 applied to a frequency modulator where the modulation index is set to 0.5 as shown in Figure 3.13. While simple, this method has the drawback that the modulation index must exactly equal 0.5. In practice this analogue method is not suitable because component tolerances drift and cannot be set exactly. A second method is more widely used. Here what is known as a quadrature modulator is used. The term quadrature means that the phase of a signal is in quadrature or 90 degrees to another one. The quadrature modulator uses one signal that is said to be in-phase and another that is in quadrature to this. In view of the in-phase and quadrature elements this type of modulator is often said to be an I-Q modulator. Using this type of modulator the modulation index can be maintained at exactly 0.5 without the need for any settings or adjustments. This makes it much easier to use, and capable of providing the required level of performance without the need for adjustments. For demodulation the technique can be used in reverse. A further advantage of GMSK is that it can be amplified by a non-linear amplifier and remain undistorted. This is because there are no elements Gaussian Voltage low-pass controlled input amplifier filter oscillator Modulation index = 0.5 Figure 3.13 Generating GMSK using a Gaussian filter and a frequency modulator with the modulation index set to 0.5 Modulation 67 Gaussian Local low-pass input oscillator filter Summation phase phase shift shift Mixer or multiplier Figure 3.14 A block diagram of a quadrature or I-Q modulator used to generate GMSK of the signal that are carried as amplitude variations. Furthermore the system is capable of providing a low level of bit errors, i.e. a low bit error rate (BER) under noisy conditions. Pulse modulation While amplitude and angle modulation, together with their many derivatives, form the vast majority of transmissions, another type of modulation, known as pulse modulation is used in many instances. The advantage of this type of modulation is that a number of dif