Downconversion and upconversion are performed in most high-frequency receivers and transmitters by means of an often-overlooked component: the RF/microwave mixer. Over the last 20-plus years, mixers have changed a great deal in appearance and are now available in a wide variety of package styles. Still, their main functionality has not changed: to translate the frequency of a signal (usually carrying some form of information via modulation) to a second frequency (either higher or lower than the original frequency).
An RF/microwave mixer is essentially a three-port component, which can be fabricated as a passive component based on diodes or an active component based on biased field-effect transistors (FETs). A mixer's three ports are commonly known as the radio-frequency (RF), local-oscillator (LO), and intermediate-frequency (IF) ports, with two serving as input ports and one as the output port. The LO port is always an input port, so that the RF and IF ports are the ones that switch functions, depending upon whether a mixer is used for frequency downconversion or frequency upconversion.
In downconversion, a high-frequency RF input signal is mixed with a high-frequency LO signalusually over a similar frequency range as the LO signalto produce a lower-frequency IF output signal. In upconversion, lower-frequency IF signals serve as inputs, and are mixed with higher-frequency LO signals to produce RF output signals. The latter have been translated higher in frequency than the IF input signals while maintaining the modulation information of the IF signals. Downconversion is normally part of a receiver; upconversion is typically used in a transmitter. The translation of frequencies follows a simple mathematical mixing function:
n(fRF fLO) = fIF
fLO = the LO signal frequency
fRF = the RF signal frequency
fIF = the IF signal frequency
n = the harmonic number
Here is a simple example of downconversion, using an RF at 2100 MHz and an LO at 2000 MHz: Fundamental-frequency mixing would yield IF sum and difference signals, with one IF signal at 2100 2000 = 100 MHz and one at 2100 + 2000 = 4100 MHz. If the lower-frequency signal product is desired, the higher-frequency product can be removedfor instance, through the addition of a lowpass filter at the mixer's IF output. It should be noted that it is the difference between the mixing signals that is important, and the difference of LO RF can also be used.
Many different types of RF/microwave mixers have been developed over the years in support of a diversity of different types of communications systems. These include single-balanced mixers (which can be designed with a single diode), double-balanced mixers, triple-balanced mixers, image-reject mixers, in-phase/quadrature (I/Q) mixers, single-sideband mixers, double-sideband mixers, harmonic mixers, and subharmonic mixers.
Traditional double-balanced mixers, for example, are typically based on four Schottky diodes in a quad ring configuration, which provides acceptable performance for many applications. When certain levels of enhanced performance are needed, a pair of these diode quads are incorporated in the mixer circuitry to form a triple-balanced mixer. Mixers that can process signals with I and Q components are ideal for use in systems employing digital modulation, while harmonic mixerswhich can extract higher harmonics from the mixing processare typically used in generating and processing millimeter-wave signals.
Ideally, when a downconverting mixer processes LO and RF input signals to produce a lower-frequency IF signal in a receiver, the received signals are mixed with the injected LO signal. But any signals appearing at a mixer's RF port that are at that target frequency range but not the desired signalsusually referred to as "image" signalswill produce IF output signals. Receivers often employ preselector filters to remove any unwanted image signals that fall into the bandwidth of the mixer's RF port. The alternative is to use an image-reject mixer which has been designed to attenuate these unwanted image signals.
Mixers are characterized by a number of performance parameterssome of which (such as conversion loss) only apply to mixers and not to other high-frequency components. Other important mixer parameters include port-to-port isolation, VSWR, noise figure, 1-dB compression, and third-order intercept point. For example, isolation describes the separation between ports, or how much power will feed through from one port to another. High isolation indicates a mixer with minimal signal leakage between ports.
A mixer's dynamic range is the difference between the maximum amplitude of signals it can handle (as determined by the 1-dB compression point) and the lowest-level signals it can process (determined by its noise figure). Of course, choosing any mixer is a matter of matching the mixer's overall performance to a required system frequency plan. This includes whether the need is for upconversion or downconversion, how the IF will be handled, the available LO power, and even the type of mixer package desired for printed-circuit-board (PCB) mounting.
If a mixer is employed for frequency downconversion, as typically used in an RF/microwave receiver, a great deal of its performance will be dependent on the available LO signals. For example, noise in the LO signals will contribute to noise at a downconverting mixer's IF port. But a minimal LO amplitude will also limit the available dynamic range of the mixer. Mixers are typically optimized for different LO signal levels, such as +7, +10, and +14 dBm; this is the power that energizes a mixer's nonlinear switching elements, whether they are diodes or transistors. At levels above the optimal LO amplitude, a mixer will start to experience compression, where an increase in LO input power no longer results in an increase in IF output power. Early signs of compression are indicated by a mixer's 1-dB compression point.
A companion parameter for determining a mixer's linearity is the third-order intercept point (IP3), which refers to a level of intermodulation distortion caused by two tones at a mixer's RF port. For mixers used in digital communication systems, for example, excellent linearity is important in maintaining the accuracy of I and Q signal components processed by the mixer, with higher third-order-intercept-point values representing enhanced linearity performance.
Passive mixers are characterized by their conversion loss, which is caused by losses due to impedance mismatches in the mixer circuit, to diode junctions, and to other circuit-connection points in a mixer design. In a mixer used for downconversion, the conversion loss is the difference in signal level between the RF input amplitude and the IF output amplitude, while in a mixer used for upconversion the difference in amplitude is between the IF input amplitude and the RF output amplitude. Conversion-loss values of 6 to 8 dB are not unusual in standard double-balanced RF/microwave mixers. Of course, it is also possible to achieve conversion gain in an active mixer, through the use of an amplification stage and active circuit devices in the mixer. But this gain will also require the addition of bias power for the mixer's active circuitry, as if biasing an amplifier.
RF/microwave mixers were once fairly large components, featuring metal housings with three coaxial connectors for the ports. For some applicationssuch as rack-mount receivers, transmitters, and test equipmentsuch mixer packaging is still a good match (Fig. 1). But as more high-frequency designers are asked to miniaturize circuits and systems, mixer packaging has followed with increasingly smaller surface-mountable and PCB-mountable packages (Fig. 2), which allow engineers to achieve frequency translation in extremely small areas of a circuit (when including the LO source).
Newer mixers can be specified for broadband or narrowband use, depending on the specific application. In terms of the performance levels possible in small packages, mixers such as the model SYM-63LH+ from Mini-Circuits is a double-balanced mixer based on a diode quad that can handle RF/LO signals from 1 to 6000 MHz. The same firm's MAC Series of mixers is based on low-temperature-cofired-ceramic (LTCC) circuit substrates for RF/LO coverage from 0.3 to 12.0 GHz in a surface-mount package that is only 0.06 in. high. And the SGS-5-17 double-balanced mixer from Synergy Microwave uses the company's SYNSTRIP multilayer circuit technology to achieve RF/LO coverage from 3 to 19 GHz in a package measuring only 0.275 x 0.200 x 0.050 in.
In addition, a growing number of integrated-circuit (IC) manufacturers are fabricating mixer functions as part of entire front-end assemblies. These also include preselector filters, amplification, matching transformers, and IF filters to greatly simplify the task of RF/microwave receiver and transmitter designers looking for a compact, frequency-translation solution.