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

LIST OF FIGURES

LIST OF TABLES

ABSTRACT

1. INTRODUCTION

2. TEST DESCRIPTION

3. TEST RESULTS

4. SPECTRUM PHOTOGRAPHS

5. SUMMARY AND CONCLUSIONS

6 • REFERENCES

iii

PAGE

iv

v

1

1

2

7

27

31

33
LIST OF FIGURES

PAGE

Figure 1.

Figure 2.

Figure 3.

Figure 4.

Block diagram of equipment.

Phase noise from FM generators.

Measured performance of an FM receiver

in Gaussian noise.

Measured performance of an FM receiver

with a single FM interferer.

3

5

9

10

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

wi th a sing Le 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.

Figure 11.

Figure 12.

Measured performance of an FM receiver

wi th two FM interferers. Peak frequency

deviation of all FM signals is e = 3 kHz.

Measurements of SII N required to produce

a SINAD = 12 dB witfl a single FM interferer

and Gaussian noise. Peak frequency

deviation of all FM signals is e = 3 kHz.

Measurements of SII N required to produce

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

18

20

21
Figure 13.

Figure 14.

Figure 15.

Figure 16.

Figure 17.

Figure 18.

TABLE 1.

TABLE 2 •

TABLE 3.

Threshold of perceptible interference for

an FM receiver with a single FM interferer.

Peak frequency deviation is 8 = 3 kHz.

Threshold of perceptible interference for

an FM receiver with equal amplitude FM

interferers. Peak frequency deviation

is 8 = 3 kHz.

Measurements of SINAD as a function of

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.

Spectra of a 171'.8 MHz FM signal modulated

with (a) a voice signal and (b) a 400 Hz

tone. Peak frequency deviation is e = 3 kHz.

Spectra of the audio output from an FM

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.

Spectra of the audio output from an FM

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

Estimates of S/NI N required to produce

aSINAD = 12 dB.

Estimates of SINAD for S/IIN Or

S/N I N = 20 dB.

Summary of S/II~ required to produce a

SINAD = 12 dB wlth 8 = 3 kHz.

v

PAGE

23

24

26

28

29

30

13

13

22

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 performance 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.r om 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
Voice "'1 LP

FILTER

FM

SIG

GEN

LP I ~I FM

Voice"'2 --1 SIG I "S , 1 3~ FILTER .. GEN I

I I RF

Input

\

FM I¥; ~I H fM

I~ :2 NOISE & LP I -I Voice-3 , .... 1 SIG L • MOBILE DISTORTION FILTER GEN REC METER

w I I

Audio

Output

I J

LP I --I fM Voice#4~ FILTER SIG

GEN

NOISE

DIOl!

BP

FILTER

RF

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 generators. 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

below the carrier, or

P¢ = -96 dB.

4

(1)

(2)
This, of course, limits the dynamic range over which offfrequency 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

(f) OJ ·0 .~

z~

-150 0

OJU

(f)

o.E

.J::.

CL -160

fc +IOkHZ

v 7

+20kHz

Receiver with

7 7 7 7 7 10 kHz BW

Tuned to

tc + 25 kHz

+30kHz

Frequency

Figure 2. Phase noise from FM generators.

Unfortunately, the test receiver had a selectivity or

adjacent channel rejection specification of -100 dBm. This

measurement, as per EIA Standard RS-204-A, is made at a

frequency offset of 30 kHz. Although a selectivity of

-100 dB cannot be tested with a test setup that is limited

to -90 dB, the tests were continued since most of the offfrequency measurement results were less than -90 dB. In

order to achieve a -100 dBm adjacent channel measurement

5
capability, screen room shielding would probably be needed,

as well as substantially improved signal generators.

The discussion so far has assumed that the phase noise

is from 1 FM generator. This test setup however, can have

up to three act i ve interfering generators and one v ict Lm

generator. Let us assume the total power of th.e three

interfering generators at the input to the receiver is

(3)

where i k is the power in the kth interfering signal source.

