 occurs when the Two signature curves are shown in Figure 50.21, one for a amplitude of the delayed ray is larger than that of the main ray. In 45Mbit/s radio and one for a 19Mbit/s radio, both using the same this case a 'bump' occurs in the differential delay characteristic type of modulation. Note that the notch depth at the centre of the r.f. while the attenuation characteristic still has a notch in it. channel does not have to be as deep to degrade the 45Mbit/s radio as it does for the 19Mbit/s case. Also, the 45Mbit/s radio is affected by notches as far away as 15MHz while the 19Mbit/s radio is 50.2.2.2 Signature curves and DFM affected over a much narrower frequency range. A signature curve should be taken for both minimum and non To determine the effect of dispersive fading on a digital radio, a minimum phase fades. Only one is shown for each of the data rates multipath notch must be simulated. A test system like that of Figure in Figure 50.21 because the minimum and non minimum signatures 50.20 is often used for the simulation. The relative attenuation of the are nearly identical for the particular system measured. two paths, and therefore the notch depth, is set by the variable Originally the signature curve was used to directly calculate the attenuators. The phase, and therefore the notch location, is set with outage time due to selective fading. More recently an intermediate the variable phase shifter. The fixed delay determines the width of step has been added. First the signature curve is used to calculate the the notch and is normally set at 6.3ns. The 6.3ns delay is considered Dispersive Fade Margin (DFM), which is then used to estimate statistically representative of most multipath fades. outage time due to selective fading for the particular path under The measurement is performed by setting the centre frequency of consideration. the notch somewhere in the r.f. channel of the particular radio The concept of DFM allows one to calculate outage time due to system under test. The notch depth is adjus