al generators available at present are not completely satisfac- Phase noise 1°rms <4°pk measured in 1 - tory. The few signal generators on the market which have fast 100kHz band enough switching times are unattractive when their price is taken into consideration. Using this analysis as a basis, a short list of Spurious signals <-50dBc required features for a signal generator which would be suitable for Fading Internal or external GSM applications can be compiled, as in Table 61.13. The principle of a typical GSM signal generator, such as that used As the I/Q modulator itself is still analogue, this circuit largely in the Rohde and Schwarz GSM test set CMTA94, is shown in determines the accuracy of the signal generator. In addition to the Figure 61.25. Digital signals are modulated by digital signal pro- usual quality criteria for signal generators, three more must now be cessing into their in phase (1) and quadrature phase (Q) components. considered as in Figure 61.27. These digital signals are then converted to analogue and modulated If excessive imbalances are present they can be clearly seen in the into the 900MHz frequency band using a frequency hopping local generator output spectrum and may be detected as errors in the oscillator. The data is then transmitted in bursts of r.f. power with receiver under test. the amplitude profile of the bursts being shaped as specified by GSM. A fading simulator may be added at the output stage. 61.10.1.2 Simulation of radio channel Fast digital signal processors are used to condition the (analogue) I and Q modulation signals. These processors replace classic 'anal- The simulation of multipath reception (fading simulation) is becom- ogue' functions such as filtering or integration by equivalent digital ing more and more important for digital radio systems. Fading processes which increase precision and stability, as a function of simulators for GSM tests must be capable of electronically simulat- time and temperature (Figure 61.26). ing the propagation conditions over variou