Conducted RF Measurements to Quantify 10.7 MHz IF Interference to FM Receivers

Transcription

1 Page 1 of 46 ERA Business Unit: ERA Technology Report Title: Author(s): Client: Client Reference: Conducted RF Measurements to Quantify 10.7 MHz IF Interference to FM Receivers B S Randhawa I Parker Ofcom Saul Friedner ERA Report Number: ERA Project Number: 7G Report Version: Final Report Document Control: ERA Report Checked by: Approved by: Steve Munday Project Manager RF Group Martin Ganley Head of EMC & RF Group May 2007 Ref. SPM/vs/62/03452/Rep-6128

2 2 Copyright ERA Technology Limited 2007 All Rights Reserved No part of this document may be copied or otherwise reproduced without the prior written permission of ERA Technology Limited. If received electronically, recipient is permitted to make such copies as are necessary to: view the document on a computer system; comply with a reasonable corporate computer data protection and back-up policy and produce one paper copy for personal use. DOCUMENT CONTROL If no restrictive markings are shown, the document may be distributed freely in whole, without alteration, subject to Copyright. ERA Technology Ltd Cleeve Road Leatherhead Surrey KT22 7SA UK Tel : +44 (0) Fax: +44 (0) info@era.co.uk Read more about ERA Technology on our Internet page at: ERA Technology Ltd

3 3 Summary A number of studies have been carried out for FM receivers in order to both quantify the magnitude of the 10.7 MHz Intermediate frequency (IF) problem, and to derive a set of planning criteria used by both independent and BBC services. Since much of the research was carried out a few years ago Ofcom commissioned ERA Technology to investigate the IF relationship further to determine how present receivers operate. A total of forty-one FM radio receivers were tested. The receivers were acquired from the BBC, ERA staff and popular high-street electrical retailers. The radios tested fell into the following categories; car radio, hi-fi, music systems, portable and walkman type. The level of interference in the FM receiver was measured objectively based on the twosignal measurement method as described in Annex I of ITU-R Recommendation BS.641. Receivers were tested for both a positive frequency offset (with the interfering signal higher in frequency than the wanted signal), and a negative frequency offset (with the interfering signal lower in frequency). The worst results were found to occur with positive frequency offsets, when lower levels of field strength were required to cause the onset of interference. From the measurement results shown in Section 4 and Appendix A and B, the portable radio was most susceptible to an unwanted signal transmitting at a frequency offset 10.7 MHz above the tuned wanted signal, followed by the hi-fi, music system radios with a CD player, walkman type radios and finally the car radio. A minimum mean interference level of 62 dbμv/m for an unwanted signal with a positive frequency offset of 10.7 MHz was required to produce a 6 db decrease in the quality of the received audio for the portable radio. The car radio was the most immune to an unwanted signal at a 10.7 MHz positive frequency offset and required a minimum mean level of 103 dbμv/m to produce a 6 db increase in the audio noise level. For unwanted signal frequencies transmitting with a positive offset of 10.6 and 10.8 MHz, an extra 10 to 20 dbμv/m in field strength was required to produce the same effect as experienced at 10.7 MHz for the majority of hi-fi, music system and portable FM receivers tested. When the FM receivers were tested for an unwanted signal with a negative frequency offset, it was generally found that the required level of field strength was greater when compared with an unwanted signal transmitting with a positive frequency offset. 95% of the receivers tested were more susceptible to a 6 db in degradation of audio signal quality for an unwanted signal transmitting with a positive frequency offset compared with an unwanted signal transmitting with a negative frequency offset. ERA Technology Ltd

4 4 The music system radio receivers tested were the most susceptible to an unwanted signal transmitting with a negative frequency offset. The car radio receiver was the most immune to an unwanted signal transmitting with a negative frequency offset. A total of 32% of the receivers tested by ERA showed a drift towards a 10.6 or 10.8 MHz IF frequency problem instead of a 10.7 MHz offset. It also interesting to note that only four radio receivers (R5, R12, R15 and R28) out of the forty-one tested managed to achieve the desired S/N of 56 db as stated in ITU-R BS.641. This level of S/N was also only achieved for the higher wanted signal levels of 66 and 80 dbμv/m. All the receivers had a lower S/N at 54 dbμv/m compared to the higher wanted signal levels of 66 and 80 dbμv/m, which in some cases as low as 10 db. This was particularly noticeable for the walkman type radio receivers. However, fifteen out the fortyone receivers (37%) did achieve an S/N ratio of 50 db or more, again only at the higher wanted signal levels of 66 and 80 dbμv/m. The findings in this report support the potential interference problem highlighted by the RTPG, and its predecessors, which has taken the 10.7 MHz ± 0.1 MHz offset into consideration during the planning of FM VHF broadcasting services. The measurements also support the guidance [2] provided by the RTPG with respect to field strength levels in overlap regions, suggesting that: For 10.7 MHz offsets only minor coverage overlaps are allowable, although careful consideration is required regarding where these overlaps occur, e.g. avoiding overlaps in built-up areas. In areas of overlap the field strength of the higher frequency service should not exceed 65 dbμv/m at 10 m above ground level in urban areas. For 10.6 and 10.8 MHz relationships coverage overlaps are allowable to some extent, although care needs to be taken so that high field strengths are not present. The transmitter of neither service should be within the service area of the other. In areas of overlap the field strength of the higher frequency service should not exceed 80 dbμv/m at 10 m above ground level in urban areas. ERA Technology Ltd

