A radio frequency (RF) mixer is generally a three-port radio frequency component that is used to change the frequency of one of the input signals. In a radio transmitter, an RF mixer may also be referred to as an upconverter. When used in a radio receiver, an RF mixer may also be referred to as a downconverter.
An RF mixer may be an active component or a passive component. To achieve a small scale size, an RF mixer typically uses an active component formed of transistors receiving a power supply so that it may be integrated into integrated circuits with other radio frequency components and devices.
Referring now to background FIG. 1, a schematic symbol for an RF mixer 100 is illustrated. The mixer 100 has two inputs ports LO, IF/RF and one output port RF/IF. If being used as an upconverter, the input ports are local oscillating input port LO and intermediate frequency input port IF and the output port is radio frequency output port RF. If the mixer is being used as a down converter, the input ports are a local oscillating input port LO and a radio frequency input port RF, and the output port is an intermediate frequency output port IF. The LO port receives a local oscillating signal from an oscillating signal source.
The purpose of a mixer is to change the frequency of a signal while hopefully keeping everything else about the signal the same. In FIG. 1, a first signal is coupled into the IF/RF port of the mixer 100 at particular frequency f1. A carrier signal is coupled into the LO port of the mixer 100 at a second frequency (f2). Two different output signals are formed at the RF/IF output port of the mixer 100 that may be selectively used. For upconversion to a higher frequency output signal, the in-phase output signal with a frequency equal to the sum of the two input frequencies (f1+f2) is selected. For downconversion to a lower frequency output signal, the output signal with a frequency equal to the difference between the two input frequencies (f1−f2) is selected.
For example, sound waves of voice are in a low frequency range of 20 to 20,000 hertz. On the other hand, carrier frequencies of cellular communications systems are in much higher frequency bands, such as 900,000,000 hertz. To talk on a cellular phone, for example, the voice frequency needs to be upconverted to the cellular carrier frequency used in cellular communications. One or more mixers are used to change the frequency band or range of human voice to the frequency band of the cellular carrier frequency.
One important characteristic of a mixer is conversion gain. Conversion gain is the ratio of the amplitude of the output signal to the amplitude of the input signal (not the local oscillating LO signal). Conversion gain may be expressed as a power ratio. If the conversion gain is less than one, a fraction, there is actually a loss through the mixer.
Another important characteristic of a mixer is its noise figure (NF). The noise figure for a mixer is determined by dividing the signal-to-noise ratio (SNR) at the input port (not the local oscillating LO input port) by the signal-to-noise ratio (SNR) at the output port of the mixer and converting the ratio into decibels.
Thus, a mixer can be improved by increasing the conversion gain and reducing the noise figure. By increasing the conversion gain and reducing the noise figure in a mixer, the requirements for other RF components may be more relaxed leading to simpler designs using less integrated circuit die area and possibly power conserving designs with the amplification of less noise.