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Measurements of an FM Receiver in FM Interference

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Measurements of an FM Receiver in FM Interference N ITA-REPORT-79-27 Measurements of an FM Receiver in FM Interference J. R. Juroshek u.s. DEPARTMENT OF COMMERCE Juanita M. Kreps, Secretary Henry Geller, Assistant Secretary for Communications and Information October 1979 TABLE OF CONTENTS PAGE LIST OF FIGURES iv LIST OF TABLES v ABSTRACT 1 1. INTRODUCTION 1 2. TEST DESCRIPTION 2 3. TEST RESULTS 7 4. SPECTRUM PHOTOGRAPHS 27 5. SUMMARY AND CONCLUSIONS 31 6 • REFERENCES 33 iii LIST OF FIGURES PAGE Figure 1. Block diagram of equipment. 3 Figure 2. Phase noise from FM generators. 5 Figure 3. Measured performance of an FM receiver 9 in Gaussian noise. Figure 4. Measured performance of an FM receiver 10 with a single FM interferer. Figure 5. Measured performance of an FM receiver 11 with two equal amplitude interferers. Figure 6. Measured performance of an FM receiver 12 with three equal amplitude interferers. Figure 7. Measured performance of an FM receiver 15 with 1, 2 and 3 equal amplitude FM interferers. Peak deviation of all FM generators is e = 3 kHz. Figure 8. Measured performance of an FM receiver 16 with a singLe FM interferer and positi ve and negative frequency offsets~ Peak deviation of generators is '8 = 3 k Hz , Figure 9. Measured performance of an FM receiver with 17 a single FM interferer. Peak frequency deviation of all FM signals is e = 3 kHz. Figure 10. Measured performance of an FM receiver 18 with two FM interferers. Peak frequency deviation of all FM signals is e = 3 kHz. Figure 11. Measurements of SII N required to produce 20 a SINAD = 12 dB witfl a single FM interferer and Gaussian noise. Peak frequency deviation of all FM signals is e = 3 kHz. Figure 12. Measurements of SII N required to produce 21 a SI~D = 12 dB witfl 2 equal amplitude interferers and Gaussian noise. Peak frequency deviation of all FM signal is e = 3 kHz. iv PAGE Figure 13. Threshold of perceptible interference for 23 an FM receiver with a single FM interferer. Peak frequency deviation is 8 = 3 kHz. Figure 14. Threshold of perceptible interference for 24 an FM receiver with equal amplitude FM interferers. Peak frequency deviation is 8 = 3 kHz. Figure 15. Measurements of SINAD as a function of 26 normalized input signal-to-interference ratio for various frequency offsets ~f. Interference is a single FM signal. Peak frequency deviation of all FM generators is 8 = 3 kHz. Figure 16. Spectra of a 171'.8 MHz FM signal modulated 28 with (a) a voice signal and (b) a 400 Hz tone. Peak frequency deviation is e = 3 kHz. Figure 17. Spectra of the audio output from an FM 29 receiver with (a) receiver background noise, (b) Gaussian noise of SiN N = 10 dB, (c) a single FM interferer of S}IIN = 10 dB, and (d) 3 equal amplitude FM interferers of S/IIN = 10 dB. Figure 18. Spectra of the audio output from an FM 30 receiver with a single FM interferer offset in frequency by (a) ~f = 0 kHz, (b) ~f = 4kHz, (c) ~f = 12 kHz and (d) ~f = 24kHz. Peak frequency deviation of all FM signals is 8 = 3 kH~. LIST OF TABLES TABLE 1. Estimates of S/NIN required to produce 13 aSINAD = 12 dB. TABLE 2 • Estimates of SINAD for S/IIN Or 13 S/NIN = 20 dB. TABLE 3. Summary of S/II~ required to produce a 22 SINAD = 12 dB wlth 8 = 3 kHz. v MEASUREMENTS OF AN FM RECEIVER IN FM INTERFERENCE John R. Juroshek* This report investigates the performance of an FM, landrnobile, receiver in FM interference. Measurements are described showing the effects of multiple interfering FM signals as a function of modulation index, signal-to-interference ratio, signal-to-noise ratio, and frequency offset between the victim and interferer. The perfor­ mance of the receiver is measured with various combinations of one to three interfering FM signals. The report also describes the spectra of the audio output during various interference conditions. Key words: frequency modulation; spectrum; interference; communications; land-mobile radio 1. INTRODUCTION An estimate of the performance of an FM system is often based on t he assumption that white Gaussian noise is the factor limiting performance. Increased demands for spectrum, however, are resulting in more instances where interfering signals are the dominant source of degradation. Thus, it becomes increasingly important to understand how an FM receiver behaves in FM interference. This report will discuss the laboratory measurements of a single FM receiver in a co-channel and off-frequency FM interference environment. The effects of one to three independent, interfering, FM signals are measured as a function of modulation index, signal-to-interference ratio, * The author is with u.S. Department of Commerce, Institute for Telecommunication Sciences, National Telecommunications and Information Administration, Boulder, Colorado 80303. signal-to-noise ratio,and frequency offset between victim and interferer. The measurements are limited to a single mobile transceiver that operates on a frequency of 171.8 MHz. The measurements also are limited to laboratory tests with nonfading signals. 