5 5 Contents Page No. 1. Introduction 9 2. Objectives Measurements Introduction Test Set-Up Test Procedure Results FM Car Radio Receivers FM Hi-Fi Radio Receivers FM Music System Radio Receivers FM Portable Radio Receivers Walkman FM Radio Receivers Analysis Conclusions Acknowledgments References 32 APPENDIX A - FM Receiver Test Results for Positive Frequency Offsets 33 APPENDIX B - FM Receiver Test Results for Negative Frequency Offsets 39 APPENDIX C Test Equipment 45 ERA Technology Ltd

6 6 Tables List Page No. Table 1: Category of FM radio receivers tested Table 2: Measured mean S/N as a function of wanted field strength for FM car radios Table 3: Measured mean S/N as a function of wanted field strength for FM car radios Table 4: Measured mean S/N as a function of wanted field strength for FM hi-fi radios Table 5: Measured mean S/N as a function of wanted field strength for FM hi-fi radios Table 6: Measured mean S/N as a function of wanted field strength for music systems Table 7: Measured mean S/N as a function of wanted field strength for music systems Table 8: Measured mean S/N as a function of wanted field strength for FM portables Table 9: Measured mean S/N as a function of wanted field strength for FM portables Table 10: Measured mean S/N as a function of wanted field strength for FM walkman Table 11: Measured mean S/N as a function of wanted field strength for FM walkman Table 12: Summary of results for FM radio receivers tested for interference with a positive frequency offset to the tuned wanted signal Table 13: Summary of results for FM radio receivers tested for interference with a negative frequency offset to the tuned wanted signal Table 14: Mean level of interference to cause a 6 db drop in S/N for a FM car radio from an unwanted signal with a positive frequency offset Table 15: Mean level of interference to cause a 6 db drop in S/N for a FM hi-fi radio from an unwanted signal with a positive frequency offset Table 16: Mean level of interference to cause a 6 db drop in S/N for a FM music system radio from an unwanted signal with a positive frequency offset Table 17: Mean level of interference to cause a 6 db drop in S/N for a FM portable radio from an unwanted signal with a positive frequency offset ERA Technology Ltd

7 7 Table 18: Mean level of interference to cause a 6 db drop in S/N for a FM walkman radio from an unwanted signal with a positive frequency offset Table 19: Mean level of interference to cause a 6 db drop in S/N for a FM car radio from an unwanted signal with a negative frequency offset Table 20: Mean level of interference to cause a 6 db drop in S/N for a FM hi-fi radio from an unwanted signal with a negative frequency offset Table 21: Mean level of interference to cause a 6 db drop in S/N for a FM music system radio from an unwanted signal with a negative frequency offset Table 22: Mean level of interference to cause a 6 db drop in S/N for a FM portable radio from an unwanted signal with a negative frequency offset Table 23: Mean level of interference to cause a 6 db drop in S/N for a FM walkman radio from an unwanted signal with a negative frequency offset Figures List Page No. Figure 1: Picture of a typical portable radio tested Figure 2: Diagram of FM transmitter modules Figure 3: Diagram of conducted measurement set up based on ITU-R BS Figure 4: Plot of the mean interference level required to cause a 6 db drop in S/N for a FM car radio from an unwanted signal with a positive frequency offset Figure 5: Plot of the mean interference level required to cause a 6 db drop in S/N for a FM car radio from an unwanted signal with a negative frequency offset Figure 6: Plot of the mean interference level required to cause a 6 db drop in S/N for a FM hi-fi radio from an unwanted signal with a positive frequency offset Figure 7: Plot of the mean interference level required to cause a 6 db drop in S/N for a FM hi-fi radio from an unwanted signal with a negative frequency offset Figure 8: Plot of the mean interference level required to cause a 6 db drop in S/N for a FM music system radio from an unwanted signal with a positive frequency offset ERA Technology Ltd

8 8 Figure 9: Plot of the mean interference level required to cause a 6 db drop in S/N for a FM music system radio from an unwanted signal with a negative frequency offset Figure 10: Plot of the mean interference level required to cause a 6 db drop in S/N for a FM portable radio from an unwanted signal with a positive frequency offset Figure 11: Plot of the mean interference level required to cause a 6 db drop in S/N for a FM portable radio from an unwanted signal with a negative frequency offset Figure 12: Plot of the mean interference level required to cause a 6 db drop in S/N for a FM walkman radio from an unwanted signal with a positive frequency offset Figure 13: Plot of the mean interference level required to cause a 6 db drop in S/N for a FM walkman radio from an unwanted signal with a negative frequency offset Figure 14: Results of BBC receiver tests (1984) Abbreviations List FM IF IMD LO PPL RF S/N Frequency Modulated Intermediate Frequency Intermodulation Distortion Local Oscillator Phase Locked Loop Radio Frequency Signal-to-Noise ERA Technology Ltd

