Jun 02, 2008

Keeping the best audio quality in mobile phone by managing voltage drops created by 217 Hz transients

Source of major noise

Whatever the protocol used by a mobile phone, GSM or TDMA, RF transmitter switching creates the most notorious noise for the power supply. The RF power amplifier switches on and off at a rate of 217 Hz. At each of these events, a high current (up to 1.7A) is drawn from the power supply, creating a sudden voltage drop on the battery equivalent serial resistance (ESR) reaching up to 500 mV.

For a System-on-Chip (SoC) embedding high-resolution audio converters with audio amplifiers, or for a high sensitivity MEMS, such an amplitude jeopardizes the overall performance of the SoC(s). More specifically, the audio quality may be deeply altered by audible buzz sounds.

Such a noise is particularly audible as it is not random. Indeed, noises with amplitudes as low as 10 μV can be heard if they occur at a fixed rate of recurrence. They could be even much more disturbing than a random noise of higher amplitude, which will be considered as a background noise.

The best choice for preventing GSM noise from degrading audio quality is to place a Linear Regulator (LR) with very Low Drop-Out (LDO) and Output Noise to supply directly the audio amplifier from the Lithium-Ion battery. Such LDO linear regulator is then used as a clean-up or filtering module for the amplifier power supply.

The usual key criterion to select the best LR to prevent GSM noise is the capability of the LR to reject noise from the input voltage, expressed as Power Supply Rejection Ratio. But PSRR figures should also be pondered with the LR transient responses and drop-out characteristics…

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