GSM Noise Reducer
Chason Carroll
In today's world, mobile accessories are becoming much more
complex. A recent problem that has
plagued many audio accessories is that of GSM noise or "buzz." The buzz consists of a transmission signal
of 217 Hz with a duty cycle of 12.5%.
This frequency causes audible noise in the voice band (300 -3k Hz). This invention addresses two problems that
effect the audio signal originating in a cell phone and progressing to an
accessory: ground shifting and RF GSM buzz.
These problems originate from
the method the phone extracts power from its power source, i.e. battery, power
supply, accessory. The phone draws
current from the power source in order to transmit and receive audio. This current produces spikes (at 217Hz) that
are converted into noise by the audio accessory. The objective of this invention is to rid the system of the noise
spikes so that they are not heard by the end user.
The system in this invention can be divided into two parts
consisting of a differential amplifier followed by a 217 Hz notch filter (See
Figure 1). The joining of these two
elements produces a system that surpasses the effectiveness of either component
by itself. The differential amplifier
is designed to remove common signals at its input ports. Normally, the inherent offsets of the
components (i.e. voltage/current input offsets of op amps, resistor/capacitor
tolerances) in the system are the only non-ideal parameters of the device. This means that the differential amplifier's
non-ideal characteristics will have less than perfect effectiveness when
external signals are applied. These
imperfections present the problem that uncommon signals may be passed and
amplified, i.e. noise. The other part
of the system, the notch filter, is designed to remove a small band of
frequencies (or a single frequency if needed).
The notch filter can be designed to "notch" or remove a single
frequency. In this case, the optimal
frequency would be 217Hz. However, due
to the fact that the signal coming from the phone is referenced to the analog
(or low current) ground, the notch filter injects noise rather than eliminates
it. The reason for this is that the
notch filter is referenced to the digital ground (high current ground) of the
accessory. The digital ground contains
higher currents that increase unwanted noise spikes. If the notch filter' inputs are referenced to the digital ground,
the increased noise spikes will be injected and amplified. If the notch filter is referenced to the
analog ground, larger currents are injected onto the analog ground, which cause
the phone to inject and amplify the noise when transmitting. This problem exists because the signal
coming from the phone is referenced to the analog ground. When the two parts of the system are used in
conjunction, the effectiveness of the invention can be seen.
First, the differential amplifier is implemented. The purpose of the differential ampl...