9 9 1. Introduction When planning new VHF FM broadcast services a number of frequency relationships need to be considered in order to reduce possible interference between both existing and planned services. One of these relationships is the intermediate frequency (IF), generated by the difference between a local oscillator (LO) within the listener s receiver, and the received frequency. Whilst the receiver intermediate frequency is expected to be 10.7 MHz this is offset in some receivers by ± 0.1 MHz and even more in a few percent of others [1]. For example if a service transmitted on 90.0 MHz happens to overlap with another service nearby transmitted on MHz (a difference of 10.7 MHz), a receiver which is tuned to 90.0 MHz could potentially suffer interference from the higher frequency service due to the IF being 10.7 MHz away. Due to the variation of individual receiver characteristics, the mechanism has been known to occur over a range of IF s between 10.6 MHz and 10.8 MHz. As a result of the potential interference problem the Radio Technical Policy Group (RTPG), and its predecessors, has taken 10.7 MHz ± 0.1 MHz into consideration during the planning of FM VHF broadcasting services. In order to reduce these IF interactions in the Band II frequency, the broadcast radio planners try to avoid allocating frequencies with these separations in the same geographic areas. The RTPG does provide guidance [2] and suggests that: For 10.7 MHz offsets only minor coverage overlaps are allowable, although careful consideration is required regarding where these overlaps occur, e.g. avoiding overlaps in built-up areas. In areas of overlap the field strength of the higher frequency service should not exceed 65 dbμv/m at 10 m above ground level in urban areas. For 10.6 and 10.8 MHz relationships coverage overlaps are allowable to some extent, although care needs to be taken so that high field strengths are not present. The transmitter of neither service should be within the service area of the other. In areas of overlap the field strength of the higher frequency service should not exceed 80 dbμv/m at 10 m above ground level in urban areas. A number of studies have been carried out in order to both quantify the magnitude of this problem, and to derive a set of planning criteria used by both independent and BBC services. Since much of the research was carried out a few years ago Ofcom would like to investigate the IF relationship further to determine how present receivers operate. ERA Technology Ltd

10 10 2. Objectives The programme of work was to consider experimental measurements to quantify the IF characteristics of 40 to 50 FM receivers, representing a range of receiver types in common use in the UK. The measurements programme used a test methodology based on ITU-R BS.641 [3] to determine the RF protection ratios for the receivers and quantify the IF characteristics of receivers in the 10.4 to 11.0 MHz frequency band. The objectives were to: Determine the protection ratio for 40 to 50 FM receivers from a range of categories. Characterise each receiver for its IF performance level for frequency offsets between 10.3 to 11.1 MHz. Prove the field strength level at which interference occurs for differing levels of wanted signal. Determine whether the interference comes from above or below the tuned (wanted) frequency. Observe any receiver functionality issues around the IF and how the audio quality was affected. A short literature review was undertaken at the start of the project to identify any other relevant standards or reports. Objective and subjective assessments of sound quality were considered using ITU-R BS and ITU-R BS respectively. However, the most suitable test methodology found for this project was ITU-R BS.641 as recommended by ITU- R [5]. 3. Measurements 3.1 Introduction When planning FM radio frequency coverage it has been normal to avoid MHz frequency spacings for co-coverage transmitters, as many receivers have been shown to produce spurious responses related to the use of a 10.7 MHz intermediate frequency. In areas where frequency availability is restricted, it is increasingly necessary to consider the use of 10.6 MHz or 10.8 MHz spacings. The local oscillator in an FM receiver is normally 10.7 MHz higher than the signal frequency. The mechanism involved in this interference is that the station on the higher frequency beats with the receiver local oscillator, with which it is nearly co-channel. It is also possible that interference will be caused to the upper frequency due to radiation from the local oscillators of receivers tuned to the lower frequency. As 10.7 MHz relationships have generally been avoided, there is little evidence regarding the practical severity of the second mechanism. ERA Technology Ltd

11 11 For the majority of FM receivers it has been proven that an unwanted signal operating 10.7 MHz above the wanted transmission produces the worst-case scenario of unwanted interference in the receiver [6]. The following section describes the test methodology used to measure the 10.7 MHz IF phenomenon particular of FM receivers. 3.2 Test Set-Up This section details the methodology used to measure potential interference levels due to unwanted IF produced by 2 nd order intermodulation products between the local oscillator of the FM receiver and an unwanted signal ± 10.7 MHz from the wanted signal. A total of fortyone FM radio receivers were tested. FM radio receivers were acquired from the BBC, ERA staff and popular high-street electrical retailers. The radios tested fell into the following categories as shown in the table below. Table 1: Category of FM radio receivers tested Category Total Number Car Radio 4 Hi-Fi 6 Music Systems 6 Portable 21 Walkman Type 4 The Hi-Fi system was classified as a large music system with separate speaker commonly found in homes. The music systems were defined as portable music systems with CD players and the portable radios were defined as just portable radios. The age of the receiver tested varied from brand new to 10 to 15 years old, allowing a broad range of FM receiver technology to be tested with respect to age and category. ERA Technology Ltd

12 12 Figure 1: Picture of a typical portable radio tested The level of interference in the FM receiver was measured objectively based on the twosignal measurement method as described in Annex I of ITU-R B.S.641 [3]. The wanted signal was a 500 Hz tone generated by the Lindos 101 audio oscillator and then transmitted using an NRG Kit Phase Locked Loop (PPL) FM transmitter. The FM transmitter comprised of a 15 khz low pass filter, a 50 μs pre-emphasis circuit, a stereo encoder and a transmitter carrier at the wanted received frequency (See Figure 2). ERA Technology Ltd

