RF Mixer Basics: Types, Applications, and Design Insights

Introduction

In modern RF and microwave circuit design, mixers play a pivotal role in signal processing. RF mixers are essential components used to convert signals from one frequency to another, enabling efficient transmission and reception in wireless communication systems. This tutorial covers the fundamentals of RF mixers, including their key terminologies, types, and applications in microwave circuit design.

What is an RF Mixer?

The RF mixer has 3 ports: RF, IF, and LO. It has two input ports and one output port.

• Up Frequency Conversion: IF and LO are used as input ports, and the output is available at the RF port.
• Down Frequency Conversion: RF and LO are used as input ports, and the output is available at the IF port.

Essentially, it’s a non-linear device used for shifting signals from one frequency to another point in the spectrum. It is considered a linear device because it keeps the properties of the input signal intact while performing frequency translation.

Figure 1: RF Mixer Spectral components

The spectral components of the RF mixer are mentioned in Figure 1. As shown, it produces $\pm m(Input1) \pm n(Input2)$, where Input1 and Input2 are input frequencies, and $m$ and $n$ range from 0, 1, 2, 3, and so on. Figure 1 mentions up to the third harmonics of the inputs.

RF mixer devices are used in the up-conversion and down-conversion modules of RF transceivers. In RF receivers, they convert RF to a lower IF frequency or to baseband to make signal processing easier. This frequency translation process is known as down-conversion.

Figure 2: RF mixer in up/down conversion circuit design

In the RF transmitter part, mixers convert a lower IF frequency or baseband frequency to a higher IF or RF frequency to provide efficient power transmission. This frequency translation process is known as up-conversion.

Low-Side LO vs. High-Side LO

• Low-Side LO System: If the LO frequency is lower than the RF frequency, then the image frequency will be lower than LO. Here, the image frequency will exist at $(f_{LO} - f_{IF})$ in the frequency spectrum.
• High-Side LO System: When the LO frequency is higher than the RF frequency, then the image frequency will be higher than both LO and RF. Here, the image frequency will exist at $(f_{LO} + f_{IF})$ in the frequency spectrum.

In down-convert mixers, the image frequency will be converted directly to the IF position along with the IF frequency itself. In up-convert mixers, the image frequency will be an unwanted sideband that has an amplitude at the same level as the desired signal. This image frequency needs to be filtered out in up-conversion. There are specially designed image reject mixers which do the function of removal of these image frequencies.

RF Mixer Terminologies

Let’s understand some common RF mixer terminologies. These are the mixer specifications used while selecting a mixer for RF circuit design:

• Frequency Range: Specified for all ports of the RF mixer device, including RF, LO, and IF ports. This is the range for which the mixer has been designed to provide optimum performance as desired.

• Power Level: The power level of the signal to be fed at the mixer ports. Usually, the LO power level is specified. As a rule of thumb for using mixers in RF circuit design, the LO power level should be 15-20 dB higher than the RF power for optimum performance. This helps determine the power level of the other input port (either RF or IF) compared to the LO input port.

• Conversion Loss: It is the ratio of the output signal to the input signal. In other words, conversion loss is the difference between input RF power and output power level.

  For example, if converting from IF to RF frequency, the conversion loss in dB is: $Conversion Loss (dB) = P_{RF}(dBm) - P_{IF}(dBm)$

  For $P_{RF} = -10 \text{ dBm}$ and $P_{IF} = -17 \text{ dBm}$, the conversion loss of the RF mixer is 7 dB. Typical values of conversion loss are between 5 and 10 dB.

• 1dB Compression: Under normal mixer operation, conversion loss remains constant irrespective of input power. If input power increases by 1 dB, output power will also increase by 1 dB. However, when the input power becomes too large, this 1 dB to 1 dB relationship no longer exists, and the output varies non-linearly with the input. 1 dB compression is defined as the input power needed to increase the conversion loss by 1 dB from the ideal. The 1 dB compression point is considered a measure of the linearity of the RF mixer. When designing power levels in the RF circuit lineup, the output power should be less than the output 1 dB compression point of the mixer device by a considerable amount. Otherwise, the device will be driven into saturation.

• Isolation: A measure of the amount of power that leaks from one mixer port to another. Desired port isolation is achieved using mixer balance circuits and hybrid junctions at appropriate places. Usually, LO/RF isolation is specified. Typical values of isolation are between 15-25 dB. Practically, some amount of power leakage exists at LO, RF, and IF ports.

