A STUDY OF CO-CHANNEL AND ADJACENT-CHANNEL INTERFERENCE IMMUNITIES OF
SUBSIDIARY COMMUNICATIONS AUTHORIZATION (SCA)

FM BROADCAST RECEIVERS

\-\-\-\-\-\-\-\-\--

Project TRB-99-3

Preliminary Report

December 1, 2000

Technical Research Branch

Laboratory Division

Office of Engineering and Technology

Federal Communications Commission

OET Report Prepared by:

FCC/OET TRB-00-1 David L. Means

Project TRB-99-3 December 1, 2000

A STUDY OF CO-CHANNEL AND ADJACENT-CHANNEL INTERFERENCE IMMUNITIES OF
SUBSIDIARY COMMUNICATIONS AUTHORIZATION (SCA)

FM BROADCAST RECEIVERS

Executive Summary

This preliminary report presents the results of a study intended to
produce objective data on the interference performance of thirteen
Subsidiary Communications Authorization (SCA) FM broadcast receivers
used for reception of Radio Reading Service. This study was primarily
intended to explore concerns raised in the record of the Commission's
proceeding *In the Matter of Creation of Low Power Radio Service*, MM
Docket No. 99-25, regarding the potential impact on the Radio Reading
Service of eliminating third adjacent channel interference protections
for 10 watt and 100 Low Power FM (LPFM) stations. The study was
conducted using a sample of fourteen FM broadcast SCA receivers provided
by National Public Radio (NPR). Thirteen of these receivers were tested
for adjacent-channel and co-channel interference performance; the
remaining receiver was deemed not testable because of severe main
channel bleedthrough problems. Because of the small sample size, the
test results presented herein were not extrapolated to the general
receiver population.

The median ratios of interfering signal-to-desired signal, *i.e.*,
undesired-to-desired (U/D) ratios, for 1% increased impairment found for
the sample receivers on third and fourth adjacent channels were 36.6 dB
and 37.9 dB, respectively. Both of these values fell short of the
Commission's 40 dB U/D criteria for third adjacent FM channels by a few
dB. Thus, some receivers in this sample might not adequately reject the
signal of a LPFM station operating on a third adjacent channel when used
in close proximity to that station. In addition, some receivers might
not be able to reject the signals of existing full power stations
operating on fourth adjacent channels consistent with the Commission's
longstanding FM separation requirements. In fact, several of the tested
receivers were more susceptible to signals on fourth adjacent channels
than to signals on third adjacent channels. It appears, however, that
most potential receiver performance problems could be resolved on a
case-by-case basis by providing the affected listener with a directional
external antenna or by selecting an SCA receiver with interference
rejection characteristics appropriate to the case at hand. Providing
outboard filters may also solve most receiver performance problems, but
this approach is not likely to be cost-effective.

Interference from the main audio program channel of the station carrying
the SCA, *i.e.*, bleedthrough, was audible in eight of the receivers,
even under unimpaired conditions. In two of these receivers, the audible
level of the main channel bleedthrough worsened with the level of the
interference impairment, and was indeed the principle audible
manifestation of the interference. In receivers such as these two, the
increased bleedthrough can mask the perception of interference or even
render it undetectable as such. Several of the receivers tested also
exhibited gross asymmetry in their selectivity characteristics, leading
to our conclusion that their adjacent channel interference performance
is compromised by misalignment.

The Receiver Sample

The 14 SCA receivers provided by NPR were solicited from member stations
who transmit radio reading services. Although the manufacturers of two
of the receivers are not known with certainty, at least nine different
manufacturers have models represented in the sample. Four of the
receivers were manufactured by McMartin; however, these four all have
slightly different model designations. All the receivers in the sample
except one are small tabletop or portable receivers, designed for
personal residential use, with an integral whip antenna and loudspeaker.
The single exception is a receiver designed for mounting in an equipment
rack in an institutional or commercial installation. All except one were
fixed-tuned to a particular host station frequency and SCA frequency.
The single exception was a receiver which had two modes of operation:
one which allowed operation only on a single parent station and SCA
frequency, and another mode in which the entire FM broadcast band could
be tuned for main-channel programming only. Twelve of the receivers had
provisions for some form of external antenna input, typically a Type F
connector or terminal screws. One receiver had an unorthodox external
antenna connection consisting of a pair of unlabeled pressure-fit lugs.

The only information provided with the receivers regarding appropriate
technical parameters for testing was the host station carrier frequency
and the corresponding SCA frequency. Attempts were made to obtain
additional information on other parameters (such as de-emphasis
characteristics, maximum modulating frequency and peak deviation) from
the station contacts provided by NPR, but complete information was not
available for most of the receivers. Of necessity, default test values
were used for maximum modulating frequency and peak deviation, and
appropriate pre-emphasis/de-emphasis for each receiver was determined
experimentally.

