1、The CCIR, CCIR VOLUME*I 70 4855232 0507745 T Rec. 332-4 31 RECOMMENDATION 332-4 SELECTIVITY OF RECEIVERS (Question 5711) (1953-1956-1959-1963-1966-1970-1974-1978) CONSIDERING (a) that the selectivity of a receiver is a measure of its ability to discriminate between a wanted signal to which the recei
2、ver is tuned and unwanted signals; (b) that economy in the use of the radio spectrum requires the maximum selectivity compatible with the technical and economic considerations relating to the particular class of receiver; (c) that the method of single-signal selectivity is used to express the perfor
3、mance of certain characteristics of the receiver. The measurements are made with sufficiently low levels of input to avoid non-linearity (e.g. overloading) affecting the results; automatic gain control, automatic frequency control, etc., being rendered inoperative; (d) that measurement of selectivit
4、y with more than one signal should be the general method for measuring the selectivity. Sometimes the non-linear effects are numerous, then it will be necessary to select the most representa- tive cases to simplify the measurements; (e) that defined methods of single-signal and multiple-signal selec
5、tivity measurements are desirable to permit comparison of receivers, UNANIMOUSLY RECOMMENDS 1. modulation of the wanted signal without significant distortion (see also Recommendation 328 Q 1.1); 2. 2.1 2.2 the limitations of the selectivity of the receiver by unavoidable amplitude non-linearity, e.g
6、. cross-modulation ; 2.3. the fact that an excessively large attenuation-slope may lead to serious distortion of the phase/frequency characteristic in the passband; 2.4 the fact that selectivity and protection ratios are different characteristics, the first being a property of the receiver only, the
7、 second being an agreed minimum value, taking into account characteristics of the emission, propagation and reception; that the bandwidth of the receiver shall be no wider than is essential for the transmission of the necessary that in establishing the selectivity of a receiver, account should be ta
8、ken of: the unavoidable spread of the spectrum of signals in adjacent channels (see Recommendation 328, Q 3); 3. that the filters which determine the selectivity shall be included as near as possible to the receiver input, and the amplifying stages preceding the filters shall be sufficiently linear,
9、 to avoid significant loss of selectivity, e.g. by cross-modulation of the wanted signal by strong unwanted signals; 4. that, for the purpose of studying bandwidth or ,single-signal selectivity, the following definitions are used: 4.1 for amplitude-modulated signals (including single-sideband and in
10、dependent-sideband emissions), the passband is the band of radio frequencies accepted by the receiver and measured at the detector input, limited by the two frequencies for which the attenuation exceeds that of the most favoured frequency by some agreed value; in general this value is 6 dB, except f
11、or high-quality radiotelephony receivers where the value is 2 dB; 4.2 for frequency- or phase-modulated signals, the modulation acceptance bandwidth of a receiver, other than those used for broadcast reception, is twice the frequency deviation of an input signal which, when applied at a level 6 dB h
12、igher than the maximum usable sensitivity level, aad measured in accordance with Recommenda- tion 331, Q 11, will produce a Signal + noise + distortion noise + distortion . ratio equal to that specified for the maximum usable sensitivity level. This is an indication of the frequency deviation which
13、the receiver will accept without excessive degradation of the ratio Signal + noise + distortion noise + distortion , -. CCIR VOLUME*I 90 4855212 050794b 1 -. 32 Rec. 332-4 4.3 - of the difference in the attenuations corresponding to two different frequencies beyond the passband, - to the difference
14、between these frequencies; 4.4 - of the input signal level at the image frequency required to produce a specified output power from the receiver, -. to the level of the wanted signal required to produce the same output power. attenuation-slope: the attenuation-slope on each side of the passband is t
15、he rati: image-rejection ratio: the image-rejection ratio is the ratio: The image frequency is the wanted signal-frequency plus or minus twice the intermediate frequency, according to whether the frequency-change oscillator is respectively higher or lower in frequency than the wanted signal-frequenc
16、y . If the receiver incorporates more than one frequency change, theregwill be more than one image frequency, and for each of these will be a corresponding image-rejection ratio; 4.5 intermediate-frequency rejection ratio: the intermediate-frequency rejection ratio is the ratio: - of the level of a
