Basic Knowledge of Various Types of Mixers

A mixer, as the name suggests, combines two input signals to generate their frequency sum or difference. It is known as up-conversion when a mixer is used to produce an output frequency greater than the input signal (the two frequencies are added), and it is known as down-conversion when a mixer is used to produce an output frequency lower than the input signal.

Ⅰ. Single/Double/Triple Balanced Passive Mixer

In general, passive mixers are noted for their simplicity and lack of need for additional setup or external DC power. Wide bandwidth performance, strong dynamic range, low noise figure (NF), and good port isolation are further noteworthy characteristics of this type of mixer. Due to the design of this type of mixer and its benefit of not requiring an external DC power supply, the mixer output has a very low noise figure. A good generalization is that a passive mixer's noise figure should be equal to its conversion loss. These mixers are perfect for situations that require a low noise figure but cannot be satisfied by active mixers. This type of mixer also performs exceptionally well in systems with high frequencies and broad bandwidths. From RF to mmWave frequencies, they offer good performance. The isolation between the various ports is another crucial feature of a mixer. This feature frequently dictates which mixer can be utilized for a specific application. Double-balanced passive mixers have strong port-to-port isolation and a simpler architecture than triple-balanced passive mixers, which often have the best isolation performance but a complex architecture and perhaps insufficient other qualities (such as linearity, etc.). Double-balanced mixers offer the finest isolation, linearity, and noise figure for the majority of applications.

In terms of the entire signal chain, linearity, which is frequently quantified by the third-order intercept point IIP3, is one of the most crucial properties of RF and microwave devices. Generally speaking, passive mixers are noted for their strong linearity performance. Unfortunately, significant LO input power is necessary for passive mixers to work at their best. It takes between 13 and 20 dBm of LO drive, which is relatively strong for some applications, to operate the majority of passive mixers, which use diodes or FET transistors. Passive mixers' greatest flaw is their high LO drive needs. The conversion loss at the mixer output is another flaw in passive mixers. Since these mixers are passive components without gain blocks, the output of the mixer frequently experiences significant signal loss. For instance, if the mixer's conversion loss is 9 dB and its input power is 0 dBm, the mixer's output will be -9 dBm. Overall, these mixers are excellent for the military and test measurement applications.

Advantages of Passive Mixers

wide bandwidth

high dynamic range

low noise figure

High port-to-port isolation

Figure 1. I/Q Mixer Block Diagram and Image Rejection Frequency Domain Diagram

Ⅱ. I/Q Image Reject (IRM) Mixer

I/Q mixers are a subclass of passive mixers that provide the benefits of traditional passive mixers as well as the added advantage of filtering out undesirable image signals internally. When used as downconverters, these mixers are also known as IRMs (Image Reject Mixers), and when used as upconverters, they are known as SSBs (Single Sideband Mixers). The LO signal is split in half and then phase-shifted by 90° (0° for one mixer and 90° for the other) in the I/Q mixer, which consists of two double balanced mixers. With the help of this phase shift, the mixer is able to generate only the desired sideband signal while filtering out undesired signals.

On the same spectrogram, Figure 2 displays the performance of the I/Q mixer (purple trace) and double-balanced mixer (blue trace). As can be observed, the double balanced mixer generates both high and low sidebands, but the I/Q mixer reduces the undesirable low sidebands by offering 45 dB of rejection.

Figure 2. Spectrum Plot of HMC773A Passive Mixer and HMC8191 I/Q Mixer with 1 GHz IF Input and 16 GHz LO Input

I/Q mixers require a high LO input power level, just like double-balanced passive mixers. The I/Q mixer normally needs around 3 dB more LO drive than two double-balanced mixers because of the way it is built. I/Q mixers are delicate to precisely balanced input amplitude and phase matching. The degree of picture rejection will be directly impacted by any deviation from 90° in phase or amplitude of the input signal, mixing structure, system board, or mixer itself. By externally calibrating the mixer to increase performance, the consequences of these inaccuracies can be reduced.

I/Q mixers are frequently employed in applications where sideband removal is required without the use of external filtering because of their sideband suppression capabilities. They also offer very good noise figure and linearity. Microwave point-to-point backhaul communications, test and measurement equipment, and military applications are typical examples of such markets.

Advantages of I/Q Mixers

Inherent Image Rejection

No need for expensive filtering

Good Amplitude and Phase Matching

Ⅲ. Active mixer

Single balanced and double balanced active mixers, commonly referred to as Gilbert unit mixers, are the two basic varieties. The built-in gain blocks at the RF output and LO port are a benefit of active mixers. Such mixers have minimal input LO power requirements and offer some conversion gain to the output signal. In contrast to the majority of passive mixers, the typical LO input power for active mixers is close to 0 dBm.

An LO frequency multiplier, which is used to multiply the LO frequency to a higher frequency, is frequently integrated into active mixers. The customer will benefit greatly from this frequency doubler's ability to drive the mixer without using a high LO frequency. Port-to-port isolation is often very good in active mixers. High noise figure and, in most situations, low linearity are its drawbacks. The noise figure and linearity of an active mixer are impacted by the demand for an input DC power source. In the communications and defense industries, where having a low LO drive and integrated conversion gain can be crucial, active mixers are frequently utilized. Active mixers are typically utilized in low-end instrumentation or as the third or final stage mixer in the IF portion of the test and measurement industry (integration and cost-effective design are more important than noise figure).

Advantages of Active Mixers

High integration, small size

LO drive requirements are low

Integrated LO doubler

Good isolation, but poor linearity and noise figure

Ⅳ. Integrated Frequency Conversion Mixer

Due to growing customer need for more comprehensive signal chain solutions, integrated frequency converters have gained popularity. These devices are made up of various functional components that are linked together to form a subsystem, simplifying the final system design for the customer. In one package or chip, these devices combine many blocks including mixers, phase-locked loops (PLLs), voltage-controlled oscillators (VCOs), frequency multipliers, gain blocks, detectors, etc. Such devices can be produced as a single die that contains all the design blocks or as a SIP (system in package), where many die are combined into a single package.

Frequency converters can provide designers many advantages by combining many devices onto a single chip or package, including: lower size, fewer components, a simpler design architecture, and—most importantly—a quicker time to market. quick.

Figure 3. Functional block diagram of the HMC6147A integrated frequency translation mixer