of yig filter 11 will be increased significantly from about 1 to about 50 ohms and most of the signals from step recovery diode 18 will be reflected back into the yig sphere in addition as a secondary effect it will appear as if there is more series resistance between step recovery diode 18 and ground reducing the current flow through and therefore the generation of harmonics by the diode thus by simply changing the bias on pin diode 26 the signal appearing on output terminal 24 can be switched from a harmonic of high frequency swept source 20 to the output of low frequency swept source 28 however at some frequencies the isolation between the high and low frequency swept sources may not be as high as would be desired this isolation can be increased by reducing or shutting off the signal from one of the sources while the signal from the other is being passed through yig filter 11 for example when pin diode 26 is forward biased the signal from low frequency swept source 28 can be reduced in amplitude by several orders of magnitude through the use of modulator 5 typically such a modulator uses one or more pin diodes connected in shunt across the output of fixed oscillator 3 when those pin diodes are forward biased the output of fixed oscillator 3 will be effectively shut off in addition amplifier 9 or leveler 13 can be shut off to reduce the amplitude of the low frequency swept signal likewise.