17、signal at the intermediate frequency applied to the receiver input and which produces a specified output power from the receiver, to the level of the wanted signal required to produce the same output power; - 4.6 the frequency of an interfering signal and a harmonic of the local oscillator frequency
18、, etc., other spurious responses can occur when the intermediate frequency arises as the sum or the difference of spurious-response rejection ratio: the spurious-response rejection ratio is the ratio: - - of the input level at the interfering frequency required to produce a specified output power fr
19、om the receiver, to the level of the wanted signal to produce the same output power: 5. that single-signal measurements be made of the passband, the attenuation slope, the image response, the intermediate-frequency rejection and other spurious-response rejection ratios as defined above and also, in
20、frequency-modulation receivers, the modulation acceptance-bandwidth. For the attenuation-slope, sufficient indication is generally obtained by considering the frequency differ- ence corresponding to attenuations of 20, 40, 60 and if possible, 80 dB, reckoned from the limit frequencies of the passban
21、d. When the values thus obtained are essentially equal for the two sides of the passband, it is sufficient to give mean values. For some purposes it is of interest to know the bandwidth at fixed levels corresponding to the above-mentioned attenuations. These figures can easily be deduced from the pa
22、ssband and the attenuation-slopes at the different levels (see Fig. 1). Since, when plotted in decibels to a logarithmic scale of frequency, the sides of the selectivity characteris- tics are often almost straight beyond a certain frequency difference relative to the mid-band frequency, the attenuat
23、ion outside the passband can also be expressed as the slope of the attenuation/frequency characteristic, in decibels per octave of the frequency difference. The frequency and attenuation at the starting point of such a slope, relative to the mid-band frequency, should be stated; 6. that, for the pur
24、pose of studying the selectivity in the non-linear region with two or more input signals the following definitions are used: 6.1 effective selectivity: the effective selectivity is the ability of the receiver to discriminate between the wanted signal (to which the receiver is tuned) and unwanted sig
25、nals (having frequencies generally outside the passband), the level of which is such as to produce non-linear effects, the wanted and unwanted signals acting simulta- neously. The effective selectivity can be investigated by measuring blocking, adjacent-signal selectivity (or adjacent-channel select
26、ivity, if there is regu!ar channelling) and radio-frequency intermodulation, as follows: 6.2 blocking: blocking is measured by the level of an unwanted signal on a nearby frequency, e.g., in an adjacent channel, which results in a given change (generally a reduction) e.g., 3 dB, in the output power
27、due to a modulated *wanted signal of specified level applied to the receiver input; 6.3 adjacent signal selectivity: one of two following principles of measurement is used: 6.3.1 The adjacent-signal selectivity is measured by the level of an unwanted modulated signal at a frequency near to that of t
28、he wanted signal which results in an output power from the receiver (sum of the power of all unwanted components) of a specified amount (e.g., 20 dB) below the power due to the modulation of the wanted signal (adjacent-signal selectivity type A). -=-ri * Except for class of emissions AIA and A1B whe
29、n an unmodulated carrier is used. CCIR VOLUME*I 90 4855212 0507947 3 Rec. 332-4 33 FIGURE 1 - Coniwsioii beriiPeri trierhods oJpresenratioti cf sitgle- signal se1ecrii.ii.r The formula: Ba+lorr = B + 20 (2t1/Pfl) can be used to convert to bandwidths at specified levels. where: a: the attenuation at
30、the edge of the passband B,+2,l: the bandwidth at a level (a + 20n) dB from the middle of the passband in kHz B: the bandnidth of the passband in kHz Ptl: slope of the attenuation in dB/kHz ri: an integer (I. 2. 3 or 4) The measurement of the level of the unwanted signal may be made with the modulat
31、ion of the wanted signal removed. In the case that the modulation is not removed, the output power due to the modulation shall be excluded from the measurement by adequate audio-frequency filtering or a wave analyzer shall be used to measure the unwanted components. With receivers for amplitude-modu
32、lated classes of emission with reduced or suppressed carrier, the wanted signal shall be modulated. 6.3.2 The adjacent-signal selectivity is measured by the level of an univanted signal* at a frequency near to that of the wanted signal which results in a degradation of the wanted modulated signal at
33、 the output of *the receiver either *: - by a specified change in the ratio: Signal + noise + distortion noise + distortion (e.g., of 6 dB) when measured at the maximum usable level of sensitivity (adjacent-signal selectivity or to a specified value (e.g., 12 dB) when measured at values above the ma
