1、I CCIR VOLUME*X-3 90 W 4855232 0503884 7 W 6 The CCIR, Rec. 559-2 RECOMMENDATION 559-2 * OBJECTIVE MEASUREMENT OF RADIO-FREQUENCY PROTECTION RATIOS IN LF, MF AND HF BROADCASTING (Question 44/10, Study Programme 44A/10) (1 978-1 982-1990) CONSIDERING that the radio-frequency protection ratio is direc
2、tly related to the audio-frequency protection ratio (see that this relationship depends on a number of technical parameters, such as: (u) Recommendation 638): (b) - - - - - - the dynamic compression: - - - the frequency separation between the wanted and unwanted carriers; the bandwidths of the trans
3、mitter and the receiver; the rate-of-cut of the band-limiting filters at the transmitting and receiving end; the type and depth of modulation; the spectral energy distribution of the modulation signal; the pre-emphasis and de-emphasis characteristics, if any; the out-of-band radiation of the transmi
4、tter; the amplitude/frequency response of the human ear, which can be simulated by the weighting network of the measuring instrument, (see Recommendation 468); the amplitude of the receiver input voltage, - UNANIMOUSLY RECOMMENDS that, once an audio-frequency protection ratio has been agreed upon, o
5、ne of the following objective two-signal methods should be used for the determination of radio-frequency protection ratios in amplitude-modu- lation sound broadcasting. 1. Objective measuring method 1.1 Principle The objective method is essentially a two-signal method which consists in modulating su
6、ccessively, with a given modulation depth, the wanted and the interfering transmitter by a standard colour noise signal, the spectral amplitude distribution of which corresponds to modern dance music programmes. The interference effect is measured at the audio-frequency output of the receiver by mea
7、ns of a single channel measurement circuit using a standardized instrument or an instrument based on a two-channel measure- ment circuit (see $ 1.2). 1.2 Output measirretnent For measuring the Wanted and interfering signals at the output of the receiver, use should be made of: - a standardized instr
8、ument which should include a network for weighting the subjective interference effect of different interfering frequencies in accordance with Recommendation 468 and a voltmeter suitable for r.m.s. measurements *; or a special instrument based on the circuit shown in Fig. 1 and having weighting filte
9、rs with an . amplitude-frequency characteristic as shown in Fig. 2. This instrument has circuits for the separation of narrow-band and wideband interference by means of retunable band and rejection filters respectively, circuits for weighting each of these types of interference with maxima in the re
10、gion of 4 kHz, and 0.5 kHz and 3.0 kHz respectively, an adder and an r.m.s. voltmeter. - ,. * * Further information is given in Recommendation 560. The use of an r.m.s. rather than a quasi-peak meter as given in Recommendation 468 permits more accurate account to be taken of the beat-note predominan
11、t with-closer frequency spacings and all other effects. This conclusion was drawn from the excellent agreement between the values of radio-frequency protection ratio obtained, either using the objective two-signal method, or from subjective listening tests for all values of frequency spacing. CCIR V
12、OLUMEUX-1, 90 4855212 0503885 9 I Rec. 559-2 7 / , I Rejection f FR 2 L- I filter -fi L 1 I- i I 1.rn.s. voltmeter FIGURE 1 - Schematic of the measuring apparatus FR1, FE: weighting networks F I lIlIIII)(II I , II I 60 100 140 200 300 500 700 1000 1400 2000 3000 5000 7000 10000 16000 Frequency f( E)
13、 FIGURE 2 - Frequency responses of the weighting networks CCIR VOLUME*X-3 90 W 4855232 050388b O D 8 . Rec. 559-2 1.3 Noise signal for modulating the signal generators Two conditions must be fulfilled by the standardized signal to simulate programme modulation: its spectral constitution must corresp
14、ond to that of a representative broadcast programme; its dynamic range must be so small as to result in a constant uniquivocal reading on the instrument. - - The amplitude distribution of modern dance music was taken as a basis, as it is a type of programme with a considerable proportion of high aud
15、io-frequencies, which occur most frequently. However, the dynamic range of this type of programme is too wide and does not fulfil, therefore, the second requirement mentioned above. A signal which is appropriate for this purpose is a standardized coloured noise signal, the spectral amplitude distrib
16、ution of which is fairly close to that of modern dance music (see curve A of Fig. 3, which is measured using one-third octave filters). * This standardized coloured noise signal may be obtained from a “white-noise” generator by means of a passive filter circuit as shown in Fig. 4. The frequency-resp
17、onse characteristic of this filter is reproduced as curve B of Fig. 3. (It should be noted that the difference between curves A and B of Fig. 3 is due to the fact that curve A is based on measurements with “one-third octave” filters which pass greater amounts of energy as the bandwidth of the filter