This report uses a convention of designating numeric values

with lower case letters and equivalent dB values in upper

case. The ratio of i I N to total phase noise power is then

given by

=

Pep PI + P2 + P3

, (4)

where Pi is the' phase noise contribution of theith

generator. However, for various ieasons, we have found it

necessary to restr ict the tests to equal amplitude

interfering signals which means that

and for all practical purposes

p 1 = p 2 = P'··3

Thu.s

(5)

(6)

i I N.

Pep = (7)

which means the ratio of interference power to phase-noise

power at the: input to the receiver remains constant with the

number of Ln t erferLnq sources.

6
Some other parameters of interest in the test setup are

the receiver specifications. These are:

Sensitivity

/ ?

12 dB SINAD (EIA method) 0.35 LA'

20 dB Quieting 0.50

Channel Spacing 30 kHz

Audio Output 12 watts at less

Oscillator Stability

Maximum Frequency Deviation

3. TEST RESULTS

than 3% distortion

to 8 ohm speaker

+ 0.0002%

+ 5 kHz

One measure of performance for an FM receiver is SINAD,

which is an abbreviation for the ratio "signal + noise

+ distortion to noise + distortion" expressed in dB. This

ratio is measured with a noise and distortion meter

connected to the output terminals of the receiver.

shows the measurement of SINAD in Gaussian noise.

Figure 3

For this

particular test, only one active FM generator is used; the

desired generator. The quantity S/N1N designates the ratio

of signal power to noise power at the input to the receiver

and e denotes peak frequency deviati~. Noise power here is

referenced to the receiver IF bandwi~th, or

(8)

where b I F is the IF noise power bandwidth, and no is the

noise power density. The value of b I F for the test receiver

is 10.1 KHz. All SINAD measurements were conducted with a

1000 Hz modulating tone on the desired FM generator.

7
Figure 3 shows the classical FM improvement-threshold

relationship. The larger value of e, provides a larger

value of SINAD provided S/N1N is above threshold. Unfortunately, the threshold requirements also increase with

increasing e. One should note that the maximum SINAD

obtainable with the test receiver is approximately 30 dB,

which is consistent with its specifications. A SINAD of

0.0 dB is used to designate a loss of capture of the desired

signal.

Figures 4,· 5, and 6 show similar measurements except

that FM interference is now substituted for the Gaussian

noise. Figure 4 shows the results with a single FM

interferer while 5 and 6 are for two and three equal

amplitude FM interferers respectively. The curves are

plotted in S/IIN' which is the ratio of signal-power to

total-interference-power at the input to the receiver.

Again, as per EIA specifications, the desired FM signal is

modulated with a 1000 Hz tone while each interfering signal

is modulated with a 400 Hz tone. All of the tests were made

at a desired input signal level of SIN = - 80 dBm and zero

frequency offset between the desired and interfering

signals.

The important difference to note in Figures 3 through 6

is that the threshold requirements change somewhat for FM

interference as compared to Gaussian noise. Although the

definition of threshold is arbitrary, it is evident that

with e = 5 kHz, an S/N1 N greater that 11 dB is required for

satisfactory operation, in Gaussian noise. However, with

single FM interference, this reduces to 5 db.

These results are summarized in Table 1, which shows an

estimate of the S/IIN or S/N1N that is required to produce

an output SINAD = 12 dB.

8
30

CD

"'0

o 20

<t:

Z

(J)

10

I

•

I

'-3 kHz

I

l~e=5kHz

I

Figure 3. Measured performance of an FM

receiver in Gaussian noise.

9

40
x----x----x---x--

x---------

---.....----~ 3 kHz

0 .........

~,/

/'

/

/"

/

,.0 //

o

/

i /0"8 =1 kHz

• 0

I //

/(

/ ......... --·-5 kHz / II

to

30

CD

"'0

ci 20

<t

Z

(/)

e to 20

S/I,N' dB

30 40

Figure 4. Measured performance of an FM receiver

with a single FM interferer.

