What is an RF Mixer: Active and Passive?

It’s well known that an RF mixer is a component that multiplies two input signals. Recently Easybom conducted research into the RF Mixer, let’s see what they found out.

Definition and Application

An RF mixer can be called a “frequency converter”. Frequency mixing refers to the process of transforming a signal from one frequency to another, and its essence is the process of the linear shift of spectrum. It has two input ports, and two signals (in most cases with different frequencies) come in and produce an output signal, which is generated by multiplying the two input signals.  If you multiply two signals with different frequencies, RF mixer produces a composite signal with two frequencies, one is the sum of the two input frequencies, and the other is the difference between the two frequencies. 

In the superheterodyne receiver, the purpose of frequency mixing is to ensure that the receiver can obtain high sensitivity, enlarge a large number of and appropriate passband, plus work stably at the same time. The RF mixer circuit consists of three components: local oscillator, nonlinear device, and bandpass filter.

In terms of applications, RF mixers usually play a crucial role in frequency conversion, phase identification, variable attenuator, phase modulator, parametric mixer, quadrature phase-shift keying modulation, image suppression mixer, and SSB modulator.

Passive Mixers and Active Mixers

There are two types of RF mixers: passive RF mixers and active RF mixers, as Easybom concluded.

The Passive RF mixer is the most common type of RF Mixer. This kind of mixer has different design styles, such as single-ended, single-balanced, double-balanced and triple-balanced, and so on. This RF mixer is welcomed by most engineers for its fabulous performance, simple structure and cost-effectiveness in design. 

One of the outstanding characteristics of the passive mixer is that it needs no external DC power which means it’s quite simpler to function. Such RF mixers also have other commendable features, including wide bandwidth performance, good dynamic range, low noise figure (NF), and good isolation between ports. The design of this kind of RF mixer and its advantage of no external DC power supply make the noise figure of the output of the mixer very low.

Another common RF mixer is active RF mixer. There are two main types of active mixers: single-balanced and double-balanced (also known as Gilbert unit) mixers. The advantage of an active RF mixer is the built-in gain module of the LO port and RF output which enables it to provide a certain conversion gain for the output signal, and the input LO power requirement is low. 

The typical LO input power of active mixers is about 0 dB, which is much lower than that of most passive mixers. Active mixers often integrate LO multipliers to multiply the LO frequency to a higher frequency. This frequency multiplier is very beneficial to customers and can drive the mixer without high LO frequency. 

Active RF mixers usually have good isolation between ports. However, its disadvantage is that the noise figure is high and the linearity is low in most cases. The demand for input DC power supply affects the noise figure and linearity of the active mixer. 

In the test and measurement market, active mixers are mainly used as the third or last stage mixer of if subparts, or for low-end instruments (integration and cost-effectiveness add esign are more important than noise figure).

Conclusion

To sum up, Easybom holds the opinion that active RF Mixer and passive RF Mixer have their own merits and drawbacks hence we ought to choose with consideration in practice according to their characteristics.