Noise, TDMA Noise and Its Suppression Technology

Noise and TDMA Noise

The term "noise" is often used broadly to describe unwanted electrical signals that distort the purity of a desired signal. Some types of noise cannot be avoided (such as actual fluctuations in the amplitude of the signal being measured) and can only be overcome through signal averaging and bandwidth reduction techniques. Other types of noise (such as radio frequency interference and "ground loops") can be reduced or eliminated through various techniques, including filtering techniques and careful wiring and component placement. Finally, there is noise that arises from the signal amplification process and can be attenuated through low-noise amplifier design techniques. Although noise reduction techniques are effective, it is always desirable to start with a system that is immune to noise interference and has the lowest possible amplifier noise. The following is a brief summary of the various types of noise that affect electronic circuits.

Thermal noise (or Johnson noise or white noise) is directly related to the temperature due to the thermal agitation of electrons in a resistor. In the case of a speaker or microphone, the noise source is the thermal motion of air molecules.

Shot noise is caused by random fluctuations in the number of charged carriers emitted from the surface or diffused from the junction. This noise is always associated with DC current and has nothing to do with temperature. It mainly exists in bipolar transistors.

Flicker noise (or 1/f noise or pink noise) is mainly caused by traps associated with silicon surface contamination and lattice defects. These traps randomly capture and release carriers with a process-dependent time constant, generating a noise signal with energy concentrated at low frequencies.

Popcorn noise is caused by the presence of heavy metal ion contamination and is found in some integrated circuits and discrete resistors. In some bipolar integrated circuits, popcorn noise is caused by too much doping in the emitter region. Reducing the doping level may completely eliminate popcorn noise. This is another type of low-frequency noise.

Avalanche noise is a type of noise generated by the Zener phenomenon or avalanche breakdown phenomenon in a pn junction. When avalanche breakdown occurs, holes and electrons in the depletion layer of a reverse-biased pn junction collide with silicon atoms to obtain enough energy to generate hole-electron pairs.

TDMA noise ("hum") originates from a 217 Hz frequency waveform generated in GSM cellular phones, which produces audible noise when it couples into the audio path and passes to the speaker, earpiece or microphone. A detailed description of this type of noise is given below.

This application note will clearly address the TDMA noise challenges that customers encounter when driving a single-channel speaker in a GSM cell phone design. Before delving into how to minimize this noise, a background description of bridge-tied load (BTL) single-channel amplifier operation will be reviewed. In the following application diagram, all resistors have equal R values ​​(Figure 1).

Figure 1. Single-channel amplifier with bridge-tied load.

In this configuration (Figure 1), an input signal, VIN, is applied to the inverting input of amplifier A1 and amplified with a gain of 0dB. The output of A1 is connected to one side of the speaker and to the inverting input of amplifier A2, also amplified with a gain of 0dB. The output of A2 is connected to the other side of the speaker. Because the output of A2 is 180 degrees out of phase with the output of A1, the resulting difference, VOUT, between A1 and A2 is twice the amplitude of the output of a single amplifier. When given a sinusoidal input signal, the BTL configuration effectively doubles the output voltage compared to a single-ended amplifier, resulting in a four-fold increase in output power at the same load (Figure 2).

Figure 2 Output voltage for bridge-tied load
[page] As GSM cell phone manufacturers discovered, BTL single-channel architectures are susceptible to radio frequency interference (RFI). This interfering signal couples directly into the audio path, distorting the desired waveform and audibly creating a "hum" known as TDMA noise. GSM cell phones use TDMA (time division multiple access) time-slot sharing to generate high-power RF signals from 800MHz to 900MHz or 1800MHz to 1900MHz. Transmitted currents can exceed 1A, with a pulse repetition rate of 217Hz and a pulse width of approximately 0.5ms during a call. If the current pulses couple into the audio circuit, the large 217Hz harmonics can create an audible "hum".

What causes the audible "hum"? Energy in the audio range, including the 217-Hz TDMA repetition pulse rate and its harmonics, exists in the channel in two ways: as current variations in the DC power supply, and in the modulation envelope of the RF signal. The large current drawn by the RF power amplifier during the transmit gap and the smaller current drawn by the RF circuit during the receive gap form the DC power supply current pulse waveform (Figure 3).

Figure 3 Periodic transmission and reception current pulse waveform

The two main mechanisms for coupling current waveforms into the audio circuits are power supply ripple current and ground line ripple current, both of which exist at a frequency of 217 Hz. In addition, a portion of the transmitted RF energy will also be coupled into the audio circuits.

