Commercial automotive AM/ FM radios must perform dependably under a variety of operating conditions. Because designers of these radios employ digital-signal-processing (DSP) algorithms to overcome the effects of reflections, signal multipath, and fading, they often spend weeks in the field analyzing the effects of different signal conditions. A more practical and less time-consuming solution is the use of actual recorded radio signals to simulate the conditions faced by an automotive AM/FM radio design in the field.

A particularly challenging test of AM/FM radio performance is an evaluation of adjacent- and/or alternate-channel performance in which the radio must receive a moderately weak RF signal in the presence of a strong adjacent RF signal. To evaluate the subjective reception quality between radios, OEM car radio manufacturers often apply test plans based on specific in-field routes at different locations. When driving in cities, for example, it is common to have an RF environment called “urban canyon” where large buildings will create a complex pattern of multipath and shadowing specific for FM stations. Channel simulators are often used to recreate typical multipath models for a specific channel. The approach is no longer practical, however, when multiple simulators are needed to simulate adjacent interferers— one of the reasons that the automotive industry still performs radio optimization using field test drives.

The test drive approach presents major repeatability problems, however. Since propagation conditions change due to weather conditions and the proximity of large vehicles, the results are never the same from one test drive to the next. Also, since test drives are performed over a wide range of locations, test repeatability is critical for evaluating an AM/FM radio design for use in different areas. Fortunately, due to advances in RF signal recording and storage, recording of actual FM broadcast signals offers a viable alternative to traditional test drives for evaluating commercial AM/ FM radio designs.

For example, the RF Record & Playback System from Averna can capture the full FM band (20-MHz bandwidth) with 14-b resolution. Based on a PXI hardware architecture from National Instruments, the digital recorder can also make parallel recordings of the GPS location, the radio’s audio, and a video of the drive test from an onboard camera. The system has an 80-dB spurious-free dynamic range (SFDR) that may seem large, but FM receivers can handle a much wider range of signal levels from -2 to over +110 dBuV (-109 to +3 dBm).

The Ann Arbor area of Detroit represents a “hard-to-reproduce,” extreme-dynamic-range test case, which can be used to evaluate a radio’s capability to mitigate the effects of very strong adjacent interference. In the test drive, a local 3-kW transmitter at 107.1 MHz interferes with a signal from Detroit at 106.7 MHz. During the test drive, the interferer will range from 65 to 95 dBV while the desired channel will vary from 25 to 50 dBV. Besides multipath fading, there is a shadowing effect from a tall building that at times blocks line-ofsight reception. This type of fading is not well represented by models found in RF signal generators and is better suited for record-and-playback signal simulation.

Figure 1 shows this test drive superimposed on a map, with the 3-kW transmitter located on top of a tall building. While the antenna is in direct line of sight, the interferer is at its peak level and the reception of the desired frequency at 106.7 MHz is seriously degraded. Figure 2 shows a typical frequency spectrum of the signal strength while Fig. 3 shows the strength of both signals over the duration of the test drive.

Although the dynamic-range specification of the RF Record & Playback System is 80 dB, practical recording has shown the usable dynamic range to be around 60 to 65 dB. This reduction can be explained by digitizer saturation and the peak-to-average ratio (PAR) of multicarrier signals. To avoid possible saturation of the digitizer (signal clipping), recorded data must be at least 5 dB below saturation. For multicarrier signals, such as FMband or COFDM signals in an urban environment, the recorded signal will commonly have a 10 to 15 dB PAR due to vector addition of multiple RF signals present within the 20-MHz passband of the RF signal chain. The combination of the two factors leaves 60-to-65 dB of usable dynamic range for good-quality recording of multiple carriers.

Despite this limitation, the RF Record & Playback system has proven to be very effective in capturing multipath and weak signals if strong interferers (greater than 40 dB) are not present in the band of interest. The system employs a low-noise amplifier with better than 2-dB noise figure to capture weak signals in rural areas with no noticeable degradation, taking into account impedance mismatches. The RF Record & Playback system is designed for a 50-Ohm impedance while an automotive FM antenna and/or a radio input are traditionally matched to 75 Ohms for FM and to a high-impedance (above 1.5 kOhms) for AM.

Figure 4 presents a block diagram of the proposed solution that uses the Universal Receiver Tester (URT) to replicate a test condition such as the radio environment found in Ann Arbor; a weak signal is received in the presence of a strong interferer and combined with an impediment such as multipath effects. The first generator (Gen 1) provides the strong interferer. Since the noise floor of this source is high at the frequency of the weak signal, a notch filter is required to suppress the noise before combining it with the weak signal from a second generator (Gen 2) to simulate the desired signal. The notch filter, a three-pole cavity filter that can be tuned across a 10-MHz range, has at least 50-dB rejection (Fig. 5).

Gen 2 is configured with a Dynamic Range Extender (DRX), which is a programmable attenuator. By providing attenuation of the desired signal, the noise floor is also attenuated. Also, the effect of multi-path fading can be imposed on the weak signal since the DRX response time is sufficient to react to the fading profile (nominally, 40 times per second). For the purpose of the test, the radio is mounted within a shielded enclosure to suppress potential interference from external signals.

With the hardware configuration and the performance as described, an accurate representation of the RF signal condition must be provided from the output of the generators. The strong interferer is provided through Gen 1 from an on-site RF recording of the FM band. The Universal Receiver Tester (URT) RF recorder was used to capture the signal on a test drive during typical weather conditions. The weak signal is generated through Gen 2 by using the FM Fading Simulator, a .wav file format music track, and playback of the power profile derived during a field recording of the weak signal.

Multipath fading is a common type of signal impairment on mobile signal reception. In the case of FM signals, it will cause deep dropouts of the signal, to several times per second, depending on the speed of the vehicle. The FM Fading Simulator applies a flat fading response to the test signal compared to the more comprehensive effects of a standard channel simulator. However, this approximation of multipath is normally sufficient for testing FM tuner performance.

The FM Fading Simulator allows test tones to be generated as a standard AM/FM generator, or a given audio track in the form of a .wav file to be modulated. Additionally, the envelope of the generated signal can be modulated through playback of a tab-delimited text file. The power pattern envelop can be extracted through simple signal processing of any prerecorded FM signal by use of a simple utility within the playback toolkit.

Figure 6 illustrates the basic functional aspects of the test setup. The capability to play back any given audio signal or track, with the impairment of multipath flat fading on the desired signal, provides an additional degree of freedom not possible during field test. This is important considering that some DSP algorithms are designed to react differently, according to the type of audio program.

For the sake of an example analysis, assume that the desired weak signal is at a level of 30 dBµV and a strong interferer exists at 110 dBµV, 400 kHz away from the desired signal. This would be one of the most stressful tests that can be applied to an FM radio, and typical of what one would encounter in the worst part of the Ann Arbor test drive (Fig. 7). With these experimental assumptions, it is now possible to calculate the effective dynamic range of the system. With an in-field ADC recording dynamic range of about 65 dB and FM cavity notch filter with about 50 dB suppression, the test system dynamic range is about 115 dB at the frequency of interest.

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