18、 increases with frequency. The spectrum beyond the required bandwidth of the standardized coloured noise should be restricted by a low-pass filter having a cut-off frequency and a slope such that the bandwidth of the modulating signal is approximately equal to half the standardized bandwidth of emis
19、sion. The audio-frequency amplitude/frequency characteristic of the modulating stage of the signal generator shall not vary by more than 2 dB up to the cut-off frequency of the low-pass filter. O IO 20 30 40 20 50 1 IO0 200 500 Frequency (Hz) FIGURE 3 Curve A: I:rcqiiciicy spectrum of standnrdized n
20、oise (nicasurcd with one-third octave filters) Curve 1): Frequency respoiisc cliarsctcristic of filter-circuit * Recommendation 571 proposes a different coloured noise signal. The use of that signal instead of the one proposed in this Recommendation would lead to different relative values of the rad
21、io-frequency protection ratio which would only be justified if the characteristics of a typical programme signal would better be simulated by the coloured noise of Recommendation 571. (See Report 798.) CCIR VOLUME*X-1 90 LI855212 0503887 2 = Rec. 559-2 6000 068pF r-v Y ! 9 Rr5 kn FIGURE 4 - Filter-c
22、ircuit 1.4 Measitring arrangements Figure 5 shows a schematic diagram of the measuring arrangements, in which the elements of fundamental importance are drawn in thick outlines. The other elements are measuring and control devices which are required for putting the investigations into practice, or f
23、or facilitating them. . A: B: C: D: E: F: G: H: J: K: L: I- r FIGURE 5 - Schematic of the measuring apparatus 1 kHz audio-frequency generator (for M: calibrated attenuator calibration of the depth of modulation) calibrated attenuator noise generator P: artificial antenna (according to noiseshaping f
24、ilter (see Fig. 4) calibrated attenuator Q: receiver under test N : frequency meter for measuring the frequency difference between signal generators G and K Recommendation 331) low-pass filter signal generator (wanted signal) modulator calibrated attenuator signal generator (interfering signal) modu
25、lator R: instrument for measuring r.m.s. values in accordance with 1.2 oscilloscope (for monitoring purposes) selector switch for the modulation (I kHz tone or standardized noise signal) change-over switch ror the modulation (signal generator G or K) S : T: U: CCIR VOLUME*X-3 90 m Li855232 0503888 4
26、 m 10 Rec. 559-2 1.5 Depth of modulation of the test transmitters The depth of modulation of the wanted or interfering signals is determined by the following procedure. The signal generator is first modulated to a depth of 50% with a sinusoidal tone at 1 kHz from the generator A, adjusted by means o
27、f the attenuator B and verified by oscilloscope S at the radio-frequency outputs of modulators H or L, and the required audio-frequency voltage is measured at the modulator inputs (switch U) by means of the instrument R. The amplitude of the noise signal (C + D), which is measured with the same meas
28、uring instrument R, should then be adjusted (by means of the attenuator E) to read 6 dB lower than the value obtained with the sinusoidal signal; provided that the instrument has a time constant of 200 ms. This corresponds to a depth of modulation of 50% measured with a programme meter with quasi-pe
29、ak indication. Deeper modulation is not appropriate, because, on account of its very small dynamic range, the noise would have a more disturbing effect than any real programme. 1.6 Audio-frequency signal:to-interference ratio The signal generator (wanted signal) (G + H + J) modulated with noise acco
30、rding to 9: 1.3 and 1.5 produces a signal at the audio-frequency output of the receiver under test Q, which represents, measured with the instrument R, the reference level “zero”. The noise modulation is then transferred by means of the switch U, from the audio-frequency input of the modulator H of
31、the signal generator (wanted signal) to the audio-frequency input of the modulator L of the signal generator (interfering signal). After suppression of the modulation of the wanted signal, the radio-frequency level of the interfering signal generator (K + L + M) is then adjusted so that the unwanted
32、 voltage, as measured by means of the instrument R at the receiver output, results in the required audio-frequency signal-to-interference ratio ; for example, 20, 30 or 40 dB below the reference value. 1.7 %e radio-frequency level of the wanted signal at the receiver input The.radio-frequency output
33、 voltage of the wanted-signal generator (G + H + J) should be, to begin with, as small as possible, so that only linear receiver characteristics enter into the measurement. The level of the unmodulated signal generator should, however, be chosen high enough for the noise voltage (mostly the inherent