13 13 Audio In 15 khz LP Filter 50 μs Preemphasis Stereo Encoder RF Signal Generator FM Out Figure 2: Diagram of FM transmitter modules The unwanted signal was created by FM modulating a standard coloured noise signal, i.e. pink noise generated by a HP 33250A arbitrary wave form generator modulated on a RF carrier, produced by a Rohde and Schwarz (R&S) signal generator with a 50 μs preemphasis circuit. The spectrum beyond the required bandwidth of the standardized coloured noise was restricted by a low-pass filter having a cut-off frequency of 15 khz, before being transmitted using the signal generator. The various pieces of equipment used in the conducted measurement set-up described in Figure 3 below were connected via screened cables. The measurements were performed in a screened room to prevent any unwanted FM broadcast influencing the results of the radio receivers tested (See picture above). Also, the 1 W FM transmitter was placed in a fully anechoic room, ensuring that any spurious radiation emanating from metal transmitter enclosure was not directly coupling onto the radio receiver being tested. The receivers were tested for wanted field strengths of 54, 66 and 80 dbμv/m. ERA Technology Ltd

14 Test Procedure Audio Oscillator FM Stereo Transmitter Attenuator Fully Anechoic Room 15 khz LP Filter Pink Noise Generator Signal Generator Band Pass Filter Combiner Screened Room Spectrum Analyser Splitter Audio Analyser FM Receiver Matching Pad Figure 3: Diagram of conducted measurement set up based on ITU-R BS.641 Using the set-up in Figure 3 above, the following procedure was used to set the frequency deviation and audio levels of the wanted and unwanted signals: ERA Technology Ltd

15 15 1. The Lindos L101 audio oscillator was set to a sinusoidal frequency of 500 Hz and its left and right audio output ports were connected to the FM stereo transmitter audio input ports. 2. The wanted RF carrier level C of the FM stereo transmitter as observed on the spectrum analyser was to set to -60 dbm, by varying level of the attenuator. This power level equated to a minimum electric field strength of 54 dbµv/m being received by a FM whip antenna. The wanted signal level was measured in a 50 khz Resolution Bandwidth (RBW) using peak detection on the spectrum analyser. 3. Using the spectrum analyser procedure outlined in ERC Recommendation 54-01, the wanted FM stereo signal (from the FM transmitter) was set to a deviation of +/- 75 khz by controlling the audio level on the audio oscillator. 4. The reference level of the wanted audio signal S from the audio oscillator was then measured using the Lindos L102 audio analyser without any weighting, by directly connecting the two units together. 5. The FM transmitter was then switched to mono and the audio level output from the audio oscillator was adjusted to produce a deviation of ± 32 khz on the spectrum analyser. The FM transmitter was then switched back to stereo transmission. 6. The audio-frequency level at the input of the unwanted transmitter before the preemphasis was set by adjusting the voltage output of the pink noise generator until the same peak-reading of the wanted signal S was observed on the audio analyser. Thus, giving an equivalent (quasi) peak deviation of ± 32 khz for the noise signal N. Next, the following procedure was used to determine the level of interference that a FM receiver under test could tolerate for an unwanted signal ± 10.7 MHz from the wanted received channel: 1. A 500 Hz sinusoidal tone generated by the audio oscillator was FM modulated with a RF carrier at a frequency 90.0 MHz using a PPL FM stereo transmitter. 2. A shielded BNC lead with a crocodile clip attached to base of the FM receiving antenna was used to inject the wanted signal into the receiver under test. 3. The audio level of the FM receiver phono-output was adjusted to produce the same audio line-out at the audio analyser, for a deviation ± 32 khz. 4. The wanted audio signal S was switched off and using CCIR weighting a quasi-peak measurement of the noise N was recorded. The S/N ratio between the audio levels of the wanted S and noise of the receiver without interference was then calculated. 5. Next, the interfering base band noise signal was created using an Agilent 33250A function/arbitrary waveform generator and filtered with the appropriate shape as recommended in ITU-R BS.559 (i.e. pink noise). ERA Technology Ltd

16 16 6. Using the R&S SMIQ-03 signal generator, the base band noise signal was then modulated with a RF carrier at an unwanted frequency of MHz, with a 50 µs pre-emphasis, via a separate 15 khz low pass filter. 7. Both the wanted and unwanted signals were coupled together using a combiner. The RF signals were then split, with one port going to the R&S FSIQ7 spectrum analyser and the other port to FM receiver via a ohm matching pad. 8. The RF carrier level I of the interferer transmitting the unwanted pink noise was increased until a 6 db increase in noise level was measured by the audio analyser. This level of I was recorded on the spectrum analyser as the level of unwanted RF to cause interference to the FM receiver. The level of audio noise generated by the unwanted signal was measured using a weighted CCIR filter with quasi-peak detection. 9. Step 8 was repeated for 0.1 MHz unwanted frequency increments until the unwanted RF frequency was MHz. 10. Steps 8 and 9 were then repeated for wanted electric field strength values 66 and 80 dbµv/m, corresponding to -48 and -34 dbm received power levels respectively. 11. Steps 8 to 10 were then repeated by reversing the wanted and unwanted RF carrier frequencies. 4. Results The following results sections show the FM radio receivers tested by ERA as shown in Table 1 above. Results are shown as the mean interference level recorded for all receivers tested for each of the five categories. Tabulated results can be found in Appendices A and B. ERA Technology Ltd

