interference in the signal from the splitter 24 i e when the rssi exceeds the voltage on the line 78 the sample and hold circuit 80 samples the output of the voltage sweep generator 82 and holds the sampled voltage level thereby providing a fixed voltage to the summer 84 which in turn provides a fixed output voltage to the vco 64 because a fixed voltage is provided to the vco 64 the frequency output by the vco 64 does not change and the signal from the splitter 24 is no longer scanned but is frequency shifted so that the narrowband interference is moved to the if which is the notch frequency of the notch filter 66 additionally when the comparator 76 indicates that narrowband interference is present the switch 72 opens and the only path for the signal from the splitter 24 to take is the path through the mixers 62 68 and the notch filter 66 the threshold voltage on the line 78 may be hand tuned or may be generated by filtering some received signal strength either way the voltage on the line 78 should be set so that the comparator 76 does not indicate that interference is present when only a wideband signal such as the signal shown in fig3 is present but only indicates interference when a signal having narrowband interference is present for example the frequency spectrum 42 shown in fig4 shows three narrowband interferers 44 46 48 only one of the interferers would be needed for the.