Testing FM receivers is not that much different than testing other pieces of audio equipment. Appropriate tests include the big six: level, frequency response, THD+N, phase, interchannel crosstalk, and signal-to-noise ratio. You make the measurements just like you would when testing other audio gear—by connecting an audio analyzer to the device’s output and measuring the signals to see what effect the device has on them.
However, in practice, there are a couple of significant differences between testing a usual line-level audio device and an FM receiver. This has to do with how you format the test signals and how you get them into the receiver.
Audio recorders and processors typically are connected directly to the inputs and outputs of an audio analyzer. But an FM receiver accepts its audio input over the airwaves as a multiplex encoded FM radio signal. What’s more, since most FM receivers are capable of receiving radio data system (RDS) signals, the on-board systems for decoding such signals also should ideally be part of your test routine.
RDS is a communications protocol for sending small amounts of digital information within a conventional analog FM radio broadcast. Typically, RDS signals are used to transmit program-related information or emergency messages directly to the front-panel display of FM receivers.
FM Broadcasting
FM was invented in 1933 by Edwin H. Armstrong, one of the founding fathers of radio technology, and is a technique whereby a baseband message signal m(t) varies or modulates the frequency of a carrier wave. In broadcasting, FM is used to provide high-fidelity sound over broadcast radio. In this case, the message signal is music and speech, and the carrier wave is at a radio frequency within the VHF radio band, usually 87.5 to 108.0 MHz.
FM radio was first used for monaural broadcasting in the 1940s, and FM stereo was introduced in the 1960s. One of the key requirements for stereo FM broadcasting was to maintain backward compatibility with the large existing base of monophonic FM receivers. This goal was achieved by using a scheme to multiplex the stereo signal channels as shown in Figure 1.
The left (L) and right (R) audio signals are first low-pass filtered at 15 kHz, and then the sum (L+R) and difference (L-R) signals of the two stereo channels are formed. The (L+R) signal is broadcast in the lower portion of the baseband spectrum (0 to 15 kHz) so that a monophonic receiver correctly receives the signal as a monaural transmission.
The (L-R) difference signal then is amplitude modulated onto a suppressed 38-kHz subcarrier in the 23- to 53-kHz region of the baseband spectrum. A 19-kHz pilot tone is added to the multiplexed signal: one-half the frequency of the 38-kHz subcarrier and with a precisely defined phase relationship to it. This pilot tone is used by FM receivers to detect a stereo transmission and reconstruct the stereo signals from the multiplexed signal.
To improve the signal-to-noise ratio, it is a common practice to apply a pre-emphasis filter to the audio signals before breaking them into L+R and L-R components to boost the higher frequency portion of the signal. The pre-emphasis filter has the high-pass characteristic of a simple RC filter circuit.
The amount of pre-emphasis is defined by the time constant of the RC circuit. In North America, a 75-µs time constant is used. In the rest of the world, 50 µs is used.
FM receivers incorporate a reciprocal de-emphasis filter, which removes the effect of the pre-emphasis. This restores the message signal’s flat frequency response after transmission. By using pre-emphasis and de-emphasis, high-frequency headroom is reduced.
The multiplex signal shown in Figure 1 also contains an RDS signal. To add an RDS message to an FM stereo broadcast signal, an RDS encoder is used, as illustrated in Figure 2. The bit rate is 1,187.5 bits per second, 1/48th of the 57-kHz RDS subcarrier frequency, the third harmonic of the 19-kHz broadcast pilot tone. As this bit rate is quite low, RDS only is suitable for sending small amounts of text or other data that are updated with a time frame in seconds.
The RDS message (Figure 2a) first passes through a differential encoder, the equivalent of a logical exclusive OR (Figure 2b). It then is converted from non-return-to-zero coding to pulses (Figure 2c) and to a series of biphase symbols to minimize the power of the data signal near the 57-kHz subcarrier (Figure 2d). The biphase symbols are filtered with a data-channel spectrum shaping filter to achieve the required band-limited spectrum (Figure 2e). Finally, the filtered data waveform is used to modulate the 57-kHz subcarrier frequency and then summed with the rest of the multiplex stereo signal. The waveform of the modulated signal is seen in Figure 3. A decoder in the FM receiver is used to decode the RDS message, usually so the text can be displayed.