17 FM Car Radio Receivers 140 Interference I (dbuv/m) dbuv/m 66 dbuv/m 80 dbuv/m Frequency Offset (MHz) Figure 4: Plot of the mean interference level required to cause a 6 db drop in S/N for a FM car radio from an unwanted signal with a positive frequency offset From the plot above it can be seen that a car radio is most susceptible to interference from an unwanted signal operating 10.7 MHz above the wanted transmission. The results also suggest that the higher the wanted field strength received by a car radio the less immune it is to interference. This effect is most noticeable between a wanted field strength of 54 dbμv/m and 80 dbμv/m, where 21 dbμv/m less field strength is required to cause a 6 db degradation in the quality of the received audio, for a frequency offset of MHz. Receiver 36 required the lowest level of interference of 91 dbμv/m at a frequency offset of MHz for a wanted field strength of 80 dbμv/m. The table below shows the maximum S/N achieved whilst testing was far greater for a higher wanted RF signal compared with a low wanted RF signal level. Also, the standard deviation of the S/N improved with the higher wanted signal level. ERA Technology Ltd

18 18 Table 2: Measured mean S/N as a function of wanted field strength for FM car radios Wanted Field Strength (dbμvm) Mean S/N (db) Standard Deviation (db) The plot below shows that a car radio is less susceptible to interference from an unwanted signal operating with a negative frequency offset compared with the positive frequency offset response as shown in Figure 4. The results show that an average field strength of 123 dbμv/m is required to cause a 6 db degradation in the quality of the received audio. Also, a higher interference level is required as the wanted signal strength increases. 140 Interference I (dbuv/m) dbuv/m 66 dbuv/m 80 dbuv/m Frequency Offset (MHz) Figure 5: Plot of the mean interference level required to cause a 6 db drop in S/N for a FM car radio from an unwanted signal with a negative frequency offset Receiver 36 required the lowest level of interference of 113 dbμv/m at a frequency offset of MHz for a wanted field strength of 80 dbμv/m. The table below shows the maximum S/N achieved whilst testing was far greater for a higher wanted RF signal compared with a low wanted RF signal level. Again, the standard deviation of the S/N improved with the higher wanted signal level. ERA Technology Ltd

19 19 Table 3: Measured mean S/N as a function of wanted field strength for FM car radios Wanted Field Strength (dbμvm) Mean S/N (db) Standard Deviation (db) FM Hi-Fi Radio Receivers 130 Interference I (dbuv/m) dbuv/m 66 dbuv/m 80 dbuv/m Frequency Offset (MHz) Figure 6: Plot of the mean interference level required to cause a 6 db drop in S/N for a FM hi-fi radio from an unwanted signal with a positive frequency offset From the plot above it can be seen that a hi-fi radio is most susceptible to interference from an unwanted signal operating 10.7 MHz above the wanted transmission. The results also suggest that the higher the wanted field strength received by a hi-fi radio the less immune it is to interference. This effect is most noticeable for a frequency offset of MHz, between a wanted field strength of 54 dbμv/m and 80 dbμv/m, where a mean interference level of 86 dbμv/m is required compared to 66 dbμv/m to cause a 6 db degradation in the quality of the received audio signal respectively. Receivers R14 and R25 required the lowest level of interference of 51 dbμv/m at a frequency offset of MHz for a wanted field strength of 80 dbμv/m. The table below shows the maximum S/N achieved whilst testing was far greater for a higher wanted RF ERA Technology Ltd

20 20 signal compared with a low wanted RF signal level. Also, the standard deviation of the S/N improved with the higher wanted signal level. Table 4: Measured mean S/N as a function of wanted field strength for FM hi-fi radios Wanted Field Strength (dbμvm) Mean S/N (db) Standard Deviation (db) The plot below shows that the car radio is on average 12 dbμv/m less susceptible to interference from an unwanted signal operating with a negative frequency offset compared with the positive frequency offset response as shown in Figure Interference I (dbuv/m) dbuv/m 66 dbuv/m 80 dbuv/m Frequency Offset (MHz) Figure 7: Plot of the mean interference level required to cause a 6 db drop in S/N for a FM hi-fi radio from an unwanted signal with a negative frequency offset The results again show that the higher the wanted field strength received by a hi-fi radio the less immune it is to interference. This effect is most noticeable between a wanted field strength of 54 dbμv/m and 80 dbμv/m, where a mean interference level of 98 dbμv/m is required compared to 85 dbμv/m to cause a 6 db degradation in the quality of the received audio respectively for a frequency offset of MHz. ERA Technology Ltd

21 21 Receiver R14 required the lowest level of interference of 53 dbμv/m at a frequency offset of MHz for a wanted field strength of 80 dbμv/m. The table below shows the maximum S/N achieved whilst testing was far greater for a higher wanted RF signal compared with a low wanted RF signal level. Also, the standard deviation of the S/N improved with the higher wanted signal level. Table 5: Measured mean S/N as a function of wanted field strength for FM hi-fi radios Wanted Field Strength (dbμvm) Mean S/N (db) Standard Deviation (db) FM Music System Radio Receivers Interference I (dbuv/m) dbuv/m 66 dbuv/m 80 dbuv/m Frequency Offset (MHz) Figure 8: Plot of the mean interference level required to cause a 6 db drop in S/N for a FM music system radio from an unwanted signal with a positive frequency offset From the plot above it can be seen that a music system radio is most susceptible to interference from an unwanted signal operating 10.7 MHz above the wanted transmission. The results also suggest that the higher the wanted field strength received by a music system radio the less immune it is to interference. This effect is most noticeable for a ERA Technology Ltd

