1、14 Rec. 612 RECOMMENDATION 612 * MEASUREMENT OF RECIPROCAL MIXING IN HF COMMUNICATION RECEIVERS IN THE FIXED SERVICE (Question 1/3, Study Programme 1M3) (1986) The CCIR, CONSIDERING (a) (b) (c) receivers ; that frequency synthesizers are widely used in HF communication receivers; that reciprocal mix
2、ing is an important characteristic of such receivers; that it is desirable to be able to compare the reciprocal mixing performance of different models of (d) (e) that to facilitate (c) it is necessary to standardize methods of measurement of reciprocal mixing; that the methods of measurement should
3、be independent of receiver noise factor, UNANIMOUSLY RECOMMENDS that the reciprocal mixing performance of HF communication receivers should be established, using the methods of measurement specified in 0 4 of Annex I. ANNEX I* 1. Introduction Frequency synthesizers are widely used in modern high qua
4、lity HF communications receivers. Besides having high frequency stability and accuracy, a frequency synthesizer is easy to operate and control. At present, however, the frequencies provided by the synthesizer are not always sufficiently pure, so that a considerable number of spurious components may
5、accompany the wanted signals in its frequency spectrum. At the same time, on both sides of the wanted output there are noise skirts which degrade the interference rejection and noise characteristics of the receiver. In recent years, a new requirement has therefore appeared in the specifications of H
6、F receivers, i.e. reciprocal mixing, defined as the degradation of the receiver output signal-to-noise ratio due to the mixing of strong interfering signals with the noise skirts of the synthesizer. Relevant reciprocal mixing effects are described in Report 704. This Annex provides a quantitative re
7、lationship between the synthesizer out-of-band noise characteristics and receiver reciprocal mixing so that the requirements for synthesizer out-of-band noise characteristics can easily be specified and performance comparison between various receivers facilitated. * * This Recommendation should be b
8、rought to the attention of Study Groups 1 and 8. This Annex contains text from Report 856 which is hereby deleted. y- COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesCCIR VOLUME*III 30 m 4855212 0509485 1 W Rec. 612 15 2. Effects of re
9、ciprocal mixing Reciprocal mixing in a receiver occurs when, during the reception of a wanted signal, a strong out-of-band interfering signal mixes with out-of-band skirt noise from the synthesizer, producing mixing products which fall into the receiver IF band, causing the receiver output signal-to
10、-noise ratio to be degraded (see Fig. 1). v, i L FIGURE 1 - Reciprocal mixing B: bandwidth of the receiver (Hz) fI: first intermediate frequency VL: wanfed output of the synthesizer- Vi: out-of-band noise density . VI: a strong interfering signal at the receiver input V,: wanted signal From Fig. 1,
11、an equation showing the relationship between VI and the following items, i.e. receiver output signal-to-noise ratio So/ N, synthesizer output signal purity VA/ 6 as well as the wanted signal V, can be derived Gao, 19771: (1) Where So is the signal at the receiver output and N, represents the recipro
12、cal mixing products only when V, is sufficiently large so that the front-end noise of the receiver can be neglected. Assuming the noise density at 20 kHz away from the wanted output of the synthesizer of a given receiver is -120 dB/Hz relative to the wanted output of the synthesizer, B = 2800 Hz, V,
13、 = 40 dB(pV), So/N, = 20 dB, the VI = 105.5 dB(pV). It can be seen from the above, given the wanted signal V, the signal-to-noise ratio So/N, at the receiver output, and the bandwidth B, the allowable level of interference VI rises as the out-of-band noise density of the synthesizer V;. is reduced.