Both receiver characterization and interference testing were attempted
on all 14 receivers, although one receiver proved untestable for
interference immunities because of severe main-channel bleedthrough into
the demodulated SCA audio.

Characterization Tests

Because the interference measurements essentially constituted a test of
the selectivity of the receivers, and because the performance of the
receivers' audio sections was not germane to the issues at hand, the
only characterization test deemed necessary was quieting sensitivity.
This characteristic is determined by the RF input signal power necessary
to produce 50 dB of quieting in the audio output. For receivers that
were incapable of achieving 50 dB quieting, the maximum quieting
achievable and the quieting level at --55 dBm RF input were measured and
recorded. To the extent they are applicable to SCA receivers, the
procedures outlined in IEEE Standard 185-1975 were followed.

As noted in the data tables, for receivers without external antenna
inputs of known impedance, the RF input values stated are test bed power
output values only.

Interference Testing Methodology

The appropriate methodology for quantifying the manifestation of
interference in the audio output of radio broadcast receivers has been a
matter of some controversy in the LPFM rule making proceeding. FCC
Laboratory engineering staff maintains that interference is best
quantified in terms of relative degradation of the audio output signal
as compared to the unimpaired condition, in contrast to methods which
target an absolute signal quality goal expressed in terms of
Signal-plus-Noise to Noise \[(S+N)/N\]. Further, (S+N)/N methods require
that the modulating signal be removed from the desired signal in order
to perform the measurement, eliminating the possibility of capturing any
audio intermodulation interference effects which might be present.

For these reasons, the methodology chosen for these tests is the
measurement of the undesired-to-desired (U/D) RF signal ratios necessary
to produce degradation of the unimpaired baseline Total Harmonic
Distortion plus Noise (THD+N) of the audio output by an additional 1%
and 3%. This method captures all interference energy, regardless of how
it is manifested, but the noise term N is typically dominant.

Interference Testing Procedure

The interconnection of the test equipment and the equipment under test
is diagrammed in

Figure 1.

![](media/image1.wmf){width="6.850694444444445in" height="4.7in"}

The desired signal's carrier was generated on the parent station's
nominal frequency for each receiver under test using a Hewlett Packard
8642B signal generator. This carrier was modulated by the composite
baseband output of a CRL SG-800A stereo generator. To exercise the main
stereo channel of the desired signal, "easy listening" musical
programming was fed to the main inputs of the stereo generator from a CD
player. The programming used was Natalie Cole: [Take a
Look]{.underline}, Track 1: "I Wish You Love."

The SCA input of the stereo generator was fed either a 67 kHz or 92 kHz
modulated subcarrier, as appropriate, generated by FM modulating the
Hewlett Packard 8664A signal generator with a

1 kHz tone provided by the Audio Precision System One audio analyzer.
Deviation produced by the

1 kHz tone was set at +/- 6 kHz. Appropriate pre-emphasis on the SCA
baseband was applied using the Rane GE-14 graphic equalizer. The
appropriate pre-emphasis for each receiver was determined by
substituting a pink noise signal for the 1 kHz tone as the modulating
signal input to the HP8664A signal generator, and then adjusting the
equalizer settings to produce a flat audio output response from the
receiver under test with the tone control, if provided, set in the
nominally flat position. Flatness of the audio response was measured
using an Audio Control SA-3050A real time spectrum analyzer. No attempt
was made to boost the audio response below 100 Hz or above 5 kHz, and
all equalizer bands above 6.3 kHz were set for maximum attenuation.

The stereo generator and HP 8642B signal generator were adjusted to
provide 105% total modulation of the desired signal carrier, with the
stereo pilot and SCA subcarrier injection levels set at 10% each. Main
channel modulation was set at a relatively low level, with the
programming used illuminating the 100% modulation indicator only on
occasional peaks.

Similarly, the modulated carrier for the undesired signal was generated
by another HP 8642B signal generator, with the composite stereo baseband
input provided by another CRL SG-800A stereo generator. The modulating
signal was pink noise provided by a Hewlett Packard 8057A precision
noise generator. The pink noise was band-limited using the filter
section of the Orban 8455A Optimod processor, and compressed and limited
using the DBX 166A compressor/limiter to simulate heavily processed
program material.

In order to provide as accurate a simulation as possible of real-world
interference from a heavily modulated station with highly processed rock
music programming, such a station was selected from the local
off-the-air FM broadcast spectrum, and the undesired signal's modulation
levels, compression, and limiting were adjusted to accurately duplicate
the off-the-air station's spectral envelope.

Co-channel and adjacent channel interference tests on all upper and
lower adjacent channels up to the fourth adjacent channel were conducted
on each receiver in the sample by combining the desired and undesired
modulated carriers in a Minicircuits ZFSC-2-1 splitter/combiner, and
providing the combined signal to the RF input of the receiver under
test, using a matching transformer if necessary. In all cases, the
desired signal level provided to the receiver was --55 dBm, which is
approximately the signal level delivered to a receiver by a dipole
antenna located at an FM broadcast station's 60 dBu protected contour.