22 22 frequency offset of MHz, between a wanted field strength of 54 dbμv/m and 80 dbμv/m, where a mean interference level of 85 dbμv/m is required compared to 72 dbμv/m to cause a 6 db degradation in the quality of the received audio respectively. Receiver R7 required the lowest level of interference of 57 dbμv/m at a frequency offset of MHz for a wanted field strength of 80 dbμv/m. The table below shows the maximum S/N achieved whilst testing was far greater for a higher wanted RF signal compared with a low wanted RF signal level. Also, the standard deviation of the S/N remained constant for different wanted signal level field strengths. Table 6: Measured mean S/N as a function of wanted field strength for music systems Wanted Field Strength (dbμvm) Mean S/N (db) Standard Deviation (db) The plot below shows that the music system radio is on average 6 to 8 dbμv/m less susceptible to interference from an unwanted signal operating with a negative frequency offset compared with the positive frequency offset response as shown in Figure Interference I (dbuv/m) dbuv/m 66 dbuv/m 80 dbuv/m Frequency Offset (MHz) Figure 9: Plot of the mean interference level required to cause a 6 db drop in S/N for a FM music system radio from an unwanted signal with a negative frequency offset ERA Technology Ltd

23 23 The results again show that the higher the wanted field strength received by the music system radio the less immune it is to interference. This effect is most noticeable between a wanted field strength of 54 dbμv/m and 80 dbμv/m, where a mean interference level of 91 dbμv/m is required compared to 80 dbμv/m to cause a 6 db degradation in the quality of the received audio respectively for a frequency offset of MHz. Receiver R12 required the lowest level of interference of 61 dbμv/m at a frequency offset of MHz for a wanted field strength of 80 dbμv/m. The table below shows the maximum S/N achieved whilst testing was far greater for a higher wanted RF signal compared with a low wanted RF signal level. Also, the standard deviation of the S/N remained constant for different wanted signal level field strengths. Table 7: Measured mean S/N as a function of wanted field strength for music systems Wanted Field Strength (dbμvm) Mean S/N (db) Standard Deviation (db) FM Portable Radio Receivers Interference I (dbuv/m) dbuv/m 66 dbuv/m 80 dbuv/m Frequency Offset (MHz) Figure 10: Plot of the mean interference level required to cause a 6 db drop in S/N for a FM portable radio from an unwanted signal with a positive frequency offset ERA Technology Ltd

24 24 From the plot above it can be seen that a portable radio is most susceptible to interference from an unwanted signal operating 10.7 MHz above the wanted transmission. The results also suggest that the higher the wanted field strength received by a portable radio the less immune it is to interference. This effect is most noticeable for a frequency offset of MHz, between a wanted field strength of 54 dbμv/m and 80 dbμv/m, where a mean interference level of 70 dbμv/m is required compared to 62 dbμv/m to cause a 6 db degradation in the quality of the received audio respectively. Receiver R10 required the lowest level of interference of 43 dbμv/m at a frequency offset of MHz for a wanted field strength of 80 dbμv/m. The table below shows the maximum S/N achieved whilst testing was far greater for a higher wanted RF signal compared with a low wanted RF signal level. Also, the standard deviation of the S/N improved with the higher wanted signal level. Table 8: Measured mean S/N as a function of wanted field strength for FM portables Wanted Field Strength (dbμvm) Mean S/N (db) Standard Deviation (db) The plot below shows that the portable radio is between 27 to 35 dbμv/m less susceptible to interference from an unwanted signal operating with a negative frequency offset compared with the positive frequency offset response as shown in Figure 10. ERA Technology Ltd

25 25 Interference I (dbuv/m) dbuv/m 66 dbuv/m 80 dbuv/m Frequency Offset (MHz) Figure 11: Plot of the mean interference level required to cause a 6 db drop in S/N for a FM portable radio from an unwanted signal with a negative frequency offset The results show that there is little difference between the interference levels measured for high wanted field strengths compared with low wanted field strengths received by a portable radio. Receiver R33 required the lowest level of interference of 50 dbμv/m at a frequency offset of MHz for a wanted field strength of 80 dbμv/m. The table below shows the maximum S/N achieved whilst testing was far greater for a higher wanted RF signal compared with a low wanted RF signal level. Also, the standard deviation of the S/N improved by 2 db for a higher wanted signal level. Table 9: Measured mean S/N as a function of wanted field strength for FM portables Wanted Field Strength (dbμvm) Mean S/N (db) Standard Deviation (db) ERA Technology Ltd

26 Walkman FM Radio Receivers Interference I (dbuv/m) dbuv/m 66 dbuv/m 80 dbuv/m Frequency Offset (MHz) Figure 12: Plot of the mean interference level required to cause a 6 db drop in S/N for a FM walkman radio from an unwanted signal with a positive frequency offset From the plot above it can be seen that a portable radio is not the most susceptible to a particular positive frequency offset. On average an interfering field strength of 99 to 109 dbμv/m is required cause a 6 db degradation in the quality of the received audio respectively. This applies for all three wanted signal levels tested. Note, that only one walkman type radio managed to receive a wanted signal of 54 dbμv/m. Receiver R9 required the lowest level of interference of 99 dbμv/m at a frequency offset of MHz for a wanted field strength of 66 dbμv/m. Once again, the table below shows the maximum S/N achieved whilst testing was far greater for a higher wanted RF signal level compared with a low wanted RF signal level. Also, the standard deviation of the S/N measured was the highest for a wanted field strength of 66 dbμv/m. Table 10: Measured mean S/N as a function of wanted field strength for FM walkman Wanted Field Strength (dbμvm) Mean S/N (db) Standard Deviation (db) ERA Technology Ltd

