Abstract :
Group-delay equalisers operating at microwave frequencies may find application in multicharinel-telephony radio-relay systems, e.g. in systems without translation to an intermediate frequency in repeaters. Studies to ascertain the feasibility of such equalisers operating in the 4 Gc/s band are described. Several equalisers have been designed and constructed in standard waveguide WG11, using a pair of identical bandpass ladder networks terminating a waveguide hybrid or a 3dB directional coupler. In all cases except one, the equalising networks had delay characteristics that were arbitrarily assumed; the exception was a network designed to equalise the group-delay performance, over a bandwidth of 20 Mc/s, of a pair of bandpass filters in tandem. Basic equaliser insertion losses of 0.5¿2dB have been observed, the figure depending on the complexity of the network. The effect of temperature changes on the equaliser and filters in tandem resulted in the midband frequency of the equaliser shifting in step with the frequency of the filters, so that the group-delay performance remained equalised over a temperature range of 20degC. It is concluded that, although the performance of the equalisers is satisfactory, they are less flexible in operation and are mechanically bulky compared with intermediate-frequency equalisers.
