GSM Noise Reducer

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...