Car stereos get benched

Automotive stereos returned to a stereo manufacturer's service center for repair. Stereos include several models installed into numerous vehicles from several manufacturers.

Automate testing of stereos. Replace analog meters and stand-alone instruments with an integrated, computer-controlled system. Perform some 30 measurements on AM, FM, and weather-band (WB) carrier frequencies. Measurements include 3-dB limiting sensitivity, signal-to-noise ratio (SNR), total harmonic distortion (THD), and intermediate frequency (IF) rejection ratio. Eliminate manual setups of test equipment, automate measurements, and provide test reports.

• Mini-Circuits: passive low-pass filter. www.mini-circuits.com.
• National Instruments: PXI instrument chassis with embedded controller, PXI RF signal generator module, PXI dynamic signal analyzer module, PXI RF switch module, graphical programming language, test executive. www.ni.com.

Bloomy engineers' PXI-based system generates RF signals and measures audio outputs.

When a car stereo experiences trouble, a dealer's mechanic will replace it and send the defective unit to the manufacturer's service center. For years, technicians at one stereo manufacturer's service centers tested stereos manually, using an RF signal generator to create modulated signals for the stereo's antenna input. They used DMMs or analog dial meters to measure the audio output power. The technicians performed all the tests by hand, adjusting analog RF signal characteristics (including carrier frequency and power level), adjusting the meter ranges, and recording the measurements.

When the stereo manufacturer wanted to automate the tests, it turned to Bloomy Controls (www.bloomy.com) to develop a test system. Bloomy engineers developed a PXI-based system (figure 1) that generates RF signals and measures audio outputs. The signal analyzer measures audio signals in rms volts across a 4-ohm, 50-W resistor.

Figure 1.  A PXI chassis contains all the instruments needed to test automotive stereos.

Once the stereo stops scanning, the technician adjusts the stereo's volume control until the system measures 2 Vrms. The system then gradually drops the stereo's RF signal power until it detects a 3-dB drop in audio power. The RF signal power that causes the 3-dB decrease in audio output is the 3-dB limiting power.

While the limiting-sensitivity test requires a custom algorithm, others such as signal-to-noise ratio (SNR) and total harmonic distortion (THD) can take advantage of preprogrammed functions. The system also measures the stereo's IF rejection ratio, which produced some unexpected results.

During the IF rejection test, the system sets the RF signal's carrier frequency to the stereo's 10.7-MHz IF frequency with the stereo still tuned to 98.1 MHz. A discrepancy appeared between the audio response measured by the new system and that measured by the old system. RF signals from the analog and PXI generators contained noise spikes and elevated noise floors proportional to the IF signal power. Bloomy president Peter Blume added a 12-MHz low-pass filter (LPF) to reduce the noise level, but doing that required both the new and old test setups to produce more IF signal power to get an audio response from the stereo. Blume discovered that, all along, the stereo responded to noise instead of to the IF signal.

The filter produced a measurement about 20 dB higher than the customer expected, and the actual IF rejection ratio wasn't measurable using the traditional test procedure. Specifically, when the FM band noise was reduced, the stereo's audio output was unresponsive to IF signals above 100 dBµV, the maximum power for both signal generators. The new test system uses a power limit to evaluate the pass/fail test result instead of automatically performing a test procedure that would provide an invalid measurement.