A mixer is a non-linear device which is used to perform the beating between frequencies. Usually, it is made of a diode ring (but also active devices can be used) biased by a Local Oscillator (LO). Due to the non-linearity, applying a signal at the RF port, the frequencies present at the output (IF port) will be:
1)
fIF = ±m*fRF ±n*fLO
where both m and n = 0, 1, 2...
Normally, the desired response corrensponds to values of m and n of 1..
For example, if we want to convert an RF signal at 1 GHz to an intermediate frequency of 120 MHz, the convertion can be realised:
In the above mentioned example, it must be noted that if at mixer of case (1) is applied an RF signal at 760 MHz, the beating fLO - fRF will also be converted to a frequency of 120 MHz, then falling in the bandwidth of the IF filter. In case (2), the same will happend for an RF at 1240 MHz for the product fRF - fLO.
The above mentioned frequencies are called image frequencies, and are always located at a distance 2*fIF from the intentional received frequency. They are specially important unwanted responses, having normal mixers, for these frequencies, the same insertion loss as on the wanted response. Is then very important to filter out these frequencies before the conversion.
Undesired responses are also generated by the harmonics of the RF and/or the LO frequencies (corrensponding to values of m and n different from 1 in the (1)), even if at lower level than the fundamental frequency.
When defining the conversion frequencies is then necessary to account for these unwanted responses in order to guarantee that RF frequencies which ganerates responses in the IF bandwidth are far enough from the intentional received RF to be properly filtered out. These considerations often led to the use of multiple conversions.
Last Updated Oct, 06, 02