Although customers expect better sound quality from digital phones, audio specs for digital phones are generally more lenient than those for analog ones. Specifications for CDMA handsets1 are silent on the topic of audio quality. GSM standards2 do contain requirements for frequency response, loudness, echo return loss, and distortion, but these requirements are not tied to RF performance. This approach checks for basic functionality of the microphone, amplifier, and ADC, but not for properties such as frequency response and sensitivity. Yet audio quality is far too important to ignore completely in manufacturing final test.
Acoustic problems can arise as a result of errors during the surface mount or assembly process.
The test loops a signal back between the microphone and speaker. GSM standards, for example, specify a loop from the microphone, through the ADC and DAC, and back to the speaker. Typically, you would transmit a command to put the phone in test mode, and then place the phone on an artificial head. The artificial head will then source sound into the mobile unit's microphone at a certain sound pressure, and it will measure the echo from the speaker. Additionally, data may be sent and received from electrical connections to the phone.