10
30

rn

'U

q 20

«

z

U5

10

I

~5kHZ

Figure 5~ Measured performance of an FM receiver

with two equal amplitude interferers.

11
30

CD

ci 20

<t

Z

u;

10

,~x-''-.----x---x-'---x--'----x--....--.-

5kHz

Figure 6. Measured performance of an FM receiver

with three equal amplitude interferers.

12
TABLE 1. Estimates ofS/NINRequired to

Produce a SINAD = 12 dB

S/IIN or S/N1N

Interference e = 1 kHz e = 3 kHz e :;:: 5 kHz

Gaussian noise 7.0 dB 8.5 dB 11.0 dB

1 FM Signal 7.5 dB 2.0 dB '5. 0 dB

2 FM Signals 6.0 dB 3.5 dB e.5 dB

3 FM Signals 6.5 dB 4.0 d"B 8.0 dB

Note that, contrary to what one might ~nitially expect, the

SINAD does not necessarily increase with increasing 8. The

reason is that a SINAD = 12dB is below threshold in most

instances. Above t.hr esho Ld , however I S INAD does Lnc r e ase

wi th increasing e. This is shown in Table 2 whe re. SINAD is

tabulated for a constant S/IIN (or S/NI N) of 20 dB.

TABLE 2. Estimates of SINAD for S/IIN

or S/N1N = 20 ~B

SINAD

Interference e = 1 kHz e = 3 kHz e :;: 5 kHz

Gaussian Noise 26.0 31.0 31.5

1 FM Signal 24.0 26. 0 30.0

2 FM Signal 24.0 27.5 29. 5

3 FM Signal 24.5 28. 0 29.0

13
Frequency offset between the victim and interfering

signals is denoted ~f, where a positive Af meanS the

interfering signals are higher in frequency than the desired

signal. Figure 7 shows a plot of S/IIN that is required to

produce a S!NAD of 12 dB for various 8f. The test setup is

basically the same as for the previous tests described in

Figures 4 through 7, except that 8 is set to 3 KHz. It

should be pointed out that a SINAD = 12 dB corresponds to

marginal performance. Also note from the 8 == 3 curves in

Figures 4, 5, and 6, that the performance rapidly changes at

the SINAD = 12 dB point with. small changes in S/I 1 N• As can

be seen, the difference in performance due to one or three

interferers is only a few dB.

The response of the rece i ver when the interferer's

frequency is less than the v ict Lm (nega t i veti f )i9 shown in

Figure 841 As can be seen, the results are asymme trical with

the two response curves offset by 1.2 kHz. This offset is

due toa combination of receiver tolerances and oscillator

stability.

So far, the tests have all used 1000 Hz tone modulation

for the desired FM signal and 400 Hz modulation on the

interfering generators as specified in the EIA standard.

The response with voice modulated interference, however, is

similar, as shown in Figures 9 and 10. These figures

compare the interference produced by 400 Hz modulation with

that pr-oduced by voice modulation of the interfering PM

signal. Plotted in these figures is the S/1 IN required to

insure a SINAD of 12 dB as a funct ion of 8 f. The pe ak

frequency deviation f or both cases is e == 3kHz. Figure 9

shows the results for a single FM interferer, while

Figure 10 shows the resu.lts wi th two equal amplitude

interferers. Although the results for three interferers are

hGt shown, they are nearly identical. Therefore, one can

coneludethat the interference does not change significantly

14
10

o

-10

-20

-30

(D

"'0

-40

-50

-60

II

o

II

o o

o

x

8

x

C

o

o

o 1 FMlnterfer

C 2 FM Interfers

X :3 FMlnterfers

x

o

-70

o X

D

-80

-90

'X

o ,0)(

-000

Estimated Dynomlc ~qe7-­ of Test 'Setup

2 4 6 8 to 12 14

trfJkHz

16 18 20 22 :24 26

Figure 7. Measured perf,orman-ceof an FM recei-ver with

1,2 and 3 equal'amplitudeFM interferers.

Peak deviation ofallFM generatorsi.s '-e--;;;-3 --'kHz;.

