1、 Rec. ITU-R SM.1838 1 RECOMMENDATION ITU-R SM.1838 Test procedure for measuring the noise figure of radio monitoring receivers (2007) Scope This Recommendation belongs to a set of Recommendations describing the test methods to determine technical parameters of radio monitoring receivers that are imp
2、ortant for the users of these receivers. When the described methods are followed by manufacturers, comparing different receivers is made easier. This Recommendation specifies the test procedure for the determination of the noise figure of a monitoring receiver. This test procedure definition is reco
3、mmended to all the manufacturers with the advantage for the users of such receivers, that an easier and more objective assessment of product quality is possible. The ITU Radiocommunication Assembly, considering a) that ITU-R has published the Typical recommended specifications for analogue and digit
4、al monitoring receivers in the Spectrum Monitoring Handbook (2002), but that nothing is said about the test procedures behind such specifications; b) that the specification of the noise figure (NF) strongly depends on the test procedures applied; c) that the noise figure of a receiver is a parameter
5、 which determines its ability to receive weak signals and to produce output signals with a usable level and an acceptable quality; d) that the noise figure specified in the data sheet of a receiver depends extremely on the test frequencies used, the receiver operating conditions and the ambient temp
6、erature prevailing during the tests; e) that the noise figure characteristic has a direct influence on the suitability of a receiver to fulfil certain monitoring tasks; f) that a unique test procedure for the measurement of the noise figure is required to make the specifications published by various
7、 manufacturers comparable because it will not be possible to convert specifications based on different test procedures with arguable effort; g) that a defined test procedure for the noise figure must be independent of the receiver design; h) that a well-defined test procedure for the noise figure, i
8、f adopted by all the manufacturers of radio monitoring receivers, will have the advantage for the users of such receivers, that an easier and more objective assessment of products from different manufacturers is possible; j) that supplementary information about these noise figure measurements can be
9、 found in Report ITU-R SM.2125 Parameters of and measurement procedures on H/V/UHF monitoring receivers and stations, recommends 1 that the measurement method in Annex 1 should be used to specify the noise figure of a radio monitoring receiver. 2 Rec. ITU-R SM.1838 Annex 1 Test procedure for measuri
10、ng the noise figure of radio monitoring receivers 1 General aspects The noise figure depends mainly on the following parameters: the frequencies used for the test; the receiver settings (e.g. preamplifier, attenuator); the temperature prevailing during the tests. Furthermore, to correctly assess the
11、 noise figure: the measurements have to be done over the whole frequency range of the receiver; a maximum value for the noise figure must be specified and published by the manufacturer in the data sheet over the receivers whole operating range. Since noise figure values are frequency dependent the m
12、anufacturer can choose to additionally specify the noise figure for selected frequency bands or ranges; a mean value (the arithmetic mean of a number of test measurements) may also be indicated; the published noise figure values have to be valid over the entire temperature range indicated in the dat
13、a sheet. Limitations, if any, have to be mentioned in the data sheet. 2 Basics of the noise figure measurements The noise figure is one of the main specifications of a monitoring receiver. The noise figure is closely tied to the sensitivity of the monitoring receiver. The noise figure of a monitorin
14、g receiver is the factor by which the noise power delivered by the monitoring receiver increases when a reference noise is applied to the input. The noise figure is referenced to the input of the monitoring receiver and is measured at the output. The noise figure of a monitoring receiver can be meas
15、ured by several methods: “gain” method “Y-factor” method (noise source method) “sensitivity” method. The measurements must be done over the whole frequency range by tuning the receiver to the test signals with the frequencies f1, f2, . fn. Per octave at least two frequencies being evenly distributed
16、 over the full frequency range of the receiver have to be chosen. The receiver must be set up under normal operating conditions. All the attenuators of the monitoring receiver, if any, are to be set to their minimum attenuation. The automatic gain control (AGC) must be switched off during the tests.
