of exit from the structure. More frequent identification may be advisable when cables are b

31、uried in a duct. Connectors and grounding kits should be fitted in accordance with manufacturers instructions. Connector fitting should be carried out under laboratory condition and feeders should be installed in accordance with manufacturers recommendations, with connectors already fitted to their

32、upper ends and suitably protected from water ingress. 5.9 Sealing On completion, connectors should be wrapped with PolyIsoButylene (PIB) self-amalgamating tape. Over-wrapping with petroleum jelly impregnated waterproof tape should be avoided since PIB is attacked and gradually dissolved by petroleum

33、 based products. Where PolyVinylChloride (PVC) covers are provided for connectors, they should be removed and the connectors taped as described. Feeder and cable entries, external cable or feeder terminations, and Earth connections to feeders on towers or gantries should be suitably sealed or protec

34、ted against the ingress of moisture using non setting pastes, self amalgamating tapes, neoprene paints as appropriate and in accordance with manufacturers instructions. Particular attention should be paid to the shedding of surface water. 5.10 Inspection for moisture In cases where the mast is expos

35、ed and there is a possibility of moisture gathering at the outer jacket of the copper case of the incoming cables, it is wise to remove the outer insulating jacket at a point well inside the equipment room where it can be inspected for traces of moisture. 6 Antenna systems 6.1 Introduction Noise rec

36、eived by an antenna is generated by a large number of sources, coming from a large number of directions. The noise power, measured at the connector of an antenna, is the sum of all received powers from independent noise sources with their own directions. Each received noise power component has its o

37、wn pointing vector, and coupled to this pointing vector a free-space E and H field strength and polarization plane. This makes that the received power at the antenna connector cannot be correlated to a field strength in a deterministic physical sense as we use to do for a single radio signal. Thus i

38、n a true scientific approach only a noise power at an antenna connector can be defined and measured, and for example expressed in a noise figure or noise temperature as is done in the relevant ITU documents. However, in the application field of radio engineering, the relevance of noise measurement l

39、ies in the comparison between a radio signal and the background noise level. As we express the signal strength of the radio signal in a field strength (generally for frequencies below 30 MHz), we can 16 Rep. ITU-R SM.2055 also express the background noise power in an equivalent field strength, assum

40、ing all the noise power at the antenna connector is received from the same direction (azimuth and elevation) as the radio signal was coming from. It is this approach that gives us opportunities to describe antennas properties, used for noise measurements. 6.2 Antenna properties The antenna used for

41、noise measurements in a particular frequency range should be fit for the purpose of receiving radio signals in that frequency range. This means that the directivity is matched to the radio signals to be received. For example in a frequency range used for near vertical incident signal (NVIS) communic

42、ations the directivity of the antenna shall be optimized for the reception of high elevation angle signals. The K-factor can be derived from a antenna gain value averaged over the relevant elevation angles. Another example is the use of a beam antenna for long range communication. As we can assume t

43、hat this antenna will be directed at the source direction of the radio signal to be received, we can use the antenna gain in the optimal direction to calculate the K-factor. We can conclude that there is no universal antenna for all type of noise measurements as well as for all frequency ranges. The

44、 radiation pattern of the antenna needs to have a relationship with the type of noise to be measured. For antennas placed in an environment where noise sources are distributed evenly around the antenna, the antenna pattern is less relevant than in cases where the noise is received from a defined ang

45、le. In the first case only the antenna efficiency or average gain over the total antenna aperture needs to be used as a correction factor. This is particular the case with VHF and UHF measurements. The lower the frequency the more relevant the 3D properties of the antenna diagram are. The same is va

46、lid for polarization, an ideal antenna for this type of measurement is polarization independent or is sensitive in all relevant polarization planes at the same time. 6.2.1 Calculation of the antenna factor (K-factor) There is a direct relationship between the antenna gain of a passive and loss-free

47、antenna (in a certain direction), Gi, and the antenna factor K (see also Annex 1): rxMHzrxrxRgfRgVEK=303004304(5) In dBs: )log(108.12)log(20rxiMHzRGfK = 8.29)log(20 =iMHzGf = 50rxR Losses in the antenna have to be subtracted from the antenna gain. 6.2.2 Gain versus frequency (bandwidth and compensat

48、ion) Loss-free antennas are in general tuned antennas, for example a dipole or inverted V antenna. The antenna bandwidth reduces progressively, where the antenna dimensions decrease with wavelength. In case of swept measurement the sweep width can be wider than the bandwidth of the antenna. The meas

49、urement result should corrected for the resonance curve of the antenna. The measurement of Rep. ITU-R SM.2055 17 the resonance curve can be done using a small auxiliary antenna, a non resonant small dipole or a magnetic loop, fed by the tracking generator output of a spectrum analyser or a noise source. It may be necessary to insert an amplifier between the tracking generator or noise source output and the auxiliary antenna. The coupling to the measurement antenna may be in the near field. 6.2.3 Measurement sensitivity matters What are th