Testing, Testing…
Testing an FM receiver requires an FM transmitter or exciter, a small transmitter that typically is used to drive an RF power amplifier to create a more powerful signal. The transmitter takes the monaural or multiplexed stereo signal with or without RDS and modulates an RF frequency within the FM band. Transmitters are available from various vendors, with solutions varying from inexpensive do-it-yourself kits to professional-grade broadcast equipment.
The audio quality of the transmitter must be better than the receiver under test or the measurements will reflect the transmitter’s limitations and not the receiver’s. When comparing transmitter specs, remember that some companies may be more conservative in their numbers than others and that testing methodologies can differ. Sometimes the testing conditions are not stated, which makes comparison difficult.
For testing, the RDS signal must be added to the audio signal after the FM multiplex stage. Stand-alone hardware encoders are available to do this. But for testing purposes, software solutions can do the job much more affordably, and some are freeware.
Audio Precision offers an RDS encoder utility for use with our APx Series Audio Analyzers. The utility creates sine wave test signals and multiplexes and combines them with RDS messages. The completed waveforms can be uploaded to the analyzer’s generator section and then output directly to an FM transmitter.
Figure 4 illustrates a complete FM receiver test setup. Only one mono channel is needed for the input to the FM transmitter because the multiplexed waveform already includes both the left and right stereo channels.
From the transmitter, the signal is fed to the FM receiver via a direct connection to minimize outside interference and reduce spurious radiation from the transmitter. The main RF output often is an N connector, in which case an N-to-BNC adaptor will be needed with a 50-Ω inline BNC terminator to provide an appropriate load for the transmitter. A 50-Ω BNC cable adapted to the receiver at one end completes the RF connection.
To calibrate the system, the transmitter is driven briefly by the audio test system to 100% modulation (±75-kHz deviation). This establishes the level of the FM stereo pilot and RDS signal. The audio signal should be reduced to the desired test level while leaving the pilot and RDS level unchanged.
The receiver now is tuned to the transmitter frequency, and the transmitter’s RF output is adjusted to provide a sufficient level for full FM quieting, yet not so high as to overload the receiver’s front end. Since the transmitter in this suggested setup is directly connected to the receiver, a minimal RF level is needed. Many receivers have an FM stereo indicator, an RDS indicator, or a signal strength meter to help determine sufficient signal strength. If an RDS message is being received and decoded by the receiver, it should be visible on the receiver’s display.
Finally, the audio outputs of the radio receiver should be connected to the inputs on the audio analyzer being used for the tests. Typically, the audio outputs are unbalanced analog on consumer tuners and receivers and balanced on professional FM monitors. If a car radio without line-level outputs is being tested, the speaker outputs should be connected to dummy loads and then to the balanced inputs on the audio analyzer.
Since an FM stereo signal is just two channels, it is only necessary to connect the first two channels of a multichannel receiver to analyze the FM reception quality. A home receiver usually provides many ways to input a signal, and its multichannel functions are best tested independently of the FM reception.
However, in the case of a car radio that has multiple outputs and no way besides FM to input test signals, a multichannel audio analyzer that can simultaneously take measurements on all the outputs can be highly useful for both speeding up measurements as well as revealing any crosstalk or interactions that may occur between channels.
Making Measurements
With the hookups completed, the generator and transmitter at the right level, and the receiver tuned to the correct frequency, it’s time to make measurements. Common audio test measurements include level, total harmonic distortion plus noise, phase, crosstalk, signal-to-noise ratio, and frequency response. All frequency response measurements taken from an FM receiver will show a sharp high-end roll-off just before 19 kHz due to the FM multiplex filter (Figure 5). When making crosstalk/stereo separation measurements, double-check the transmitter specifications because some transmitters have limitations in this area that may skew the test results.
Significant crosstalk in the transmitter or receiver also can affect THD+N measurements. This effect can be seen by selectively driving one or both channels with audio.
About the Authors
Joe Begin is director of technical support at Audio Precision and a registered professional engineer in Oregon. He has 18 years experience as an engineer and seven years experience as a consultant providing acoustics and vibration engineering services. Mr. Begin has written many Technotes for the AP website and holds a B.S. and a master’s degree in engineering.
Adam Liberman is a technical writer who has been with Audio Precision since 2008. Mr. Liberman has worked as an audio engineer for 30 years with extensive experience in broadcast and theatrical sound.
Audio Precision, 5750 SW Arctic Dr., Beaverton, OR, 503-627-0832
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