15
o

-10

-20

-30

m

"U

-40

-50

-60

-10

-80

-90

Negati ve .1f

Estimated DynomicRange~-­ of Test Setup

12 14

± ~f) kHz

16 18 20 22 24 26

Figure 8. Measured performance of an FM receiver

with a singleFM interferer and positive

and negative frequency offsets. Peak

deviation of generators is e = 3 kHz.

16
20 ..---.....--....-----.---.,..----+---.-----,----.----r---r---r---,------,

10

o

-10

x

o

x

o

x

o

X Interfer - Voice Modulotion

o Interfer - 400 Hz Modulation

-20

X

-30 0

co

-0

~-40 ...... X <,

(f)

-50 0

X

-60

0

-70 X

-80

-90

o

--~7~- Estimated Dynamic Range/

of Test Setup

2 4 6 8 10 12 14

Llf J kHz

16 18 20 26

Figure 9. Measured performance of anFM receiver with

a single FM interferer. Peak frequency

deviation of all FM signals is e = 3 kHz.

17
to

o

-10

-20

-30

CD

"0

~-40

-50

-60

-70

-80

-90

X Interfer - Voice Modulation

o Interfer- 400 Hz Modulation

x

o

o

X

o

x

o

X

o

x

X X

o 0

Estimated Dynami;Range7--

of Test Setup

1

10 12 14 16 18 20 22 24 26

6f, kHz

Figure 10. Measured performance of an FM receiverw'i, th two FM interferers. P·eakfrequency

deviation of all FM signals is e = 3 kHz.

18

,-
if the FM signal is voice modulated in place of the 400 Hz

tone modulation.

Tests also were conducted to show the effects of the

receiver being subjected simultaneously to Gaussian noise

and FM interference. In Figures 11 and 12, a plot of S/IIN

that is required for a SINAD of 12 dB is plotted as a

function of ~f. This time, however, Gaussian noise is also

included in the tests. Results are shown for Gaussian

signal-to-noise ratios of 20, 15, and 12 dB. The quantity

S/IIN represents signal power to total interference power

and does not include the Gaussian noise.

One can see that the Gaussian noise degrades the

system's ability to withstand interference. Figure 11 shows

a change in S/IIN of approximately 5 dB is required to

compensate for the effects of S/N1 N of 12 dB. Figure 12 is

for the case of two equal amplitude interferers. No tests

were made with S/N1N less than 12 dB since the receiver's

performance rapidly degrades at this point. Again the tests

with three interferers are almost identical and, therefore,

are not shown in the report.

These results are summarized in Table 3, where the

S/IIN required for a SINAD = 12 dB is shown for various

interference conditions and frequency offsets of 12, 20, and

25 kHz. The .fi r s t; three entries are for the tests without

Gaussian noise, while the last two entries includes Gaussian

noise. Note the large change that occurs at ~f = 12 kHz.

19

20 r---.---.-----.----.---~-.__-__r__-__,__-____r-- __-_- -_

10 0

0 X [J 0

(S X 0 SIN =20 dB

0 0 X

0

0 0 X SIN =15 dB

X 0 SIN =12 dB

0

0

-10

-20

-30

CD

-c

-40 t--4

<,

(f)

-:-50

-60

-70

-80

x

o

o

X

o

o

X

o c c

-90

Estimated Dynami;Range/--

of Test Setup

2 4 6 8 10 12 14

Llf, kHz

16 18 20 22 24 26

Figure 12. Measurements of S/IIN required to produce ~a

SINAD = 12 dB with 2 equal amplitude interferers

and Gaussian noise. Peak frequency deviation

of all FM signal is 8 = 3 kHz.

