Updating IEEE 185-1975

IEEE 185-1975, Standard Methods of Testing Frequency Modulated Broadcast Receivers, has long been used to test and evaluate consumer FM products in the U.S. While its IEEE status now is withdrawn standard, perhaps due to the shift overseas of consumer electronics design and manufacturing, it continues to be used as an informal reference because of its wide acceptance and familiarity.

However, some sections of the standard impose difficulties when used with more modern tuners, even those designed in the 1980s. Other sections specify tests that poorly represent typical FM broadcast signals, tests that inadequately probe worst-case signals, tests that are inherently inaccurate, tests that are too restrictive, and tests that are unnecessarily inconvenient.

Frequency

IEEE 185-1975 §5.2 calls for 98.0 MHz or 98.1 MHz to be used as the standard midband test frequency. However, interference from a local FM station that leaks into the tuner or test setup may preclude use of these frequencies, especially for sensitivity measurements. I use the clear channel nearest 97.5 MHz, the geometric center of the band, which at my location is 96.9 MHz.

Stereo Modulation

§5.3 defines L = -R as the standard stereo modulation. This is a curious choice, one not explained in the standard. It puts all of the modulation in the stereo subband and none at baseband. It is not representative of typical FM broadcast signals. In fact, no uncontrived sounds exhibit L = -R. I use L-only, R = 0 for stereo modulation. I do this mainly because it minimizes distortion in my Sound Technology 1000A FM Alignment Generator. But single-channel modulation also better spreads the test signal over the composite-stereo spectrum and is more typical of actual broadcast modulation.

Selectivity

§6.14 calls for the use of a 1-kHz bandpass filter when testing selectivity "to reduce errors from noise." But the filter only increases measurement error. For conventional tuners with ceramic IF filters, when the interfering signal leaks into the desired channel, its 1-kHz modulation is accompanied by dozens of high-level harmonics. The harmonics are distinctly audible and their power contribution ought to be included. On the air, interference from an adjacent or alternate channel similarly manifests itself as irregular bursts of impulses with high harmonic content. For DSP tuners with digital IF filters, local-oscillator phase noise limits selectivity. Neither the modulation fundamental nor its harmonics ever appear. Instead, the background noise suddenly rises. Using a 1-kHz bandpass filter with these tuners yields an entirely misleading and inaccurate measurement.

Capture Ratio

§6.13 defines monophonic capture ratio in a convoluted way, one that involves an approximation. Apparently this was done to facilitate use of the test equipment available at the time. I use a direct definition, which is simply how far below an unmodulated signal a 100%-modulated, 1-kHz interfering signal must be for a specified level of suppression, normally 30 dB. Sometimes I also report the figure for 50-dB suppression.

At my location the primary limitation for FM reception is low levels of co-channel interference and multipath distortion in stereo, for which there is no IEEE 185-1975 test. Tuners are much more susceptible to these problems in stereo than in mono. I define stereo capture ratio in the same way as the monophonic figure, but with both signals in stereo.

I make both capture ratio measurements at 65 dBf.

Modulation Acceptance

RF Aberration Tests

Sections that deal with RF aberrations, §6.15 Spurious Responses and §6.16 RF Intermodulation, impose several difficulties when used with more modern tuners. One problem is that monophonic usable sensitivity is the reference signal level for unwanted responses. This is the RF level at which audio output for a 100%-modulated, 1-kHz tone drops 30 dB when measured through a 1-kHz notch filter. This is equivalent to 3% THD + noise. Not only is a notch filter required for this measurement, which is inconvenient, but its Q is unspecified. Notch bandwidth can affect measurement results for tuners that use adaptive noise reduction, which concentrates residual noise near a test tone. (But no tuner yet uses adaptive noise reduction for monophonic signals.) A more fundamental problem is that tuners with narrow IF filters exhibit modulation-induced noise at low signal levels. This can cause the usable sensitivity figure to be several dB worse than the figure for 50-dB quieting sensitivity, a perplexing reversal of the results for wide IF filters. In addition, a signal at usable sensitivity is far too noisy for enjoyable listening by today's standards. It is an unrealistically weak signal. A further problem is that the standard calls for unwanted-signal levels to be measured using residual unmodulated noise. This makes the measurement susceptible to tuner or signal generator phase-noise sidebands, which are strong enough even for some nonsynthesized oscillators to invalidate the tests. Another inconvenience is that all results are given as ratios with respect to usable sensitivity. Determining the absolute signal level at which RF aberrations occur requires the addition of two numbers. Finally, since due to its limitations you may not ordinarily measure usable sensitivity, the RF aberration tests may impose unnecessary additional work. To address these problems, I've modified the tests.

I define RF intermod as the 50-dB quieting level for a third-order intermodulation product. I use three signal generators for this measurement. First, I modulate a signal generator 92% (69 kHz) at 400 Hz. This yields the same audio level after deemphasis as a 100%-modulated, 1-kHz tone. I set this generator 1600 kHz above the tuned frequency, sum it with an unmodulated generator 800 kHz above, and turn off both RF outputs. Next I set a third generator at the tuned frequency at the 50-dB quieting level for a 100%-modulated, 1-kHz tone. I enable the modulation, turn on the two other generators, and adjust their equal RF levels until the levels of the 400-Hz and 1-kHz components in the audio output are equal. The tuned-frequency generator and the intermodulation product compete for capture, and no amount of phase noise will alter the relative audio-component levels.

I define RF spur as the 50-dB quieting level for an untuned signal. I tune a signal generator from 88 to 108 MHz, increasing its level (to a 10-mW maximum of 130 dBf) until I find a spur at the tuned frequency. Because the spur may be the result of high-order internal intermodulation, I set the deviation of a 400-Hz tone to yield the same audio output level as a generator at the tuned frequency 100%-modulated at 1 kHz (sometimes less than 10 kHz deviation suffices). I do this at a high enough signal level that the audio level is independent of RF level. Then I reset the tuned-frequency generator to the 50-dB quieting level and sum the signal from the other generator, adjusting its RF level until the levels of the 400-Hz and 1-kHz components in the audio output are equal.

I define image rejection as the ratio of image- to tuned-signal levels at 50-dB quieting. I measure it the same way as RF spur.

If phase noise is low enough, you can use one less signal generator and a simplified procedure. Just set the unwanted response to the 50-dB quieting level.

In all cases, the 50-dB quieting level is that defined in §6.4: the RF signal level at which the audio output drops 50 dB when 100% 1-kHz modulation is removed. It is not the unmodulated RF signal level that quiets the background noise 50 dB.

For all measurements I use the 200-15000 Hz audio bandpass filter specified in §3.8.

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Updated May 6, 2008