Image Frequency

The image frequency or image response typically occurs in receivers that are based on the superheterodyne method. In this method, the wanted frequency or band is mixed with a Local Oscillator (LO) frequency and results in an intermediate frequency (IF). The mixing of these two frequencies results in a sum and a difference component at the output of the mixer processing. One of the output signals is required and processed through A/D conversion, while the other signal is unwanted and suppressed. It is not possible to distinguish between a positive or a negative frequency. Therefore, the output can be assumed to be absolute values. For more details on the superheterodyne receiver techniques, refer to the MICRF001 Theory of Operation Application Note.

The transceiver/receiver generates an LO frequency according to the following equation:

fLO=fRF×fLO_IF1+fLO_IF
Table 1. fLO, fRF Ratio
fRF(MHz) fLO_IF
310-318 1225
418-477 1728
836-954 3457

The intermediate frequency can be calculated using the following equation:

fIF=fRFDIV_IF×6

Table 2. fIF Calculation Factor
fRF DIV_IF
310 MHz to 318 MHz 204
418 MHz to 440 MHz 288
441 MHz to 477 MHz 306
836 MHz to 870 MHz 576
871 MHz to 956 MHz 612

The IF frequency range is from 237 kHz to 260 kHz.

As the LO frequency is used in the transceiver/receiver to get the IF, apply the two equations to get the intermediate frequency (fIF) and the local oscillator frequency (fLO). Consider the following scenarios as the mixing of frequencies will result in a sum and a difference component.
The following figure illustrates how the mixing of frequencies will result in the target IF:

Two different frequencies can be mixed down to the target IF with the given LO. To avoid unwanted reception, suppression of the unwanted frequency is implemented within the device. However, due to the small distance between the two frequencies, only limited suppression is possible. The channel spacing must be selected carefully, specifically for multi-band applications, otherwise the unwanted frequency will result in inverted message content.