21
TABLE 3. Summary of S/II~ Required to Produce a

SINAD = 12 dB wlth e = 3 kHz

Interference ~f= 12 kHz ~f= 20 kHz ~f = 25 kHz

1 FM Signal

2 FM Signals

3 FM Signals

1 FM Signals

plus S/N i n = 15 dB

2 FM Signals

plus S/N i n = 15 dB

<,

-50 dB

-40 dB

-37 dB

-27 dB

-21 dB·

-84 dB

-77 dB

-77 dB

-79 dB

-75 dB

-88 dB

-88 'dB

-84 dB

-86 dB

-75 dB

All of the previous tests used SINAD as the measure of

performance. This requires that the desired FM generator be

modulated with a 1000 Hz tone so that the measurements can

be made with a noise and distortion meter. It is

interesting to compare these results with "audio" tests

using the receiver's speaker. For the audio tests, the

value of S/IIN was decreased until the interference was just

perceptible in the receiver's speaker. This point was then

defined as the "threshold of perceptible interference."

Although the limited nature of the project required that the

listening tests be conducted with only one person. Ideally

this type of measurement would be made with a panel of

listeners.

Results of the audio tests with one and two equal

amplitude interferers is shown in Figures 13 and 14

respectively. The peak frequency deviation of both desired

and interfering FM generators is e = 3 kHz. Also shown in

the figures is the SINAD = 12 dB curves that were described

previously. In Figure 13, the threshold of perceptible

22
X Threshold of Perceptible Interference

as Determined by Listening Tests

o 12 dB SINAD

15

5

-5

-85

-75

-65

-15 x

\

\

-25 \

CD \ 'U \ -35 X

\ <,

(/) \

-45 \

\

-55

2 4 6 8 10 12 14

tlf, kHz

16 18 20 22 24 26

Figure 13. Threshold of perceptible interference for an

FM receiver with a single FM interferer. Peak

frequency deviation is 8 = 3 kHz.

23
15

-5

-15

-25

CD

-0

-35 H

<,

(J)

-45

-55

-65

X Threshold of Perceptible Interference

as Determined by Listening Tests

o 12 dB SINAD

-75

-85

2 4 6 8 10 12 14

ilF, kHz

16 18 20 22 24 26

Figure 14. Threshold of perceptible interference for an FM

receiver with equal amplitude FM interferers.

Peak frequency deviation is e = 3 kHz.

24
interference and 12 dB SINAD curve cross. This discrepancy

should not occur and is probably due to a combination of the

accuracy of the measurements and receiver stability.

The SINAD = 12 db curve can essentially be considered a

lower performance bound that corresponds to the point where

the receiver is just on the verge of losing capture. The

threshold of perceptible interference, in contrast, corresponds to an upper limit where the effects of interference

are fi~st observed. Note that when ~f = 0 , these curves are

separated by more than 20 dB. However when ~f =,14 kHz, the

separation is only a few dB. This difference implies a

change in threshold characteristics as a function of ~f that

is explored further in the following graphs.

Figure 15 shows a plot of SINAD versus input signal-tointerference ratio for different values of frequency offset.

The curves shown here, however, have been normalized so that

they can be easily compared. The normalized input signalto-interference ratio is defined as SjIIN and is given by

S/IIN = S/IIN - S/Io (9)

where S/Io is the signal-to-interference ratio required to

produce a SINAD = 0.0 dB. This normalization procedure

shifts the curves so that the shape of the curves can be

compared.

The results in Figure 15 are for a single FM interferer. Again the peak frequency deviation of all FM

generators is e = 3 kHz, with tone modulation on the desired

and interfering signals. Note that the SINAD versus S/IIN

curves have a sharper transition when ~f = 10 kHz as

compared to when ~f = 0 kHz. This difference is·more than

likely due to a combination of the IF characteristics and FM

threshold phenomena. For all practical purposes, the curves

25
30

CD

o 20

<t

2

(f)

10

10 kHz~~

X---=::'-:A:;;;=:::a[]P'( """'----..-~ X

~f =0 kHz and 20 kHz

x------.......

10 20

S/IIN ) dB

30 40

Figure 15. Measurements of SINAD as a function of 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 e = 3 kHz.

26
with ~f = a and 20 kHz are identical, and therefore, only

the ~f = 0 kHz curve is shown in the figure.

