Designers are finding new techniques to improve system performance with Mini-Circuits’ patented reflectionless filters all the time. Among these, supplementing anti-alias filters in wideband ADCs to minimize the effect of switching transients has emerged as a popular use case.
Directional couplers are important devices in RF systems. Their ability to sample either the forward or reverse direction of signal propagation allows a wide range of applications in test, measurement, monitoring, feedback and control. This note should help system designers understand the function, architecture and performance of the coupler, to select a suitable type for their particular application.
A Power Splitter/Combiner is a passive device that can be used for two reciprocal functions: a single signal may be divided into multiple outputs, or in the opposite direction, multiple input signals are combined into a single output. In case of an N-port splitter, the input signal will be divided into N output ports. When used as an N-port combiner, the N inputs will be combined into an output signal from a single port.
Ultra-Wideband (UWB) radio is defined as any RF technology utilizing a bandwidth of greater than ¼ the center frequency or a bandwidth greater than 500 MHz  . While UWB has been a known technology since the end of the nineteenth century, restrictions on transmission to prevent interference with narrow-band, continuous wave signals have limited its applications to defense and relatively few specially licensed operators . In 2002, the FCC opened the 3.1 to 10.6 GHz band for commercial applications of Ultra-Wideband technology, and since then UWB has become a focus of academic study and industry research for a promising variety of emerging applications. To prevent interference with neighboring spectrum allocations like GPS at 1.6 GHz, the FCC has imposed specific rules for indoor and outdoor transmission of UWB signals, limiting transmissions in the permitted frequency range to power levels of -41 dBm/MHz or less.
Baluns and ununs are essential in RF signal chains for many applications. RF balun designs are most commonly associated with core-and-wire transformers, but can also be realized through coaxial and coupled stripline technologies. The behavior of baluns and ununs was introduced in Part 1 of this series, where we established that both these devices are designed for impedance matching purposes. The major difference between them is that baluns are designed to match impedances between balanced and unbalanced circuits, whereas ununs provide impedance matching between two unbalanced circuits.
Frequency translation devices such as multipliers and dividers are used to convert frequencies from lower spectrum regimes to higher frequencies, and vice versa. As these devices are intrinsically non-linear, they generate spurious harmonics, which are often filtered to prevent harmonics from appearing in-band. Using conventional, reflective filters creates an undesirable scenario where the out-of-band harmonics are reflected back to the multiplier. The multiplier is also affected by the reactive loading exhibited by a reflective filter at harmonic frequencies (see Figure 1). Given that multipliers have poor output return loss, this combination of effects leads to large ripples in the conversion efficiency of a multiplier chain, and hence, susceptibility to environmental factors.
This issue can be solved by leveraging the unique capabilities of reflectionless filters. To demonstrate this solution, an experiment was conducted with a doubler test circuit and a 4X multiplier chain (see Figure 2). Each experiment was conducted using comparable reflective and reflectionless filters, and the results were then analyzed.
In essence, a transformer is merely two or more conductive paths linked by a mutual magnetic field. When a varying magnetic flux is developed within the core by alternating current passing through one conductive path, a current is then induced in the other conductive paths. This induced current is proportional to the ratio of magnetic coupling between the two conductive paths. The ratio of the magnetic coupling of the conductive paths with the core determines the induced voltage in the additional conductive paths, providing both an impedance transformation and a voltage step-up or step-down. Additional conductive paths, potentially all with different coupling ratios, may be added to realize various functions, which is why RF transformers are such varied and versatile devices used widely throughout the RF/microwave industry.
GNSS (Global Navigation Satellite Systems) has become an increasingly prevalent RF application for both military and consumer use. Early GNSS technology, or location-based services (LBS), was developed exclusively for military use and operated with a margin of error of about 10-yards (9.14m), which was sufficient at the time, but limited its suitability for end uses requiring a higher degree of precision. In the time since the first launch of GPS more than 40 years ago, the evolution of the technology has improved precision to the order of 2-3 yards (1.83-2.74m). This advancement of the technology combined with significant miniaturization and cost reduction of LBS-enabled devices has opened up a large and growing commercial market for GNSS services. For example, GNSS is now used in the agricultural market to calculate statistics for weather, soil conditions, and crop health to help farmers maximize their yields and profits. Such innovations have stimulated demand for components that support military, industrial and consumer applications.
Before discussing filter designs and differentiating any given filter topology from another, it’s important to review the fundamentals of filter structures and their function. A filter is a two-port, passive, reciprocal device that allows frequencies within a given band to pass through while blocking signals outside the desired band.
There are many filter types available to the system design engineer including RLC filters, active RC filters, crystal filters, cavity filters, ceramic resonator filters and SIW, SAW and BAW filters. Filters may be fabricated using lumped elements, thin and thick film microstrip and stripline, LTCC and other manufacturing technologies. This article will focus on lumped element filters.
The impedance of a component or transmission line is a major concern when designing RF/microwave systems. At the circuit level, optimum performance is obtained when devices are matched to the desired system impedance, typically 50Ω or 75Ω. At the system level, each building block must be matched to the system impedance to maintain performance along the signal path.