Figure 4 RF energy coupled into audio circuits

The potential for RF energy to couple into the audio circuitry is most likely to occur when there are long traces connecting the amplifier output to the speaker, where the traces act like antennas. Good layout should prevent RF energy from coupling into the audio and power traces that connect the baseband section or audio circuitry in a phone. These subsystems must be designed to block or shunt the RF signal to ground so that it does not reach the nodes of the semiconductor active audio devices. RF energy can be transferred from the RF circuitry to the audio circuitry through different paths:

* Radiated from the antenna to audio or power devices, or the traces or devices connecting them.

* Conduction from RF devices to audio devices through traces.

* Conduction via ground to the audio subsystem.

* Line-to-line coupling between row lines, or coupling from row lines to the ground terminal of the same layer or adjacent layers.

* Coupling from line to device or device to device.
Prevention methods include shielding, ground design and careful overall layout practices. Some prevention methods are as follows:

* Shield the audio section and the associated power management and baseband sections to isolate stray RF signals. Shield the RF section to minimize stray energy.

* Connect the shield to ground to allow large dynamic currents to flow in unhindered.

* Isolate the large continuous audio ground below the audio circuit section from the pulse current.

* Do not allow traces on the same layer to separate ground lines.

* Connect the device to the ground plane through multiple vias.

* Do not place traces carrying power or audio signals in parallel with traces containing RF signals or large dynamic power currents. Maximize the spacing between sensitive traces and potential sources of interference.

* For trace designs that must remain perpendicular or (90''), minimize noise coupling.

* Create a Faraday shield with a ground plane that contains enough vias to isolate audio traces from non-audio traces on inner layers.

* Do not place traces containing RF signals or dynamic DC currents directly under audio devices.

[page] Place audio feedback and signal path components as close to the audio amplifier as possible to isolate the components from RF energy sources.

Despite all precautions, some RF energy will still couple onto the audio traces. A single-pole, low-pass filter formed by a bypass capacitor to ground further attenuates the RF energy conducted to the semiconductor junctions of the audio amplifier. The RF energy must be bypassed with a small capacitor so that it does not affect the audio signal. Because the frequency band of GSM cellular phones ranges from approximately 900MHz to 1800MHz, the best capacitor is one that resonates at these frequencies; typical capacitor values ​​of 10pF to 39pF have negligible effect on the audio signal. The RF energy generated should be bypassed with a separate capacitor at each audio amplifier input, output, or power pin that is sensitive to RF energy. If further isolation is required, an inductor (or ferrite bead; a ferrite bead is a combination of an inductor and a resistor) should be added to form a two-pole, low-pass filter, and the device should be placed as close to the amplifier output as possible. Figure 5 shows a practical application of the LM4845 single-channel output. By implementing a two-pole low-pass filter with a -3dB cutoff frequency of 1MHz, customers can experience the audio buzz of a single-channel speaker, which is far beyond the audio range but far below the frequency band of GSM frequencies. The audio buzz is attenuated by 30dB, which is an acceptable level for hearing.

Figure 5: External two-pole low-pass filter at the output of the isolation amplifier

[page] While GSM cell phone manufacturers experienced difficulties with TDMA noise when using the LM4845, other customers did not. After troubleshooting and troubleshooting customer circuits, it was determined that poor component placement and poor circuit wiring were the primary causes of audio buzzing. To help system designers minimize noise sensitivity, the LM4845 was redesigned to a differential single-ended input circuit with a proprietary RF suppression circuit at the amplifier output. This improved device is the LM4946. Figure 6 shows a comparison of the LM4845 and LM4946 under the same circumstances. Without the RF suppression circuit, RF energy can propagate to the LM4845 and couple into the audio path through the repetition of the 217Hz TDMA pulse carried on the RF modulation envelope. Despite the presence of the same 217Hz TDMA repetitive pulse in the LM4946, the RF suppression circuit can increase the attenuation of the RF energy from 20dB to 30dB. Figure 6 also shows the modulation envelope that is fully attenuated in the LM4946.

Figure 6 Measured TDMA noise

Currently, only the LM4884 and LM4946 include proprietary RF suppression circuitry, and more advanced products that utilize this technology are under development.

Conclusion

As the old saying goes, "prevention is better than cure"; we can apply the same philosophy to the design of GSM cell phones. Attempting to suppress TDMA noise after the design is complete is expensive, time consuming and unproductive, so good prevention techniques should precede the actual circuit layout; device positioning, power trace location, ground location, shielding and many of the previously listed prevention techniques. The LM4946, LM4884 and future products with RF suppression techniques can adequately minimize TDMA noise. There is currently no single solution to prevent TDMA noise.

Note: In this application note, the terms “TDMA noise,” “RF energy,” “audio buzz,” and “hum” are used interchangeably.