INTERMODULATION DISTURBANCE DETECTING CIRCUIT
There are provided a frequency converting circuit 21 for inputting a broadband IF signal which includes a disturbing wave and carrying out a frequency conversion with an oscillating signal having a frequency of a desirable wave, and outputting a signal including a sum frequency component of a frequency component of a disturbing wave which is included in the IF signal and a frequency component of a desirable wave of the oscillating signal and a difference frequency component therebetween, and a low-pass filter 22 for attenuating the sum frequency component to output a signal of the difference frequency component, and a presence of an intermodulation disturbance is detected based on a frequency relationship between two difference frequency components output from the low-pass filter 22 . Consequently, it is possible to easily detect the intermodulation disturbance irrespective of a level of a received signal or a desirable wave included therein without carrying out a processing for amplitude modulating the received signal.
1 . An intermodulation disturbance detecting circuit comprising:
a frequency converting portion for inputting an intermediate frequency signal converted from a radio frequency received signal to carry out a frequency conversion with an oscillating signal having a frequency of a desirable wave;
a low-pass filter or a band-pass filter connected to an output stage of the frequency converting portion; and
a detecting portion for detecting a presence/absence of an intermodulation disturbance based on a frequency relationship of a signal output from the low-pass filter or the band-pass filter.
2 . The intermodulation disturbance detecting circuit according to claim 1 , wherein the frequency converting portion carries out the frequency conversion to output a signal including a sum frequency component of a frequency component of a disturbing wave which is included in the intermediate frequency signal and a frequency component of a desirable wave of the oscillating signal and a difference frequency component therebetween,
the low-pass filter or the band-pass filter attenuates the sum frequency component of the signal output from the frequency converting portion to output a signal of the difference frequency component, and
the detecting portion detects a presence/absence of the intermodulation disturbance based on a frequency relationship of the difference frequency component included in the signal output from the low-pass filter or the band-pass filter.