34、 noise of the receiver), measured at the audio-frequency output of the receiver, to lie at least 3 dB below the noise voltage in the presence of the modulated interfering signal according to 0 1.6. The radio-frequency output of the signal generator (G +H + J) should then be increased in steps to inc
35、lude the non-linear characteristics of the receiver (cross:modulation). 1.8 Influence of non-linear distortion in the signal generators The non-linear distortiofi occurring during the modulation process in the signal generator has components which widen the radio-frequency spectrum and thus give ris
36、e to increased radio-frequency wanted-to-interfering signal ratios in the region of the adjacent channel and the adjacent-channel but one. The non-linear distortion in the signal generators should not, therefore, exceed 1% to 2%. 1.9 Accuracy The results obtained with the objective method have been
37、compared with the results of corresponding subjective tests. From these tests, it has been found that objective measurements give a first approximation to those obtained with the subjective method. In cases where the wanted programme is particularly susceptible to interference (e.g. speech with long
38、 pauses) the difference between the objective measurements and the subjective tests may amount to more. than 5 dB CCIR, 1974-781. The results of protection ratio measurements depend to a comiderabre extent on the receiver passband. For measurements or calculations by the single-channel method, the e
39、rror in protection ratio measurements A A versus frequency difference Af for two receiver passband values on the intermediate frequency 2Af = 9 kHz and 2Af = 5 kHz (at -6 dB) is shown in Fig. 6 CCIR, 1986-901. 2. Numerical method 2.1 Principle To determine the relative radio-frequency protection rat
40、io, ie physical processes underlying the objective methods, viz., the determination of the weighted noise power by single- or two-channel methods (see 9: 1.2), are simulated by means of a mathematical model. CCIR VOLUME*X-1 90 4855212 0503889 b W 20 15 10 5 h m 2. P 4 O -5 -50 -15 r . Rec. 559-2 7 c
41、 2Af =9kH: t 1 11 O 1 2 3 4 5.6 7 a 9 10 Af (kHz1 FIGURE 6 - Error inprotection ratio measurements AA versus frequency dwerence 4 Two channels with the carrier frequencies fT and fR, whose frequency difference is A f are assumed. The transmitter power density spectrum related to the incremental band
42、width Befl is simulated by the function FT depending on the relative frequency If I. This function is formed by multiplicative sub-functions (e.g. attenuation, spectral energy distribution, pre-emphasis of the higher audio-frequencies) and additive sub-functions (e.g. out- of-band radiation) or it i
43、s built up by polygonal traces. In a similar way, the overall response of the receiver, including weighting of the noise power with the aid of single-channel or two-channel methods, was represented by means of the function FR or FR and FR, respectively, depending on the relative frequency I A f - f
44、I. In double-sideband modulation, FT and FR are symmetrical with respect to the respective carrier frequencies. Figure 7 shows the fundamental shape of the functions FT and FR, the most important sub-functions as well as the meaning of the designations used. The transmitter power density spectrum wi
45、th the carrier frequency fT produces an interference power APT in the received channel with the carrier frequency fR, which, at a given frequency difference AL can be calculated by integrating the product FT x FR. As a result, one obtains: When performing the integration according to (i), however, f
46、or Af = O (receiver exactly tuned to the transmitter frequency), one obtains the useful receiver power A PM. The relative radio-frequency protection ratio Ar, is the ratio of interference power to useful power, in the channel under consideration. CCIR VOLUME*X-L 90 q552L2 0503890 2 m 12 Rec. 559-2 3
47、0 20 10 O -1 o -20 -30 -40 - 50 -60 -70 -80 (Ar-f) 4 AfF? Af O Frequency f 2 FIGURE 7 - Fundamental relatiomhips BT BR Beff : incremental bandwidth to which the power spec- : 3 dB bandwidth of the transmitter (overall) : 3 dB bandwidth of the receiver (overall) trum is related and within which the p
48、ower density and the receiver attenuation response niay be assumed to be constant, in each case, for the caicu- lation f : carrier frequency of the transmitter f : carrier frequency in the received channel f : frequency separation I fR -fT I f, ; f, : lower and upper limits of integration ) : spcctr
49、al energy distribution in tlie sideband : filter attenuation characteristic for band limitation : prc-cmphasis (at high frequencies) at tlie trans- at the transmitter mitter : out-of-band radiation of the transmitter : function representing the transmitter power-den- sity spectrum : attenuation of the intermodulation products of the transmitter in the case of measurement with two tones : relative level of maximum power-density in the sideband, related to the carrier level : filter attenuation characteristic for band limitation at the receiver : weighting curve of