27 27 The plot below shows that the portable radio is similar in susceptibility to interference from an unwanted signal operating with a negative frequency offset compared with the positive frequency offset response as shown in Figure Interference I (dbuv/m) dbuv/m 66 dbuv/m 80 dbuv/m Frequency Offset (MHz) Figure 13: Plot of the mean interference level required to cause a 6 db drop in S/N for a FM walkman radio from an unwanted signal with a negative frequency offset Figure 13 shows, that a walkman radio is 5 to 6 dbμv/m less susceptible to interference for a frequency offset of MHz compared with the positive offset frequencies measured. Receiver R32 required the lowest level of interference of 76 dbμv/m at a frequency offset of MHz for a wanted field strength of 80 dbμv/m. Once again, the table below shows the maximum S/N achieved whilst testing was far greater for a higher wanted RF signal level compared with a low wanted RF signal level. Also, the standard deviation of the S/N measured was the highest for a wanted field strength of 66 dbμv/m Table 11: Measured mean S/N as a function of wanted field strength for FM walkman Wanted Field Strength (dbμvm) Mean S/N (db) Standard Deviation (db) ERA Technology Ltd

28 28 5. Analysis A total of forty-one FM radio receivers were tested. FM radio receivers were acquired from the BBC, ERA staff and popular high-street electrical retailers. The radios tested fell into the following categories; car radio, hi-fi, music systems, portable and walkman type. From the measurement results shown in Section 4 and Appendix A and B, the portable radio was most susceptible to an unwanted signal transmitting at a frequency above the tuned wanted signal (positive frequency offset). It can be concluded that the FM portable radio was the most susceptible to interference from an unwanted signal with a positive frequency offset of 10.7 MHz from the tuned wanted signal. The most immune to an unwanted signal with a positive frequency offset with respect to the tuned wanted signal was the walkman type radio receiver. The table below summarises the minimum mean field strength required for the various receiver types tested to produce a 6 db decrease in S/N for an unwanted signal with a positive frequency offset compared to the tuned wanted signal. Table 12: Summary of results for FM radio receivers tested for interference with a positive frequency offset to the tuned wanted signal Category Wanted Field Strength (dbμv/m) Frequency Offset (MHz) Mean S/N (db) Mean Unwanted Field Strength (dbμv/m) Car Radio ± ± ± ± ± ± 10 Hi-Fi ± 9 86 ± ± 7 71 ± ± 5 66 ± ± 7 85 ± 9 Music Systems ± 7 80 ± ± 7 72 ± 19 Portable ± ± ± 9 65 ± ± 5 62 ± ± Walkman Type ± ± ± ± 7 1 Only one walkman type radio managed to operate at 54 dbμv/m ERA Technology Ltd

29 29 When the FM receivers were tested for a 6 db decrease in S/N for an unwanted signal below the tuned wanted signal (negative frequency offset), it was generally found that the required level of field strength was greater when compared with an unwanted signal transmitting above the tuned wanted signal. The music system radio receivers tested were the most susceptible to an unwanted signal with a negative frequency offset. The car radio receiver was the most immune to an unwanted signal transmitting with a negative frequency offset. The table below summarises the minimum mean field strength required for the various receiver types tested to produce a 6 db decrease in S/N for an unwanted signal with a negative frequency offset compared to the tuned wanted signal. Table 13: Summary of results for FM radio receivers tested for interference with a negative frequency offset to the tuned wanted signal Category Car Radio Hi-Fi Music Systems Portable Walkman Type Wanted Field Strength (dbμv/m) Frequency Offset (MHz) Mean S/N (db) Mean Unwanted Field Strength (dbμv/m) ± ± ± ± ± ± ± ± ± ± ± 6 85 ± ± 8 91 ± ± 8 87 ± ± 8 80 ± ± 7 99 ± ± 5 97 ± ± 5 98 ± ± ± ± ± ± ± 27 Both sets of tables show a large spread of results from the mean values calculated for each category. The largest spread of results can be seen for hi-fi, music system and walkman type radio receiver. This is probably due to only four to six receivers tested in these categories compared to twenty-one receivers tested in the portable radio category. Ideally, at ERA Technology Ltd

30 30 least twenty radio receivers should have been tested for each category. However, this was limited due to time constraints. The 95% of the receivers tested were more susceptible to a 6 db in degradation of audio signal quality for an unwanted interference signal with a positive frequency offset from the wanted signal transmission, compared with a negative frequency offset. The measured results summarized in Table 12 for the hi-fi, music system and portable radio compare well with the median levels described in Figure 14 for the FM receivers tested by the BBC in The BBC receiver tests described on Ofcom s archived website (formally Radio Agency website) considered the question of spurious receiver responses to transmissions at 10.7 MHz intervals. As for the protection ratio curves, a wide spread of results was found. The BBC results are shown in the figure below: Figure 14: Results of BBC receiver tests (1984) It can be seen that there is an almost 50 db spread in results across the sample. Of particular concern is the fact that some receivers (such as that identified as worst in the plot) exhibit a response that is worst at frequency-offsets of a 10.6 or 10.8 MHz, rather than 10.7 MHz. ERA s results also show a similar pattern for frequency offsets of 10.6 and 10.8 MHz when compared to the BBC figures. A total of 32% of the receivers tested by ERA showed a drift towards 10.6 or 10.8 MHz IF frequency problem instead of a 10.7 MHz offset. It also interesting to note that only four radio receivers (R5, R12, R15 and R28) out of the forty-one tested managed to achieve the desired S/N of 56 db as stated in ITU-R BS.641. This level S/N was also only achieved for the higher wanted signal levels of 66 and 80 dbμv/m. All the receivers had a lower S/N at 54 dbμv/m compared to the higher wanted signal levels of 66 and 80 dbμv/m, which in some cases as low as 10 db. This was particularly noticeable for the walkman type radio receivers. However, fifteen out the fortyone receivers (37%) did achieve an S/N ratio of 50 db or more, again only at the higher wanted signal levels of 66 and 80 dbμv/m. ERA Technology Ltd