4. SPECTRUM PHOTOGRAPHS

Photographs of the spectra of the interfering FM

signals are shown in Figure 16. Figure l6(a) shows the

spectrum of a voice modulated, interfering PM signal with a

peak frequency deviation of 8 = 3 kHz, while 16 (b) shows

the spectra with 400 Hz tone modulation.

The spectra of the audio output from the receiver is

shown in Figures 17 and 18. In Figure l7(a) the audio

output spectrum is shown with only receiver background

noise. The fundamental and harmonic output due to the

1000 Hz modulation can be clearly seen. In Figure l7(b)

Gaussian noise of S/NI N = 10 dB is the interfering source.

Figure 17 (c) is similar except that now a single FM

interferer of S/IIN = 10 dB replaces the Gaussian noise.

The last photograph in this series shows three equal

amplitude interferers of S/IIN = 10 dB. The peak frequency

deviation of all FM signals in Figure 17 is e = 3 kHz. Note

the s i mi I ari t.y between Gaussian noise and FM interference as

shown in (b), (c), and (d).

Figure 18 shows similar results as a function of

frequency offset ~f. Spectra of the audio output signal are

shown for a single FM interferer at frequency offsets of 0,

4, 12, and 24 kHz. All spectra are the result of 1000 Hz

modulation on the desired FM signal and 400 Hz modulation on

the interfering FM signal. The peak frequency deviation is

e = 3 'kHz. The value of S/IIN was varied during these tests

to maintain a constant output SINAD.

27
10dS 1

io ee I

5 kHz

5 kHz

( b)

Figure 16. Spectra of a 171~8 MHz FM signal modulated

with (a) a voice signal and (b) a 400 Hz tone.

Peak frequency deviation is e = 3 kHz.

28
lOdB I

(a)

k--+I

O.5kHz

10dB I

(b)

0.5 kHz

l\J

\0

lOdB I

(c)

0.5 kHz

10dB I

(d)

J+--+I

0.5 kHz

Figure 17. Spectra of the audio output from an FM receiver with (a) receiver

background noise, (b) Gaussian noise o;f S/N1N = 10 dB, (c) a single

FM interferer of S/IIN = 10 dB, and (d) 3 eqbal amplitude FM

interferers of S/IIN = 10 dB.
w

o

toas I

(a)

ioes I

(c)

.~

0.5 kHz

.~

'0.5 kHz

ladB I

(b)

lOdB I

(d)

0.5 kHz

0.5 kHz

Figure lB. Spectra of the audio output from an FM receiver with a single FM

interferer offset in frequency by (a) ~f = 0 kHz, (b) ~f = 4 kHz,

(c) ~f = 12 kHz and (d) f = 24 kHz. Peak frequency deviation of

all FM signals is e = 3 kHz.
5. SUMMARY AND CONCLUSIONS

This report describes a limited set of laboratory

measurements that investigate the performance of an FM

receiver in FM interference. These measurements were made

on a single FM mobile transceiver at a frequency of

171.8 MHz. The performance of the receiver was studied with

both single and multiple FM interferers, as well as Gaussian

noise.

The test results show that above threshold the

receiver's performance in Gaussian noise and FM interference

is similar. For example, the output SINAD that was measured

for various interference conditions is as follows:

(a) Gaussian noise only S/NI N = 15 dB SINAD = 29.0 dB

(b) 1 FM interfe rer , S/IIN = 20 dB SINAD = 26.0 dB

( c) 2 FM interferers, S/IIN = 20 dB SINAD = 26.0 dB

(d) 3 FM interferers, S/IIN = 20 d-B SINAD = 26.0 dB

These results are all for a peak frequency deviation of

e =3 kHz and a frequency offset between victim and

interferer of ~f =0 kHz.

~~e value of S/IIN or S/NI N, where threshold occurs,

differs somewhat with the various interference conditions.

