GSM handsets use high-power amplifiers to achieve maximum communication distance to the base station. These amplifiers consume large currents bursting at relatively low frequency rates that can interfere with the typical operation of a handset, resulting in what the human ear commonly hears as an objectionable repeated humming noise.

This phenomenon can often be heard as a handset is placed in close proximity to a speaker phone or other electronic device.

Problem description

How often have you been in your company’s conference room and placed your handset near a speakerphone and have later heard heard an intrusive, repeated humming sound – sometimes even when the handset is turned on but not being used? Move the phone further away from the speakerphone and the problem is reduced.

A similar situation can be observed when placing a GSM handset near an AM or even FM radio. This unpleasant repeated humming sound is often referred to by handset design engineers as “GSM buzz”.

The problem can occur within a handset completely by itself, especially if the RF power amplifier chip is at its maximum output level.

Handsets contain efficient batteries that must provide on-demand power to the RF power amplifier chip.

Power amplifier chips deliver large amounts of RF energy within the 850MHz to 950MHz or 1800MHz to 1900MHz GSM bands.

The +33dBm GSM power is delivered within these bands at 217Hz modulated bursting intervals. Figure 1 (above right) illustrates the burst content within the audio range typical in a GSM phone.

Creating audio buzz

The 217Hz bursting results in large power excursions from the handset battery. These excursions are typically one to two amperes, depending on the efficiency of the power amplifier.

Because of these large current transients, slight amounts of resistance and inductance result in 217Hz voltage transients throughout the entire cell phone. Figure 2 (above) illustrates a typical voltage transient resulting from the 217 RF power transients. It illustrates the power at an audio chip.

It is interesting to note that both the power supply rail and the ground reference have shifted. This suggests there is impedance in each of the nodes. The 217 in effect square-wave envelope may also contain harmonics that exist well into the audio band.

This parasitic resistance and inductance can be identified in numerous places within a handset. These are often found in within the battery itself, within PCB inter layer vias, traces resistance and even connectors.



Bypass and decoupling

The first, most logical, solution is to implement both bypass and decoupling capacitors adjacent to the power amplifier chip, and then also to the audio components themselves supporting audio paths. It is good practice to focus first on the power amplifier and then retest after these components have been added.

Because of the large currents, the size of the decoupling capacitor adjacent to the RF power chip often needs to be very large. Size and cost limitations mean the size of this capacitor must be reduced, resulting in greater-than-normal voltage excursions.

Bypass capacitors (0.01µF typical) should also be used because the edge rates of the power transients can well exceed the bandwidth of a decoupling capacitor. These capacitors must be placed close to the PA chip.

A large decoupling cap may be placed in parallel with the battery to reduce the transients from the internal resistance of the battery. However, the drops associated with the PCB and flex circuit still exist, and the possibility for GSM buzz remains.

Power amplifier chip

The PA chip must have large, low inductance and low-resistance ground and power paths. This includes connectors, flex circuits, printed circuit boards and solder pads.

The routing must be as direct as possible, with minimum excursions. Ideally, both power and ground are designed as planes through to the PA device.

However, this is often not the case because of space limitations. It is also important to not run a power path to the PA chip directly over audio paths. Modulated power rails can easily couple into audio circuits and be amplified.

Via utilisation

Poor power and grounding through vias remain the biggest contributor to difficult GSM buzz problems. If possible, route traces through PCBs, flex circuits and connects to minimise via utilisation. If vias must be used, it is good practice to double up or use quadruple vias all in parallel, and ensure that both power and ground paths contain adequate protection.

High resistance points

More expensive multi-meters precisely measure resistance using four-wire measurement. This technique uses four wires and normally four terminals of the meter, called source and sense, to effectively remove the resistance of the meter leads, leaving only the measured resistance.

Voltage drops

Knowing that the transient currents can be 1-2A, a resistance of only 50mohm can result in a voltage drop of 100mV in each path, both ground and power. It is important to recognise that sometimes the drop may be in a power path, but not a ground path.