2. TEST DESCRIPTION A block diagram of the test setup is shown in Figure 1. As can be seen, the output from four FM signal generators is added in a 50 ohm summing circuit. One of these generators represents the desired signal, while the other three are the interfering signals. Modulation for the FM signal generators is obtained either from the internal sine wave sources within the units or from external voice signals. Each of the external voice modulating signals is independent and is obtained from cassette recordings of spoken phrases. All voice signals are low-pass filtered to eliminate high frequency components f.rom the cassette recorders. These are "Butterworth" filters with a cutoff frequency of 5 kHz and attenuation of 24 dB per octave. White Gaussian noise is included in the tests as shown in the lower part of Figure 1. The output from a noise diode is filtered with a bandpass filter centered at 171.8 MHz and bandwidth of 8 MHz. The filtered noise is amplified in a 40 dB amplifier and sent to the summing circuit. The purpose of the noise filter is simply to prevent overloading of the RF amplifier by the out-of-band noise. The s ummed vs i qnal containing desired and interfering signals, and noise is connected to I t he RF input terminals of the victim FM receiver. All RF connections use double shielded 50 ohm coax. The audio output from the receiver is directed either to a speaker or to a noise and distortion 2 FM Voice "'1 LP SIG FILTER GEN FM Voice"'2 --1 LP I ~I SIG FILTER .. I "S , GEN 1 I 3~ II RF Input \ NOISE & LP FM fM Voice-3 , .... 1 FILTER I -I SIG ~I L H• MOBILE ~ DISTORTION GEN I¥; REC I~ :2 METER w II Audio Output ~ I J fM Voice#4~ LP FILTER SIG I --I GEN NOISE BP RF DIOl! FIL TER AMP Figure 1. Block diagram of equipment. meter whose input is terminated with a noninductive resistor equal to the nominal speaker impedance. Whenever possible, the tests were conducted according to the procedures recommended in EIA Standard RS-204-A (Electronic Industries Association, 1972). The only major deviation from the recommended standard occurs when the FM signal generators are voice modulated. The standard requires that the FM generators be tone-modulated and thus does not define test procedures for voice modulation. Although most of the tests were conducted with tone modulation, it was felt that at least a few tests should use voice modulation to see if there were any major differences. The FM generators were modern units whose output could be frequency locked to insure a stability of better than 5 x 10-8• The accuracy of the counter used to set the generators, on the other hand, was only one part in lOG. Thus, the frequency that the generators are tuned to is known to an accuracy of 500 Hz. This accuracy, however, is sufficient for these tests. One problem that was discovered during the tests is the off-frequency phase noise that is emitted from the FM gener­ ators. The generator specifications state that the phase noise, 20 kHz offset from the carrier, be less than 136 dB below the carrier in a 1 Hz bandwidth. A plot of the phase noise specifications for the FM generators at other frequencies is given in Figure 2. Thus, a receiver with 10 kHz equivalent rectangular bandwidth, at Fe +20 kHz, could expect to receive a signal of approximately P¢ = -136 + 10 log 10 4 dB (1) below the carrier, or P¢ = -96 dB. (2) 4 This, of course, limits the dynamic range over which off­ frequency tests can be made. In practice, the dynamic range of off-frequency testing should be limited to approximately -90 dB below the carrier since the interference from adjacent channel effects must be distinguishable from the - 96 dB phase noise. Thus, throughout the report we shall assume that the dynamic range of the test setup is limited to -90 dB relative to the carrier for frequency offsets of 25 kHz < ~f <30 kHz. .~ -120 I .-5 3: OJ CD (LN -13D £DI -0 .- C c -140 OJ ~ Receiver with (f) OJ ~ ·0 .~ v 7 7 7 7 7 7 10 kHz BW z~ Tuned to 0 OJU -150 tc + 25 kHz (f) o.E .J::. CL -160 fc +IOkHZ +20kHz +30kHz Frequency Figure 2. Phase noise from FM generators. Unfortunately, the test receiver had a selectivity or adjacent channel rejection specification of -100 dBm.
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