Impairment was quantified by measuring the increase in THD+N on the 1
kHz demodulated audio tone produced by introduction of the undesired
signal, using the Audio Precision System One audio analyzer. A baseline
value for THD+N was first determined for each receiver with no undesired
signal or main channel modulation on the desired signal present. The
main channel modulation was then turned on, and the level of the
undesired signal was slowly increased until a level was reached where
the measured THD+N exceeded the baseline value by one percentage point
on peaks of frequent recurrence. The undesired to desired signal ratio
(U/D) was recorded at that point. The undesired signal was then further
increased until the measured THD+N exceeded the baseline value by 3%,
and the U/D was again recorded. This process was repeated for each
co-channel and adjacent-channel situation for each receiver, and the
results tabulated and graphed.

To provide an audio record for subjective evaluation using program
material, CD recordings of the B-52s "Love Shack" and an *a cappella*
vocal by Suzanne Vega were substituted for the undesired and desired
(SCA) signal modulations, respectively. These recordings were chosen to
represent a potentially worst-case interferer as well as affected
program material on which the presence of interference would be easily
audible. For each receiver, a 60-second digital recording was made of
the unimpaired audio, the audio impaired at the U/D ratio which produced
a 1% increase in THD+N, and the audio impaired at the U/D ratio which
produced a 3% increase in THD+N. These recordings were made for the
upper third adjacent channel interference situation only. An index to
these recordings is presented in Appendix C.

Reasonable precautions were exercised throughout the data gathering
effort to ensure against the incursion of unwanted ambient signals and
testing artifacts. Measurements were conducted inside a shielded room,
and ferrites were placed on each end of the cable connecting the signal
combiner to the antenna input terminals of the receiver under test. 6 dB
pads were placed on the outputs of the desired and undesired carrier
signal generators to reduce interaction. An isolation transformer was
used on the power mains for the receivers under test. Audio output
levels from the receivers were adjusted to minimize distortion and avoid
overload in the receivers' audio amplifiers.

Observations on the Data

Because of the relatively small test sample (13 receivers actually
tested), caution is urged in extending conclusions from the data to the
general population of SCA receivers.

The data for all the receivers tested is summarized in a table in
Appendix A in the form of median values for the U/D ratio necessary to
produce each degradation level for each of the interference situations.
For the adjacent-channel situations, these values are the medians of the
combined upper and lower adjacent-channel measured values. It should be
noted that for all table entries derived from an even number of measured
values, the table entry represents the average of the two median
measured values.

The measured data for each individual receiver is tabulated and graphed
in Appendix B.

The tabulated data and graphs for Receiver B show measured U/D values of
65.5 dB for the upper third adjacent channel at 1% additional impairment
and for both 1% and 3% additional impairment on the fourth adjacent
channel. It should be noted that the actual values probably exceed 65.5
dB. The 65.5 dB value represents the upper limit of the capability of
the test bed to generate the undesired signal.

Conclusions

Due to the small sample size, we are not extrapolating the test results
to the general receiver population. However, some observations may be
made for the sample at hand.

Section 73.215 of the Commission's rules provides that the predicted
field strength of a potentially interfering station can be no more than
40 dB stronger than the protected field strength along a station's
protected contour. This protection was eliminated for the third adjacent
channel situation for LPFM stations authorized under the *R&O* in MM
Docket 99-25. Noting that the median U/D for 1% increased impairment is
36.6 dB for the third adjacent channel, one may reasonably conclude that
most of the receivers in this sample could experience some interference
when used in close proximity to an LPFM station that is operating on the
third adjacent channel. Only three of the receivers tested exceeded 40
dB U/D for a 1% increase in THD+N on both the upper and lower third
adjacent channel.

While the median U/D for all fourth adjacent channel interference
situations (37.9 dB) exceeded the median U/D for all third adjacent
channel situations (36.6 dB), not all receivers performed better at the
fourth adjacent channel than the third. Six of the receivers were less
capable of rejecting interference at the fourth adjacent channel than
the third on either the upper or lower adjacent channel situation or
both.

Given that SCA receivers do exist that exhibit adequate third and fourth
adjacent channel interference performance, and that many of the
receivers fell short of the 40 dB U/D criterion for third adjacent by
only a few decibels, it appears possible to resolve most interference
problems on a case-by-case basis by providing the affected listener with
a directional external antenna or by selecting an SCA receiver with
interference rejection characteristics appropriate to the case at hand.
Providing outboard filters may also solve most interference problems,
but this approach is not likely to be cost-effective as the cost of
commercially available filters providing adequate performance approaches
the cost of the typical receiver.