17、 If a switchable preamplifier exists, the measurements must be done in the condition “preamplifier on”. Additionally the measurements can be done in the condition “preamplifier off”. The condition “preamplifier on” may also be expressed as “high sensitivity mode” or “low noise mode”. Rec. ITU-R SM.1
18、838 3 3 Definition of the test procedures for measuring the noise figure of receivers The noise figure measurements have to be performed following the guidelines in 1 and 2. 3.1 “Gain” method 3.1.1 Principle The noise figure formula at 25 C is the following: NF = Pout+ 174 Gain where: NF: noise figu
19、re of the system to be measured (dB) Pout: noise power density on the output of the system (dBm/Hz) Gain: gain of the system to be measured (dB). 3.1.2 Measurement set-up The measurement set-up in Fig. 1 should be used for the gain method. Measurement procedure Step 1: Connect a signal generator to
20、the monitoring receiver input and tune the monitoring receiver to the measurement frequency. Apply a CW tone with a level providing an SNR 30 dB. Step 2: Using the spectrum analyser, measure the level of the input power, Ne(dBm), then of the output power of the monitoring receiver, Ns(dBm). The gain
21、 is then Gain = Ns Ne. Step 3: Connect the 50 load to the monitoring receiver input. Using the spectrum analyser, measure the noise power density, Pout(dBm/Hz). The monitoring receiver should be set for the same parameters (manual gain, frequency, position of amplifiers or attenuators) for both the
22、gain measurement (Steps 1 and 2) and the noise figure measurement (Step 3). Step 4: Apply the formula given in 3.1.1. 3.2 “Y-factor” method 3.2.1 Principle The principle of this method is to apply a calibrated noise source to the input of the monitoring receiver. The noise density is measured, using
23、 a spectrum analyser and the noise diode in state ON or OFF. The following formula is then applied: ( )110log10)10/(=YENRNF 4 Rec. ITU-R SM.1838 where: NF: noise figure of the monitoring receiver to be measured (dB) ENR: excess noise ratio of the noise source (dB) Y: noise density difference (dB), m
24、easured when the source is switched ON and when it is switched OFF. 3.2.2 Measurement set-up The measurement set-up in Fig. 2 should be used for the Y-factor method. Measurement procedure Step 1: Connect the noise source to the monitoring receiver input, set the noise source power supply ON and tune
25、 the monitoring receiver to the measurement frequency. Step 2: Using the spectrum analyser, measure the noise density, NON, on the output (dBm/Hz). Step 3: Switch OFF the noise source power supply and measure the noise density, NOFF, on the monitoring receiver output. The parameter Y = NOFF NON.Step
26、 4: Apply the formula given in 3.2.1. 3.3 Sensitivity measurement to determine the noise figure This indirect method can be used but may give a different result than a measurement with the other two methods. The reason is that more components (IF section including demodulator, audio section and psop
27、hometric audio filter) in the receiver chain are included in the measurement. However the noise figure of receivers for analogue modulation can be properly determined using this method. 3.3.1 Principle The noise figure can be derived from the monitoring receivers AM sensitivity using the following f
28、ormula: +=221log10)log(10174mmResSNF where: NF: monitoring receiver noise figure (dB) S: sensitivity limit of the monitoring receiver (dBm), reduced by the signal-to-interference ratio including noise and distortion (SINAD) value of the sensitivity measurement (e.g. 12 dB at AM) Res: effective noise
29、 bandwidth of the filter used for the measurement (Hz) m: index of AM (A3E) modulation, used for the sensitivity measurement. Figure 3 indicates the relation between noise figure and sensitivity. Rec. ITU-R SM.1838 5 3.3.2 Measurement set-up Monitoring sensitivity is defined as the minimum input sig
30、nal required for proper demodulation of the received signal. For this measurement the audio level has to be determined by a SINAD measurement, using a psophometric filter (ITU-T Recommendation P.53) simulating the human ear. The monitoring receiver sensitivity is measured according to the sensitivit
31、y measurement method described in Recommendation ITU-R SM.1840. 3.3.3 Measurement parameters The sensitivity measurement will be done for AM only at the test frequencies. The selection of the test frequencies is done according to 2. If the sensitivity value is expressed in V it must be converted to dBm with: Value (dBV) = 20 log value (V) e.g. for 1V: 20 log 1(V) = 0 dBV Value (dBm) = Value (dBV) 107 e.g. for 0 dBV: 0 dBV 107 = 107 dBm assuming an input impedance of 50 .
