TDMA Noise

TDMA (time-division multiple-access) time-slot sharing technique that results in high-power RF in the 800MHz to 900MHz or 1800MHz to 1900MHz bands.
The transmitter current, which can exceed 1A, pulses during a phone call at a repetition rate of 217Hz and pulse width of about 0.5ms.
If current pulses couple to the audio circuitry, the harmonic-rich 217Hz signal results in the audible buzz.


TDMA Noise can get from the RF circuitry to the audio circuitry through a variety of paths
• Radiation from the antenna to the audio or voltage supply components or to traces or components connected to them

• Conduction from RF components through traces to the audio components

• Conduction through ground to the audio subsystem

• Trace-to-trace coupling between lines or from a line to ground on the same or adjacent layers

• Coupling from line to component or component to component
 
Prevention methods include shielding, ground design, and careful overall layout practice.
 
Shield the audio section and its associated power-management


and baseband sections to isolate them from stray RF.

Shield the RF section to minimize the stray energy.

• Terminate the shield on a solid ground that is free of high dynamic currents.

• Isolate solid, largely unbroken audio ground on the layer below the audio sections from pulsing current.

• Do not allow traces onto same layer to bisect ground.

• Connect components to the ground layer through multiple vias.

• Do not route traces carrying power or audio signals parallel to those containing RF or large dynamic supply currents.

•Maximize the spacing between sensitive traces and potential sources of interference.

• For traces that must maintain perpendicular or (90°) design to minimize any noise coupling.

• Isolate audio traces on inner layers from non-audio traces by a ground trace with enough via holes to act as a Faraday shield.

• Do not place traces containing RF or dynamic DC currents directly under audio components

Some RF energy will couple onto audio traces regardless of  the effort to prevent this phenomenon from occurring. Utilizing bypass capacitors to ground to create single-pole lowpass filters will attenuate this energy from conducting into the audio amplifiers’ semiconductor junctions. Small value capacitors must be used to bypass RF energy and not affect audio signals. Since GSM phone bands approximately inhabit
the 900MHz and 1800MHz range, the best capacitors are those that are self-resonant at the aforementioned frequencies; typical capacitors of 10pF to 39pF have negligible effect on audio signals. Use each capacitor to shunt RF energy operating at each audio amplifier input, output, or power pin that is sensitive to RF energy. For further isolation, add an inductor (or ferrite bead; the ferrite bead is a combined inductor and resistor) to form a two-pole low-pass filter, placing the components as close to the amplifier outputs as physically possible