In listening to the audio output of the receivers during the testing, we
noted that bleedthrough of the main channel audio signal was audible in
eight of the receivers, even under unimpaired conditions. The
bleedthrough was so severe in one case that the THD+N introduced by this
phenomenon exceeded a 3% increase over the baseline value, rendering the
receiver untestable. In two of these receivers, the audible level of the
main channel bleedthrough worsened with the level of the interference
impairment, and was indeed the principle audible manifestation of the
interference. For these two receivers, such bleedthrough can mask the
perception of interference or even render it undetectable as such.

While this bleedthrough of audio content from the main channel was
common in both impaired and unimpaired circumstances, intelligible
crosstalk from the interfering signal was not apparent. In some of the
recordings, the increased noise induced in the desired signal's audio
exhibited a rhythmic pattern that synchronized with the beat of the
undesired signal's program music. This is to be expected, as the extent
of sideband energy distribution in the undesired signal varies in
synchronization with musical programming.

To verify our assumption that noise is indeed the dominant component in
measured increases in THD+N due to interference, we conducted Fast
Fourier Transform (FFT) analysis of digital recordings from several of
the receivers under unimpaired, 1% increased impairment, and 3%
increased impairment conditions. This analysis showed relatively little
change in the levels of individual harmonic products, but a relatively
large change in the noise floor with increasing impairment, confirming
our assumptions.

For optimum adjacent channel interference immunity in receivers in
general, it is essential that any RF and IF filtering incorporated in
the design be aligned symmetrically about the tuned frequency of the
receiver. While several of the receivers displayed the expected symmetry
in the graphs of adjacent channel selectivity, others show gross
asymmetry. One could conclude that these receivers were either
improperly aligned at the factory, misaligned during servicing, or had
simply drifted out of proper alignment due to aging of components.
Regardless of the cause, such misalignment adversely affects the
interference rejection capabilities of the receiver when compared with
the intentions of the designer.

APPENDIX A

DATA SUMMARY

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height="0.9881944444444445in"}

APPENDIX B

INDIVIDUAL RECEIVER DATA

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height="4.764583333333333in"}

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height="4.764583333333333in"}

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height="2.2680555555555557in"}

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![](media/image8.wmf){width="6.659722222222222in" height="4.8in"}

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![](media/image11.wmf){width="6.659722222222222in" height="4.9in"}

![](media/image12.wmf){width="6.659722222222222in" height="5.0in"}

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height="4.829166666666667in"}

APPENDIX C

AUDIO RECORDING LOG

APPENDIX C -- AUDIO RECORDING LOG

All tracks are recordings of the demodulated audio of each receiver's
SCA channel, with impairment, when present, on the upper third adjacent
channel. All tracks are 60 seconds long.

Desired Channel Program: Suzanne Vega *a cappella* on SCA; Natalie Cole
"I Wish You Love" on main channel

Undesired Channel Program: The B-52s "Love Shack"

TRACK TITLE RECEIVER IMPAIRMENT U/D (dB)

1 AU A Unimpaired \--

2 A1% A 1% THD+N Increase 36.0

3 A3% A 3% THD+N Increase 37.1

4 BU B Unimpaired \--

5 B1% B 1% THD+N Increase 61.5

6 B3% B 3% THD+N Increase 65.3

7 DU D Unimpaired \--

8 D1% D 1% THD+N Increase 36.2

9 D3% D 3% THD+N Increase 36.9

10 EU E Unimpaired \--

11 E1% E 1% THD+N Increase 34.4

12 E3% E 3% THD+N Increase 35.6

13 FU F Unimpaired \--

14 F1% F 1% THD+N Increase 23.4

15 F3% F 3% THD+N Increase 25.6

16 GU G Unimpaired \--

17 G1% G 1% THD+N Increase 45.8

18 G3% G 3% THD+N Increase 47.1

19 HU H Unimpaired \--

20 H1% H 1% THD+N Increase 24.8

21 H3% H 3% THD+N Increase 25.3

22 IU I Unimpaired \--

23 I1% I 1% THD+N Increase 42.6

24 I3% I 3% THD+N Increase 43.5

25 JU J Unimpaired \--

26 J1% J 1% THD+N Increase 38.9

27 J3% J 3% THD+N Increase 40.7

28 KU K Unimpaired \--

29 K1% K 1% THD+N Increase 36.2

30 K3% K 3% THD+N Increase 37.0

31 LU L Unimpaired \--

32 L1% L 1% THD+N Increase 40.7

33 L3% L 3% THD+N Increase 41.0

34 MU M Unimpaired \--

35 M1% M 1% THD+N Increase 26.9

36 M35 M 3% THD+N Increase 27.4

37 NU N Unimpaired \--

38 N1% N 1% THD+N Increase 44.9

39 N3% N 3% THD+N Increase 45.4