Although the definition of threshold is arbitrary, one can

show that the S/IIN or S/NI N must be greater than the

following values in order to be above threshold:

(a) Gaussian noise only S/N1 N > 8 dB

(b) 1 FM interferer S/IIN > 2 dB

(c ) 2 FM interferers S/IIN > 3 dB

(d) 3 FM interferers S/IIN > 4 dB

31
All of these preceding results are for a frequency deviation

of 8 = 3 KHz and frequency o~fset of ~f = 0 KHz.

The tests with frequency offset ~f between the desired

and interfering signals were limited by the off-frequency

phase noise emitted from the FM signal generators. It is

estimated that this phase noise limits the dynamic range of

the tests at 20 kHz < ~f < 30 kHz to

S/IIN .:. -90 dB, (10)

Although most of the results satisfy (10), the transceiver

manufacturer does specify a selectivity of -100 dB at

~f_ = 30 kHz. Improved generators would be necessary to

confirm or deny these specifications and screen-room

shielding of the receiver also should be used. Typical

measurements of the S/IIN that was required to produce a

SINAD = 12 dB are:

(a) 1 FM interferer, ~f = 20 kHz, S/IIN = -84 dB,

~f = 25 k Hz , S/IIN = -88 dB,

(b) 2 FM interferers, ~f = 20 kHz, S/IIN = -77 dB,

~f = 25 kHz, S/IIN = -88 dB,

( c ) 3 FM interferers, ~f = 20 kHz, S/IIN = -77 dB,

l1f - 25 kHz, S/IIN = -84 dB.

Measurements also were made of the audio spectra at the

output terminals of the receiver. The spectra of the audio

noise due to FM interference was generally indistinguishable

from white Gaussian interference at the input to the

receiver.

32
In summary, the test results with FM interference

showed only minor differences between co-channel FM

interference and white Gaussian interference of equal power.

This is true, however, only when the input signal-tointerference ratio S/IIN is greater than threshold. The

value of this threshold is different for FM interference as

compared to Gaussian noise. Threshold values with a single

FM interferer were generally 5 or 6 dB lower than those with

white Gaussian noise.

6. REFERENCES

Electronic Industries Association (1972), Minimum standards for

land mobile communication FM or PM receivers; 25-970 MHz;

RS-204-A; July.

33

FORM OT-29 (3-731

U.S. DEPARTMENT OF COMMERCE

OFFICE OF TELECOMMUNICATIONS

BIBLIOGRAPHIC DATA SHEET

1. PUBLICATION OR REPORT NO 2. Govt Acce s s ion No.3. Rec-ipient's Accession No.

NTIA Report 79-27

... TITLE AND SUBTITLE

Measurements of an FM Receiver in FM Interference

7. AUTHOR(S)

John R. Juroshek

S. Publication Date

6. Performing Organization Code

9. ProjectlTask/'l"ork Unit No.

9101116 8. PERFORMING ORGANIZATION NAME AND ADDRESS

U.S. Department of Commerce

National Telecommunications and Information Administratior 10. Contract/Grant No.

Institute for Telecommunication Sciences

325-Broadway, Boulder, CO 80303

11. Sponsoring Organization Name and Address 12. Type of Report and Period Covered

U.S. Department of Commerce

National Telecommunications and Information Administratior_~

1325 G Street, NW 13.

Washington, D.C. 20005

14. SUPPLEMENTARY NOTES

1S. ABSTRACT (A 20o-word or less factual summery of most si~nificant iniormation, If document includes a si~nificant

bjbljo~raphy ot literature survey, men ti ca it here.)

This report investigates the performance of an FM, landmobile, receiver

in FM interference. Measurements are ~scribed 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 performance 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.

16. Key Words (Alphabetical order, separated by semicolons)

frequency modulation; spectrum; interference; communications; land mobile radio

17. AVAILABILITY STATEMENT

IKl UNLIMITED.

o FOR OFFICIAL DISTRIBUTION.

18. Se c uri ty Class (This report)

Unclassified

19. Se c urity Class (Thie page)

Unclassified

20. Number of pages

38

21. Price:

{r u.s. GOVERNMENT PRINTING OFFICE:.1979-0-682-011/146 l'5COMM.. OC 2;716· P73