1、CCIR VOLUME*X-1 90 4855232 0503973 b The CCIR, Rec. 703 RECOMMENDATION 703 * 95 CHARACTERISTICS OF AM SOUND BROADCASTING . REFERENCE RECEIVERS FOR PLANNING PURPOSES (1990) CONSIDERING (a) (b) (c) planning context; (d) frequency planning purposes; (e) that frequency assignment plans must of necessity
2、 take into account the characteristics of receivers ; that the range of performance of receivers used by the public is very large; that a reference receiver with characteristics based on currently available receivers may be useful in a that standards for reference receivers should therefore be defin
3、ed, which can be taken as a basis for that these standards need to be taken into account by receiver manufacturers, UNANIMOUSLY RECOMMENDS . that the receiver characteristics contained in Annex I should be used for AM sound broadcasting planning purposes. ANNEX I In deriving the recommended characte
4、ristics, the parameters contained in Annex II were also considered. 1. Sensitivity For planning purposes, “sensitivity” is understood to mean “noise-limited sensitivity”, given in terms of field strength, required to achieve a specified signal-to-noise ratio at the audio output. Most commonly availa
5、ble AM receivers are now provided with built-in antennas. In the LF and MF bands, these are usually ferrite antennas (see Note l) For the HF bands, when included, telescopic rod antennas are frequently used. Therefore, receivers using these types of antennas should be used as a reference, even thoug
6、h a variety of external antennas may occasionally be used to improve reception. For those Administrations that employ wide-band systems (see Note 2) in the MF band, the use of monopole antennas is preferred as a reference. Sensitivity should be presented as a single mean figure for each broadcasting
7、 band, from which the minimum usable field strength may be calculated taking into account other influences (e.g. man-made noise). The following values are suggested for the minimum sensitivity of an average receiver: Band 5 (LF): 66 dB(pV/m) Band 6 (MF): Band 7 (HF): These values are based upon an A
8、F signal-to-unweighted noise (r.m.s.) ratio of 26 dB and are related to a modulation of 30%. For other AF signal-to-noise ratios the corresponding minimum sensitivity can be easily calculated (see Annex II, 0 6). The AF signal-to-noise measurement is made according to IEC Publication 315-3; the fiel
9、d-strength values for the LF and MF band are measured according to IEC Publication 315-1. 60 dB(pV/m) (see Note 3) 40 dB(pV/m) (see Note 4) * The Director of the CCIR is requested to bring this Recommendation to the attention of the IEC. This Recommendation supersedes Report 617-2 (Dubrovnik, 1986)
10、which is hereby deleted. CCIR VOLUME*X-II 90 W 4855232 0503974 8 W 96 Rec. 703 Note I - A special arrangement of two ferrite rod antennas with their outputs separately received and processed up to the stage of detection has been reported to substantially reduce the effects of fading in the night-tim
11、e interference zone in band 6 (MF) and 7 (HF) CCIR, 1970-741. Note 2 - According to present day planning arrangements in the various ITU Regions, in general a “narrow- band system” refers to one in which the system bandwidth is less than 5 kHz. A “wide-band system” refers to one with a system bandwi
12、dth greater than 5 kHz. Note 3 - Values of 54 dB(pV/m) and 40 dB(pV/m) were also supported respectively by CCIR, 1982-86a and b and CCIR, 1986-90al. Note 4 - The WARC HFBC(2), Geneva 1987, adopted this value for DSB and SSB reception. 2. Selectivity Selectivity of a receiver is a measure of its abil
13、ity to discriminate between a wanted signal to which the The selectivity is understood as an effective selectivity comprising RF selectivity, IF selectivity, demodu- receiver is tuned and unwanted signals entering through the antenna circuit. lator and AF frequency response. The selectivity shall be
14、 sufficient, so that the relative RF protection ratios given in 0 2.1, 2.2 and 2.3 are met. Relative RF protection ratios are defined in Recommendation 560 and should be measured according to Recommendation 559. 2.1 For LF, MF, HF bands in case of DSB reception in a narrow band system (see Note 2 of
15、 0 i), the relative RF protection ratios of Recommendation 560, curve D, should be met for a carrier frequency separation 20 kHz a constant value of -55 dB should be met. The use of curve D is suitable for a situation where a maximum number of channels is to be planned in a given area and where qual
16、ity criteria are not considered as a priority factor. The protection ratio curves of Recommendation 560 are based on a single receiver selectivity curve. It should, however, be noted that other combinations of 3 dB bandwidth and selectivity roll-off can also meet the relative protection ratio curves
17、 shown in Recommendation 560. (Examples see Annex II, 0 2.) 2.2 For wide-band systems, in the MF band, where a wider bandwidth of the audio-frequency modulating signal is employed, the use of the relative protection ratios of Recommendation 560, curve A or By may be more appropriate. However, these
18、curves were based upon the EBU MBF reference receiver. 2.3 For HF bands, in the case-of SSB reception (after the transition period) see Report 1059. The relative protection ratios of Fig. 1 should be met for a carrier frequency separation 10 kHz a constant value of -57 dB should be met (see also Rec
19、ommendation 640). Relative RF-protection ratios A, for SSB are given with respect to the frequency difference Af between the wanted carrier fw and the interfering carrier J;: Af = fw - J;, thus, negative Af describes interference from the upper adjacent channel. 3. Performance in the presence of str