31 31 The majority of receivers tested also were more susceptible to an unwanted signal for higher wanted field strengths, especially when comparing the results at 54 dbμv/m with a wanted signal of 80 dbμv/m. This difference in susceptibility between the two wanted signal levels can probably be explained by Intermodulation Distortion (IMD), where IMD is a mixing process gone bad. Whenever two or more signals are present in a nonlinear circuit at the same time, IMD creates new, unwanted frequencies from them. If there is already a strong wanted signal then less interference is required to produce the unwanted IF around 10.7 MHz. IMD is only one of the mechanisms that can create interference. Poor IF response and poor image rejection in the VHF receiver can cause some problems that one might commonly blame on IMD. All superheterodyne receivers are subject to these problems. In a receiver using a superheterodyne circuit, the mixer circuit combines the desired signal with the local oscillator signal. The IF then selects either the sum or difference product for further processing. The presence of unwanted signals can disturb this simple scheme. The majority of FM receiver will have an IF problem around 10.6 to 10.8 MHz because of the design superheterodyne receiver used in FM radio. All radio receivers generate RF and FM receivers normally generate a small signal at 10.7 MHz above the tuned frequency. The FM receivers can mix that 10.7 MHz frequency with a signal from another FM station signal nearby and create interference approximately 10.7 MHz up or down from the station. Hence, if a strong unwanted signal radiates at MHz and the tuned wanted signal received by a FM radio (with a local oscillator at MHz) is at 90.0 MHz, interference will be received by the radio at 90.0 MHz the same as the wanted signal or at MHz (its image). In theory this also applies for local oscillator working 10.7 MHz below the tuned radio station. 6. Conclusions A total of forty-one FM radio receivers were tested. FM radio receivers were acquired from the BBC, ERA staff and popular high-street electrical retailers. The radios tested fell into the following categories; car radio, hi-fi, music systems with a CD player, portable and walkman type. From the measurement results shown in Section 4 and Appendix A and B, the portable radio was most susceptible to an unwanted signal transmitting at a positive frequency of 10.7 MHz above the tuned wanted signal, followed by the hi-fi, music system radios with a CD player, walkman type radios and finally the car radio. For unwanted signal frequencies transmitting at frequency offsets of 10.6 and 10.8 MHz an extra 10 to 20 dbμv/m in field strength was required to produce the same effect as experienced at 10.7 MHz for the majority of hi-fi, music system and portable FM receivers tested. ERA Technology Ltd

32 32 When the FM receivers were tested for an unwanted signal below the tuned wanted signal, it was generally found that the required level of field strength was greater when compared with an unwanted signal transmitting above the tuned wanted signal. The conducted measurements support the potential interference problem highlighted by the RTPG, and its predecessors, which has taken the 10.7 MHz ± 0.1 MHz offset into consideration during the planning of FM VHF broadcasting services. The measurements also support the guidance [2] provided by the RTPG with respect to field strength levels in overlap regions, suggesting that: For 10.7 MHz offsets only minor coverage overlaps are allowable, although careful consideration is required regarding where these overlaps occur, e.g. avoiding overlaps in built-up areas. In areas of overlap the field strength of the higher frequency service should not exceed 65 dbμv/m at 10 m above ground level in urban areas. For 10.6 and 10.8 MHz relationships coverage overlaps are allowable to some extent, although care needs to be taken so that high field strengths are not present. The transmitter of neither service should be within the service area of the other. In areas of overlap the field strength of the higher frequency service should not exceed 80 dbμv/m at 10 m above ground level in urban areas. 7. Acknowledgments ERA Technology would like to thank John Endicott and John Salter from the BBC, for the loan of their FM receivers and support during the project. 8. References [1] Ofcom, Office of Communication Planning of VHF FM services with 10.6 to 10.8 MHz frequency spacing, RTPG 357, September 2005 [2] Ofcom, Office of Communication RTPG policy regarding the use of intermediate frequency separations between FM services, RTPG 358, October 2005 [3] ITU-R Recommendation BS.641 Determination of Radio-Frequency Protection Ratios for Frequency-Modulated Sound Broadcasting, 1986 [4] C. Gandy, Dipole Antennas, BBC R&D White Paper WHP132, March [5] ITU-R Recommendation BS Planning Standards for Terrestrial FM Sound Broadcasting at VHF, 1998 [6] ITU-R Report 946-1, Frequency-planning constraints on FM sound broadcasting in the band 8 (UHF) [7] ERA Technology Ltd