The Basics of Mixers

Mixers are used in a variety of RF/microwave applications, including military radar, cellular base stations, and more. An RF mixer is a three-port passive or active device that can modulate or demodulate a signal. The purpose is to change the frequency of an electromagnetic signal while (hopefully) preserving every other characteristic (such as phase and amplitude) of the initial signal. A principal reason for frequency conversion is to allow amplification of the received signal at a frequency other than that of the RF.

Figure 1: A mixer presented symbolically.

Figure 1 shows the mixer’s three ports: fin1 and fin2 are the input ports while the output port is the both the sum and the difference in frequency of the inputs:

fout = fin1 ± fin2

Figure 2: Representation of downconversion and upconversion.

The three ports (Figure 2) are referred to as the RF input port, LO (local oscillator) input port, and the IF (intermediate frequency) output port. A mixer is also known as a downconverter if the mixer is part of a receiver or as an upconverter if it is part of a transmitter.

Depending upon the application in which the mixer is being used, the LO is typically driven with either a sinusoidal continuous wave signal or a square wave. In concept, the LO signal acts as a gate of the mixer in which the mixer is considered ON when the LO is a large voltage and OFF when it is a small voltage. The LO can only be an input port, while the RF and IF ports can be interchanged between the second input or output.

When the desired frequency is less than the second input frequency, the process is called downconversion. The RF is then the input while the IF is the output. When the desired output frequency is greater than the second input frequency the process is called upconversion. Here the IF is the input while the RF is the output.

In a receiver, when the LO frequency is less than the RF frequency, it is called low-side injection and the mixer is a low-side downconverter. When the LO frequency is above the RF, it is called high-side injection, and the mixer is a high-side downconverter.

Generally, a passive mixer is made of passive devices, such as diodes. An active mixer is made of active devices, such as transistors. Active or passive implementations are used depending on the application, and there are advantages and disadvantages. As an example, a passive implementation that uses diodes as nonlinear elements or FETs as passive switches, exhibits a conversion loss rather than gain. This may impact the overall noise performance of the system, so in this case an LNA is usually added prior to the mixer.

Passive mixers are widely used due to their simplicity, wide bandwidth, and good intermodulation distortion (IMD) performance. Active mixers are mostly used for RFIC implementation. They are configured to provide conversion gain, good isolation between the signal ports, and require less power to drive the LO port. They can be monolithically integrated with other signal processing circuitry and are less sensitive to load-matching.

There are three basic types of active and passive mixers: unbalanced, single-balanced, and double-balanced.

Some mixer parameters that engineers will find in a datasheet are as follows:
1. Conversion loss or gain: Measured in dB, conversion gain measures the signal gain in an active mixer, while conversion loss (known also as CL) measures the insertion loss in a passive mixer. Conversion gain is defined as the ratio of the IF output power to the RF input power. For passive mixers, CL is the most important parameter next to the noise figure. It is defined as the difference in power between the input RF power level and the desired output IF frequency power level. Of course, the lower the CL the better. Typical values of conversion loss range between 4.5 to 9 dB. Other losses that may occur are from transmission line losses, balun mismatch, diode series resistance, and mixer imbalance. In addition, the wider the frequency ranges for the three ports, the worse the CL.
2. Input intercept point (IIP3): IIP3 is the RF input power at which the output power of the unwanted intermodulation products and the desired IF output would be equal.
3. Spurious: Spurious external signals generate undesired frequencies that may fall into the IF-band. Spur tables provided by manufacturers show the relative amplitudes of each response under given LO drive conditions
4. Isolation: This is defined as the amount of power leakage from one port to another. When isolation is high, the leakage between the ports will be small.
5. Noise figure (NF): Defined as the added noise generated by the mixer and present at the IF output, the noise figure is the second important parameter (CL is the first) for the passive filter. For a passive mixer, the NF is almost equal to the loss.
6. Dynamic range: This is the signal power range over which a mixer provides useful operation.
Other parameters to become familiar with include image rejection, gain compression, intermodulation, dc-offset, two-toned third-order intermodulation, and 1 dB compression point.

Selecting a mixer

Mixer selection depends on many factors and, most of all, on the requirements of an application. Determine the LO, RF, and IF frequency ranges involved as well as the LO drive required. Some applications require a specific amount of harmonic distortion. Finally, determine the type of packaging the mixer will have.

A wide selection of mixers can be found on the Hotenda site. Below are a few examples.

Figure 3: Linear Technology LT5560.

Linear Technology offers the LT5560, a low-power high-performance broadband active mixer. This double-balanced mixer can be driven by a single-ended LO source and requires only –2 dBm of LO power. The balanced design results in low LO leakage to the output, while the integrated input amplifier provides excellent LO-to-IN isolation. The signal ports can be impedance-matched to a broad range of frequencies, which allows the LT5560 to be used as an upconversion or downconversion mixer in a wide variety of applications.

Figure 4: Maxim’s MAX2682 circuit.

The MAX2680/MAX2681/MAX2682 from Maxim Integrated Products are small, low-cost, low-noise downconverting mixers designed for low voltage operation and well-suited for use in portable communications equipment. Signals at the RF input port are mixed with signals at the LO port using a double-balanced mixer. These downconverter mixers operate with RF input frequencies between 400 MHz and 2,500 MHz, and downconvert to IF output frequencies between 10 and 500 MHz.

NXP Semiconductor offers the BGA2022 MMIC mixer that features a wide frequency range: Cellular band (900 MHz), PCS band (1,900 MHz), and WLAN band (2.4 GHz). Specifications (with reference to VS = 2.8 V; IS = 6 mA; PLO = 0 dBm; fRF = 1,800 MHz; fLO = 2080 MHz; fIF = 280MHz) include a high conversion gain of 6 dB typical, a noise figure (DSB) of 12 dB typical, and an output third order intercept point of 7 dBm.

Summary

The purpose of the mixer is to provide both the sum and difference frequency at the output port when two input frequencies are provided at the other two ports. The designer should fully understand all mixer properties to efficiently translate this frequency properly and without any distortions.

References
1. Mixer Basics Primer, A Tutorial for RF & Microwave Mixers, By Ferenc Marki and Christopher Marki, Ph.D.
2. Analog Devices “Mixer and Modulators,” MT-080 Tutorial.