Image frequency suppression measurement

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Image frequency suppression measurement [Copy link]

It is a specification for the receiver portion of an RF transceiver. For example, we need to measure the image frequency rejection of a C-band downconverter operating at an input frequency of 3700-4200 MHz and an output frequency of 52-88 MHz. We need to find two input frequencies, f1 and f2, at the receiver or downconverter that will produce the same frequency as the output (say 70 MHz). In this case, the two frequencies are called images of each other. The frequency 5785 MHz is the image frequency of 3700 MHz. This is derived from Fimage = 3700 + 2*(1042.5) = 5785 MHz. An IF of 1042.5 MHz is used here, as per the heterodyne downconverter diagram shown below.

Fimage=Fs+2*IFTo
measure the rejection, we need to enter f1 (equal to 3700 MHz) and note the output frequency power level (say P1). Then we need to enter f2 (equal to 5785 MHz image frequency) and note the power level (say P2). The difference between these two power levels is called the image frequency rejection. It is usually specified in dBc.

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