34、ximum usable level of sensitivity (adjacent-signal selectivity type B2). type BI), - * * The unwanted signal shall be modulated, except in those cases in which the modulation does not affect the result. Certain Administrations do not agree with the method of measurement described in $ 6.3.2, but Stu
35、dy Group 1 is awaiting results from the International Electrotechnical Commission (IEC) before deciding whether or not to amend this Recommen- dation. - r- CCIR VOLUME*I 90 W 48552LZ 0507948 5 34 Rec. 332-4 The measurement of adjacent-signal selectivity includes the effects of cross-modulation and i
36、nadequate intermediate-frequency filtering. The method given in 0 6.3.2 is to be preferred for frequency-modulation receivers in the mobile services (class of emission F3E) *. Note. - For single-sideband and independent-sideband emissions, a modulated signal is deemed to comprise a reduced carrier (
37、if applicable) and one sinusoidal component in only one of its sidebands. intermodulation: intermodulation is measured in terms of the levels of two unwanted signals which, when 6.4 applied together, produce at the receiver output either *: - a given level of intermodulation (e.g., 20 dB*) below the
38、 level produced by a wanted input signal (intermodulation type A), or - a specified degradation in the ratio Signal + noise i- distortion noise + distortion (e.g., of 6 dB) if measured at the maximum usable sensitivity level (intermodulation type Bl), or a degradation of the ratio - Signal + noise -
39、I- distortion noise + distortion to a specified value (e.g., 12 dB), when measured with signal levels in excess of the maximum usable sensitivity level (intermodulation type B2); when the frequencies F, and F”, of these unwanted signals have: 6.4.1 a sum equal to the intermediate frequency (firl = F
40、, + F“,), in which case, tests should be made with frequencies such that the unwanted signals will have frequencies close to, but not equal to, half the intermediate frequency; 6.4.2 a difference equal to the intermediate frequency (4f2 = F, + F”,), in which case, tests should be made with frequenci
41、es such that the unwanted signal at the lower frequency should have a frequency near to that of the wanted signal, e.g., in an adjacent channel; 6.4.3 unwanted signals should have frequencies close to, but not equal to, half the wanted signal; 6.4.4 a difference equal to the frequency of the wanted
42、signal (Fd2 = F, + F”,), in which case, the unwanted signal having the lower frequency should have a frequency near to that of the wanted signal, e.g., in an adjacent channel: 6.4.5 should have frequencies close to, but not equal to, half the image frequency; 6.4.6 a difference equal to that between
43、 the wanted signal and the nearer unwanted signal intermodula- tion product being of the third order (Fd3 = 2F, - F”,), in which case, the nearer unwanted signal should have a frequency near to that of the wanted signal, e.g., in an adjacent channel. a sum equal to the frequency of the wanted signal
44、 (Fdl = F, 4- Frrt,), in which case, the a sum equal to the image frequency ( suitable input levels may be +20 dB, +40 dB, +60 dB and +80 dB relative to 1 pV, or the maximum usable sensitivity level (see Note 2). The unwanted signals should be equal in level; in receivers for ME, they should be unmo
45、dulated, because the interference, resulting from the beat between the intermodulation product and the carrier of the wanted signal, is more severe than that due to any modulation; in receivers for R3E, B8E, J3E, F1B and F3E in the mobile services, they should also be unmodulated and the frequency o
46、f one unwanted signal should be adjusted to make the output power of the receiver have a frequency equal to or, if the signal is filtered out, near to that of the modulation initially applied to the wanted signal; 7. that, to express the selectivity in the non-linear region, it is desirable that mea
47、surements be made of the effective selectivity in terms of the blocking, adjacent-signal selectivity and radio frequency intermodulation characteristics as defined above. 8. that, with a view to the ultimate statistical treatment of the presented data, Administrations should be encouraged to provide
48、 results of measurements made on receivers of recent design, in accordance with the provisions of this Recommendation. Note I - The application of multiple-signal tests of effective selectivity to receivers for AlA, AlB, A2A, A2B and F1B signals is the subject of further study (Question 5711 (Geneva
49、, 1982). Note 2 - To enable the measurements to be made with two signal generators, the sensitivity of the receiver can be adjusted by the use of a suitable potential applied to the automatic-gain-control circuit, to correspond to the input signals recommended. In this case, one of the unwanted signals should be moddated. A correction should be made for the depth of modulation.
