RF/Microwave Bias Tees from Theory to Practice

The bias tee is an essential component for applying DC voltage to any component that must also pass RF/microwave signals, most commonly an RF amplifier that requires a DC supply. For narrowband applications, bias tee design and construction are relatively straightforward, provided attention is paid to component self-resonant frequencies (SRFs). For broadband applications, however, bias tee design and construction are nontrivial, and attention to component characteristics is paramount to a successful, high-performance design. In this article, we examine narrowband bias tee design, component SRFs, and how they impact the design, then extend those ideas to broadband bias tees.  We will also compare the electrical and physical performance attributes of different types of broadband bias tee designs including discrete circuits with conical inductors as well as MMICs. 

Fully Non-Blocking (Full Fan-Out) RF Switch Matrices

The first two articles in this series established that blocking switch matrices use switches to allow one-to-one connections between input and output ports, while non-blocking switch matrices use splitter/combiners on either the input our output ports to allow one-to-many or many-to-one connections. In this article, we’ll examine the fully non-blocking or “full fan-out” configuration in which all inputs are connected simultaneously to all outputs via splitter/combiners, sometimes with programmable attenuation on every path. Features, advantages, applications and examples will be reviewed.

Switch Matrix Configurations

Switch matrices are an essential tool for control of RF signal routing in any environment where there is a recurring need to change how systems interconnect. The addition of Ethernet and USB interfaces with flexible software and APIs (application programming interfaces) makes switch matrices particularly useful in automated test environments, allowing test sequences to be scheduled to run with no user intervention, switching between multiple devices under test (DUT), input / output ports and test equipment.

RF Blocking Switch Matrices

Blocking switch matrices are constructed using switches on the inputs and outputs, as shown in Figure 1. They are called “blocking” because once a path is set between any pair of ports, those 2 ports are not available (blocked) for use by any other path. Multiple paths can be active in parallel, up to the number of input ports or the number of output ports (whichever is fewer), with each path connecting a different pair of ports.

RF Non-Blocking Switch Matrices

Non-blocking switch matrices are constructed using switches on one set of ports and passive splitter / combiners on the other.  They are referred to as non-blocking (sometimes partially non-blocking) since the splitter / combiner component allows a single port to be connected concurrently to multiple ports on the opposite side.  Hence the path is not blocking any other ports from connecting, as would be the case with a blocking switch matrix.

Non-blocking matrices are often characterized as either fan-in or fan-out depending on the orientation of the splitter / combiners relative to the input ports.

RF Pulse Modulation: Fundamentals, Applications & Design Techniques

Pulse modulation has been around for over 125 years. In 1887, German physicist Heinrich Hertz built the first experimental spark-gap transmitters (electromagnetic pulse modulators), with which he proved the existence of radio waves.1  In 1888, using 455 MHz radio waves, he studied the ability of radio waves to be reflected from metallic objects and refracted by dielectric media.2  Hertz confirmed James Clerk Maxwell’s work from 1865, which was simplified by Oliver Heaviside in 1884.  Hertz’s work also spawned early target detection when, in 1904, a patent for “an obstacle detector and ship navigation device,” based on the principles demonstrated by Hertz, was issued in several countries to German Engineer Christian Hülsmeyer.2  Hülsmeyer’s British patent (September 23, 1904) was for a spark-gap-type, full 600 MHz pulsed radar system that he called a telemobiloscope.

Extending Power and Dynamic Range in E-Band Backhaul Test Sets

According to Ericsson’s 2022 Mobility Report, global mobile data traffic is expected to grow by a factor of 4 to 325 Exabytes total per month or 46 GB per smartphone on average by 2028, excluding Fixed Wireless Access usage.1 Network operators’ ability to support this rapid growth in consumption hinges on the speed and capacity of the backhaul systems connecting base stations to the core networks. Accordingly, the market for cellular backhaul equipment is projected to grow by 12.6% CAGR from 17.85B to 32.29B USD between 2020 and 2025. Looking more narrowly at E-Band systems still in nascent stages of development, that growth rate jumps to 22.3% (534M to 1.46B over the same period).1

LTCC Filter Innovations Enable Next Generation Aircraft Internet Links

For all the headlines and personal anecdotes lamenting how commercial air travel isn’t what it used to be, there are some clear benefits enabled by recent advances in technology we might be taking for granted. One of these is in-flight internet service. Whether domestic or international, most flights now offer internet service via satellite, allowing passengers to remain connected for personal and business use throughout the majority of their journey.

For most aircraft in service today, the satellite up/downlink connection is achieved with a mechanically steerable antenna mounted to the top of the fuselage. The antenna has a limited range of motion to maintain connection with the satellite while compensating for the movements of the aircraft during normal flight operation. These mechanical systems are now giving way to electronically steerable systems using phased array antennas to deliver more reliable connectivity with lower costs of operation and maintenance for the carriers.

Understanding RF/Microwave Push-Pull Amplifier Design

Remarkably, the concept of the push-pull connection spans three centuries. William W. Dean of the Bell Telephone Company of Missouri first described the push-pull-connected telephone transmitter in Patent No. 549,477, dated November 5, 1895.1 Next, Sir John Ambrose Fleming invented the first vacuum tube in 19042 and while Lee De Forest added the grid to Fleming’s “valve” in 1906, calling it the “audion,” it was Fritz Lowenstein in his April 24, 1912 patent application who first discovered that applying a negative bias to the grid of De Forest’s tube turned it into an audio amplifier.

Optimizing Transistor Stages for Different Operating Bandwidths

Since their invention by John Bardeen, Walter Brattain and William Shockley at Bell Labs in 1947, transistors have become a defining (if not the defining) feature of the contemporary technology landscape. Electric vehicles can have over 5 billion transistors in a given build, and today’s smartphones more than 10 billion! While their most common and familiar uses reside in the digital world, transistors are an essential element in RF circuits and systems, and while often integrated into more complex, multi-function devices, the simple transistor is still an essential tool in the designer’s toolkit.