  Isolation is measured as the difference between the input power signal and the leakage power available at other ports. For example, if an input signal is fed at the LO port and the output is obtained at the RF port, then the isolation (dB) between these two ports is expressed as: $Isolation_{LO-RF} (dB) = P_{in LO} - P_{out RF}$

  The isolation between ports is reciprocal; hence, it is measured in only one direction. The mixer isolation from port 1 to port 2 is the same as from port 2 to port 1. The three types of mixer isolation specified are L-I isolation, L-R isolation, and R-I isolation.

• VSWR (Voltage Standing Wave Ratio): If the mixer VSWR is perfect, there will be minimal reflections. Though VSWR is of least concern in RF circuit design, it is used as a matching parameter for circuits at the three ports of the mixer device.

• Intermodulation Distortion (TOI or Input and output IP3): Intermodulation Distortion (IMD) occurs when nonlinearities in a system generate unwanted frequency components, with Third-Order Intercept Point (TOI or IP3) representing the extrapolated power level where third-order intermodulation products equal the fundamental signal, measured at both input (IIP3) and output (OIP3).

• Noise Figure: The ratio of SNR (Signal-to-Noise Ratio) at the input to the SNR at the output. It is approximated by the value of the mixer conversion loss. For low-power applications, conversion loss and noise figure should be low.

• Image Rejection: For I/Q mixers and image reject mixers, this rejection value is specified in the mixer data sheet.

RF Mixer Types

Depending on devices and configurations, there are various RF mixer types. The two main types are passive (using diodes) and active (using BJTs or FETs). Further, based on configurations and the number of devices in use, there are different types such as unbalanced, single balanced, double balanced, triple balanced, etc.

Let’s understand some of these primary types of RF mixers, classified based on their design and functionality:

1. Passive Mixers:

   • Use passive components like diodes or resistors.
   • Require no external power.
   • Advantages: High linearity, wide bandwidth.
   • Disadvantages: Typically lower conversion gain, may introduce signal loss.
   • Examples: Single-diode mixer, Double-balanced mixer (commonly used for better suppression of unwanted signals)

2. Active Mixers:

   • Incorporate active components like transistors (BJT, FET, or MOSFET).
   • Require external power for operation.
   • Advantages: Provide conversion gain, reducing the need for amplification.
   • Disadvantages: Higher noise figure, limited linearity compared to passive mixers.
   • Examples: Bipolar junction transistor (BJT) mixer, Field-effect transistor (FET) mixer

3. Double Balanced Mixers:

   • Use transformers or hybrids to achieve balanced inputs and outputs.
   • Suppress both the input signal and local oscillator feedthrough, reducing spurious signals.
   • Advantages: High isolation, good suppression of unwanted harmonics.
   • Common Applications: Communication systems, radar.

4. Triple Balanced Mixers:

   • Advanced version of double-balanced mixers with three sets of balanced circuitry.
   • Provide better isolation and wider dynamic range.
   • Applications: High-performance RF systems.

5. Image Reject Mixers:

   • Include additional circuitry to reject the image frequency (a byproduct of the mixing process).
   • Advantages: Simplifies system design by reducing filtering requirements.
   • Applications: Receiver architectures.

6. IQ Mixers (Quadrature Mixers):

   • Combine an in-phase (I) and quadrature (Q) signal to perform complex mixing.
   • Advantages: Enable single sideband (SSB) modulation and demodulation.
   • Applications: Modern communication systems, software-defined radios (SDRs).

7. Harmonic Mixers:

   • Exploit higher-order harmonics of the local oscillator to mix signals.
   • Advantages: Enable operation at very high frequencies.
   • Applications: Millimeter wave systems, spectrum analyzers.

8. Phase Detector Mixers:

   • Mix signals by comparing the phase difference between two signals.
   • Applications: Phase-locked loops (PLLs), frequency synthesis.

9. Nonlinear Mixers:

   • Rely on the nonlinear characteristics of devices like Schottky diodes or varactors.
   • Advantages: Effective at high frequencies.
   • Applications: High-frequency systems.

Conclusion

RF mixers are indispensable in RF and microwave circuit design, facilitating seamless frequency conversion and ensuring efficient signal transmission. By mastering the basics of RF mixers, their types, and terminologies, you can unlock the potential to design innovative and optimized RF systems. With their wide-ranging applications in communication, radar, and broadcasting, RF mixers continue to be a cornerstone of advanced electronic systems.