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Receivers, has long been used to test and evaluate consumer FM products in the U.S. Although its IEEE status is withdrawn standard, it remains an informal reference because of its wide acceptance and familiarity. However, some sections of the standard specify tests that are inconvenient, unrealistic, or inherently inaccurate. Other tests promote confusion, inadequately probe worst-case signals, or impose difficulties with modern equipment.
IEEE 185-1975 §3.9 specifies an average-responding audio level meter. This introduces an error of −1.05 dB when measuring Gaussian noise. The standard does not compensate for this error. Sensitivity and S/N figures would be 1 dB high for older receiver specifications and test reports that used such meters. To obtain actual power ratios, I use a true RMS meter for all audio level measurements.
§5.2 specifies 90, 98, and 106 MHz (or 90.1, 98.1, and 106.1) as standard test frequencies. However, local interference that leaks into the test setup may preclude use of a frequency, especially for sensitivity measurements. Since I don't have a screen room or screened enclosure to attenuate such signals, I use the clear frequency closest to that specified.
§5.3 defines L = −R as 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 broadcast signals. In fact, no uncontrived sounds exhibit L = −R. I use L-only, R = 0 for stereo modulation, mainly because it minimizes distortion in my signal generator. But single-channel modulation also spreads the test signal over the composite spectrum more like actual broadcast modulation.
§5.5 tells how to set the output level for testing when it is adjustable, but nowhere does the standard specify an output level measurement for a standard test signal. I define output level as the voltage generated across the standard audio load (100kΩ || 1000 pF) for a 100%-modulated, 1-kHz, 65-dBf monophonic signal. When adjustable, I report the maximum output level.
§6.11.2 calls for distortion tests to 120% modulation, but this does not test a receiver's ability to handle signals accidentally or intentionally deviated much higher. Occasionally such signals do occur. I increase the deviation of a 65-dBf monophonic signal modulated with a 1-kHz tone until THD reaches 1%. I report the modulation percentage as modulation acceptance.
§6.13 gives a convoluted procedure for measuring capture ratio, one that involves an approximation. I make a direct measurement using the stated 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, a more realistic level for interference tolerance. All signals are monophonic.
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. Receivers 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 measure capture ratio with the desired signal at 65 dBf. When I have the patience, I follow the standard and remeasure at 45 dBf, reporting the worst result.
§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 receivers with an analog IF filter, a dozen or more high-level harmonics accompany the 1-kHz fundamental of an adjacent-channel test signal that leaks into the tuned channel. The harmonics are distinctly audible and their power contribution should be included. For DSP receivers with a digital IF filter, reciprocal mixing of local-oscillator phase noise usually limits selectivity. Neither the modulation fundamental nor its harmonics ever appear. Instead, the background noise rises. Using a bandpass filter in this case can yield a grossly inaccurate result.
§6.15 and §6.16 define RF spurious response and intermodulation tests that impose several difficulties, particularly for modern receivers. 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. Not only is a notch filter required for this measurement, which is inconvenient, but its Q is unspecified. Notch bandwidth can affect measurements for receivers that use adaptive noise reduction, which concentrates residual noise near a test tone. A more fundamental problem is that receivers with narrow IF filters exhibit modulation-induced noise at low signal levels. This can cause 30-dB usable sensitivity to be several dB worse than 50-dB quieting sensitivity, a perplexing reversal of the results for wide IF filters. In addition, a signal at usable sensitivity is too noisy for enjoyable listening but too quiet to degrade intelligibility. It is an inappropriate test level. A further problem is that the standard calls for unwanted-signal levels to be measured using residual unmodulated noise. Such two-signal measurements are highly susceptible to receiver or signal generator phase-noise sidebands, which can be strong enough even for nonsynthesized oscillators to invalidate a test. An unnecessary confusion is that rated spurious response ratio is supposed to be the worst result of several specified tests, one of which has the same name. It's never clear exactly what a quoted spec refers to. Another inconvenience is that all results are given as ratios with respect to usable sensitivity. Determining the absolute signal level at which an RF problem occurs requires the addition of two numbers. Finally, since due to its limitations you may not ordinarily measure usable sensitivity, the tests may impose unnecessary additional work. To address these problems, I've modified and renamed 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 one generator 92% (69 kHz deviation) 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 and sum it with an unmodulated generator 800 kHz above. Next, I set a third generator at the tuned frequency to the 50-dB quieting level for a 100%-modulated, 1-kHz tone. I sum all three generators and adjust the equal RF levels of the off-frequency generators 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 repeat the measurement with the untuned generators below the tuned frequency and average the two results.
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 RF image as the 50-dB quieting level for a mixer image. I measure it the same way I measure RF spur. Subtracting the 50-dB quieting level yields a figure equivalent to image response ratio defined in §6.15.2, but the updated measurement avoids phase noise and provides an absolute level.
Whenever phase noise is low enough, you can use one fewer signal generators 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 noise measurements I use the 20015,000 Hz bandpass filter specified in §3.8.
§6.20 specifies impedance measurements that yield SWR at the antenna port. I measure the equivalent return loss but convert it to RF mismatch loss in dB so that sensitivity degradation can be directly assessed. I give the range of values across the FM band. I use a signal level below the RF AGC threshold.
88108 MHz