In communication systems the signal transmission is at Radio Frequency(RF), which need to be translated to Intermediate Frequency(IF) to make signal processing easier. In transmitter the modulated base-band signal is translated to RF frequency whereas in receivers RF signals are converted to baseband frequency. Mixer, a non-linear device, does the job of frequency translation of signals from one frequency to another frequency in Transmitters and Receivers. Functionally it is equivalent to a multiplier.
Consider two input signals and to the mixer then the output is given as,
Out of the sum and difference frequency components, in Receiver application (where the frequency need to be down-converted) the difference components are utilized, whereas in Transmit application (where the frequency need to be up-converted) the sum components are utilized. The un-utilized other component, in either of the applications, which is to be rejected is called image component and can be filtered using bandpass filter generally.
Mixer is represented by the symbol shown in Figure. It consists of two input and one output ports, namely RF, IF and LO ports. LO port is always input port. In up-conversion application IF port is input port and RF port is output port. Whereas in down-conversion application it is vice-versa, i.e., RF port is input port and IF port is output port.
In down-conversion applications, if LO frequency is greater than input frequency(RF) it is called High side injection, whereas if LO frequency is less than input frequency(RF) it is called Low side injection.
High side injection :
Low side injection :
Choice of LO injection side, whether high or low, is often dictated by the spurs and the desired RF and IF band of operation.
Mixers are characterized by the following performance specifications, and here they are defined for down-conversion mixers.
- Conversion Gain
- Noise Figure
- Gain Compression or 1dB Compression Point
- Third-Order Intermodulation Distortion
- Port Isolation
- Port Return Loss
- Supply voltage
Usually mixers employ one of the following techniques for their implementation:
- Using LO Switching or Multiplier
- Using non-linear characteristic of the active element. Eg: diode, transistor, etc.,
- Sampling the RF signal with S/H circuit
Following are the various mixer architectures using either of the principle.
- Diode Mixer
- BJT/FET Mixer
- Single balanced Mixer
- Double balanced Mixer
- Double balanced Passive Mixer
- Transconductance Mixers
- Potentiometric Mixers
In switching mixers LO provides periodic hard limited switching waveform to mixer rather than sinusoidal. These harmonics in LO cause additional signal energy to be converted into unused spurious component called spurs unless suppressed by mixer gain at high frequencies.