14、It should be noted that, in the above calculation, the. effects of second and subsequent down conversions are not taken into account. This is justified because the oscillators used are usually fixed and would have significantly less problems in maintaining spectral purity. .- 3. Measurement of recip
15、rocal mixing Up to now, there is no internationally adopted method for measuring reciprocal mixing. The differences in the measuring methods lie in the specified level of receiver input signals and in the method of measuring reciprocal mixing products at the output. 7 F COPYRIGHT International Telec
16、ommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesI CCIR VOLUME*III 90 W 4855212 050948b 3 16 ,. Rec. 612 Methods commonly used for testing are given in Table I below: TABLE I 2 Methods of measuring reciprocal mixing Advantages Wanted signal (dB(PV) No signal No sig
17、nal Method of measuring reciprocal mixing products at receiver output Disadvantages Case 1 2 Does not require a wanted signal Dependent on noise factor. Pessimistic results compared with other methods . Increase unwanted signal level to double the noise power N, Increase the unwanted signal level un
18、til the noise power is equal to So dB(mW) obtained when a wanted signal O dB(pV) is applied to the input of the receiver Independent of noise factor. Simple measuring configuration Dependent on noise factor. If NF 15 dB, So / N, too small to measure O Increase unwanted signal level to raise the nois
19、e power by 10 dB 10 Increase unwanted signal level to reduce the original signal-to-noise ratio by 10 dB CCIR, 1982-86a Dependent on noise factor 10 - Increase unwanted signal level to make the So / N, equal to 10 dB Independent of noise factor Increase unwanted signal level to make the SJ N, equal
20、to 20 dB . Independent of noise factor Requires high level, typically + 110 dB(pV) of unwanted signal which can cause errors due to blocking, etc. 6 40 In-depth studies and experiments indicate that the methods given in cases 2 or 5 would be suitable as standard methods as they are independent of re
21、ceiver noise factor and approximate operational conditions CCIR, 1978-821. In addition, the signal levels involved fall within the normal linear dynamic range of good munication receivers. It is considered that if a higher level of wanted signal is used, the level of the unwanted signal would be so
22、large. that receiver blocking might occur. 4. Methods of measurement CCIR, 1982-86b 4.1 Method I (see case 2, Table I) The measurement shall be carried out with the receiver in a J3E mode (upper sideband with a nominal 3 kHz bandwidth), the automatic gain control inoperative, the RFAF gain control a
23、t maximum and any input attenuator adjusted to minimum attenuation. Inoperative means that either the AGC is turned off or else the AGC is not affecting receiver gain CCIR, 1982-861. I COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling ServicesCC
24、IR VOLUME*III 90 m 4855212 0509487 5 m Rei 612 17 The wanted signal shall comprise an unmodulated carrier at a level of O dB(pV) e.m, at a frequency 1000 Hz f 3 Hz above the carrier frequency to which the receiver is tuned. The wanted signal shall be applied to the receiver input and the AF gain adj
25、usted to provide a suitable level of output power, So. The wanted signal shall then be removed. The unwanted signal shall comprise an unmodulated carrier 20 kHz away from the wanted signal to which the receiver is tuned, The unwanted signal shall be applied to the receiver input and the level of the
26、 unwanted signal adjusted until the output noise power is equal to that obtained with the O dB(pV) e.m.f. wanted signal. The reciprocal mixing characteristic is represented by the level of the unwanted signal. Method II (see case 5, Table I) 4.2 The measurement shall be carried out with the receiver
27、 in a J3E mode (upper sideband with a nominal 3 kHz bandwidth), the AGC operative, the RF/IF gain control at maximum and any input attenuator adjusted to minimum attenuation. The measurement shall be made by means of the simultaneous application of two test signals, the wanted signal and the unwante
28、d signal to the input of the receiver. The signal-to-noise ratio shall be measured using a notch filter e.g. SINAD measuring equipment. The wanted signal shall comprise an unmodulated carrier at a level of 10 dB(pV) e.m.f. at a frequency 1000 Hz f 3 Hz above the carrier frequency to which the receiv
29、er is tuned. The unwanted signal shall comprise an unmodulated carrier 20 kHz away from the wanted signal. The level of the unwanted signal shall be adjusted until the output noise level is 10 dB below the wanted output signal level. It should be noted that wideband noise from the signal generator p
30、roducing the unwanted signal may affect the measurement. The reciprocal mixing characteristic is represented by the level of the unwanted signal. . 4.3 Typical values When the above two methods (case 2 and case 5) were used for measuring the performance of various receivers, it was found that the mi
31、nimum and typical values of the.reciproca1 mixing characteristic were 90 dB(pV) and 96 dB(pV) respectively. REFERENCES GAO ZHONGPING 2 March, 19771 Preliminary investigations on some main performance objectives of HF SSB receivers. Telecomm. Sci. Express. CCIR Documents 1978-821: 3/24 (China (Peoples Republic of). 1982-861: a. 3/58 (China (Peoples Republic of); b. 3/35 (United Kingdom); c. 3/59 (Japan). . COPYRIGHT International Telecommunications Union/ITU RadiocommunicationsLicensed by Information Handling Services