20、ong signals AM broadcasting receivers overloading by strong input signals may result in: - desensitization; - cross-modulation and intermodulation; . - AF signal distortion in the amplifier stages and/or in the demodulator. Limiting values of maximum input voltage to the receiver to be taken into ac
21、count in planning, cannot be recommended due to the unavoidable occurrence of the phenomenon in close proximity of AM transmitters. These difficulties may be alleviated by a careful choice of the transmitter site at the planning stage and/or by implementing case by case solutions (ruggedized receive
22、rs) when receiving locations near the transmitting station cannot be avoided. CCIR VOLUME*X-II 90 4855232 0503975 T W Rec. 703 97 -12 -10 -8 -6 -4 -2 O 24 6 8 10 12 Af (kW FIGURE 1 - Relative RF-protection ratios A, for SSB * The WARC HFBC( i), Geneva, 1984, adopted this cu b. 10/334 (Working Group
23、1OA). 1986-901: a. IWP 10/7-38 (United States of America). CCIR VOLUME*X-1, 90 m 4855212 0503976 1, m Frequency (kH4 2 5 10 98 Rec. 703 Overall frequency response (dB) -3 - 24 - 59 ANNEX II In defining the recommended characteristics given in Annex I the possible influence of the following receiver
24、parameters was takeri into account. 1. Overall audio-frequency response and system considerations The overall audio-frequency response has a strong influence on the radio-frequency protection ratio curves., 1.1 Narrow-band systems The curves defined in Recommendation 560 are based on the values show
25、n in Table I. TABLE I 1.2 Wide-band system In MF broadcasting, using 10 kHz channel spacing a standard preemphasis/deemphasis and 10 kHz bandwidth limitation has been implemented. This produces an emissionhecption system with an overall audio frequency response that is essentially flat from 50 Hz to
26、 nearly 10 kHz, limited only by the receivers choice of bandwidth. The system reduces interference caused to stations operating f 20 kHz removed in frequency and entirely eliminated undesired high-order dynamic intermodulation products that contribute to noise and interfer- ence on the MF band. This
27、 system is described in CCIR, 1986-9Oa and in Report 458. Its impact on relative RF protection ratios and, thus, selectivity of receivers is under study. 2. Relative RF protection ratios versus bandwidth and selectivity The protection ratio curves shown in Recommendation 560 for DSB systems can be m
28、et with different combinations of bandwidth and roll-off of the selectivity curve of the receiver. Some examples have been calculated using the numerical method as described in Recommenda- tion 559 (O 3). In all cases the parameters of the transmitter for the narrow-band system corresponded to, thos
29、e of curve D of Recommendation 560. For the wide-band system a transmitter bandwidth of 10 kHz was assumed. Five different combinations of receiver bandwidth B,(-3 dB) and roll-off of the selectivity curve (in dB/kHz at the steepest slope of the overall selectivity curve) have been chosen such that
30、at 9 kHz carrier difference for the narrow-band system, and at 20 kHz carrier difference respectively for the wide-band system a value of A, = -29.5 dB was reached in all cases. The value of -29.5 dB follows from curve D in Fig. 1 of Recommendation 560 and from the Rio Agreement, 1981. The results f
31、or a narrow-band system with 9 kHz channel spacing are shown in Fig. 2. 20 10 O - -10 3 c. L F -20 - 30 40 -50 -60 CCIR VOLUME*X-II 90 4855232 0503977 3 W Rec. 703 99 FIGURE 2 - Relative RFprotection ratios Arel (dB) for a narrowband system Combination Bn (kIW Roll-off (dB/kHz) . A 2.0 8 B 2.2 12 C
32、3.7 16 D 4.0 25 E 4.2 40 Bn : Roll-off: overall bandwidth (-3 dB) of the receiver (kHz) slope of attenuation of the overall selectivity characteristic of the receiver (dB/kHz at the steepest point of the slope) 3. Automatic gain control (AGC) performance Using the sensitivity values given in Annex I
33、, as a reference, it is assumed that the output level will not change by more than 6 dB for a reduction in signal level of 10 dB. Similarly, the output level will not change by more than 3 dB for an increase in signal level of 20 dB. It is assumed that the output level of an SSB receiver will not ch
34、ange by more than 3 dB when changing from reception of DSB emissions to SSB emissions with 6 or 12 dB carrier reduction and appropriate “equivalent sideband power” (see Report 1059). . 4. Automatic frequency control of SSB receivers It is assumed that the SSB receiver is equipped with a synchronous
35、demodulator, using for the carrier acquisition a device which generates a carrier by means of a suitable control loop which phase locks the receiver to the incoming carrier of the SSB emission of which the carrier can be reduced by up to 12 dB (see Report 1059). CCIR VOLUME*X-1 90 W 4855212 0503978
36、5 W 100 Rec. 703 5. Overall total harmonic distortion It is assumed that the overall total harmonic distortion does not exceed 3% at 80% modulation depth, measured according to IEC Publication 315-3. B. AF signal-to-noise ratio at higher input signal levels It can be assumed that the AF signal-to-no
37、ise ratio will improve linearly to at least 40 dB, with increasing input signal level. 7. System effects on stereophonic AM broadcasting (Under consideration.) 8. Compatibility between the main programme and additional information signals When additional signals are added, account must be taken of certain interference effects. Receiver designers should consider these in order to avoid interference to the main programme channel. REFERENCES CCZR Documenis 1986-901: a. 10/103 (United States of America).
