In the RF and microwave domain, high frequencies and stringent application specifications are the norm. As a result, engineers face a number of critical challenges, not the least of which are making precise measurements with accurate data and obtaining repeatable results.
While measurement instrumentation can play a key role in addressing these challenges, the importance of test accessories (e.g., switches, amplifiers and attenuators) in forming complete test solutions cannot be underestimated. By choosing the wrong accessory, engineers risk creating a weak link in the test system, further complicating its setup and limiting its measurement ability.
With high-performance instrumentation and accessories pushing the envelope of todays advanced high-frequency applications, it is now more crucial than ever for engineers to select the right test accessories. Doing so improves the test systems utility and automation, and eliminates any weak links in the measurement path.
Eliminating The Weak Link
RF and microwave test accessories are an integral part of any test system and can include everything from DC blocks, attenuators and couplers, to switches and system amplifiers. Such accessories complement a systems instrumentation by helping to simplify test setups and maximizing the instrumentations full potential. For the purposes of this article, only switches, attenuators, power limiters, active probes, and RF detectors will be discussed.
Regardless of the accessory in question, selecting one with the highest quality provides an easy and invaluable means of optimizing RF and microwave measurement systems, while also ensuring a more reliable test setup.
The trick lies in knowing what accessories to use in a given application. Some of the more common test accessories and their key applications are as follows:
Increasing Efficiency Using EM Switches
When used in signal routing schemes, electromechanical (EM) switches (e.g., single-pole, double-throw (SPDT), transfer or double-pole, double-throw (DPDT), multiport, and bypass) enable multiple measurements with a single connection. The ability to test multiple devices-under-test (DUTs) with a single instrument or to make multiple types of measurements on a DUT maximizes the test systems throughput.
When selecting an EM switch, a few tips are helpful to keep in mind:
1. Select a switch with good repeatability and a long operating life. Quality switch manufacturers provide a repeatability specification for their switches and specify switch lifetime as the number of cycles it can complete while still meeting performance/repeatability specifications. Other vendors specify life as the point at which the switch experiences mechanical failure; however, in this case, signal degradation can actually occur much earlier. Consequently, choosing a switch specified in this manner may result in a switching price tag over the lifetime of a test system that is quite high. Note that typical switch lifetime specification numbers are in the millions and can go as high as 10 million, depending on the switchs quality.
2. Continuous wear and tear in a switch results in debris accumulation, which increases insertion loss and measurement uncertainty. Purchasing a switch that effectively addresses this dilemma ensures better switch repeatability. Agilents switches, for example, employ a patented design with a curved top center conductor that produces friction between the jumper contact and center conductor during switch closure. The friction mimics a wiping action that removes contact point debris (Figure 1).
3. Coaxial switch datasheets are tricky to read, differing from one manufacturer to another. When reading a datasheet, watch for words like typical or guaranteed following a specification, as this may indicate a need to investigate the specification further. Also, if an important specification to the switch matrix design (e.g., repeatability) is missing from a switchs datasheet, think twice before making a purchase.
4. Never judge a switch by its price. Instead, verify that all performance specifications fit the design in question. Then, break down the switch price into units based on the switchs lifetime. Remember, having a test system go down to replace a switch can be a costly proposition. Also, note that when working with non-latching switches, it may be worthwhile to break down the switchs cost based on power consumption.
Enabling Reliable Signal Conditioning
An external variable attenuator can be used to ensure reliable signal conditioning in a range of wireless test applications, such as for fast and accurate power calibration in mobile receiver sensitivity test or for an adjacent channel interference test. In both applications, use of the attenuator eliminates power nonlinearity and inaccuracies from the source.
As an example, Figure 2 on the previous page shows two setups for a mobile receiver sensitivity test used to measure real-time power. The conventional test setup on the left offers real-time power monitoring and power-level control straight from the source. However, it is unable to measure and calibrate low power levels. The alternate setup on the right adds an external attenuator and directional coupler. Doing so provides a number of benefits. It enables easy one-time power calibration and allows the source to stay at a single power level. Additionally, the setup frees the user from power-level nonlinearity and accuracy issues from the source, and makes it easier to obtain accurate and calibrated low power levels.
Protecting Investments Using Power Limiters
When expensive instruments (e.g., a network analyzer, spectrum analyzer or signal generator) and components are mishandled or damaged by excessive RF power, DC transients or electro-static discharge (ESD) to the input port, the result is significant test system downtime and a high repair cost. Power limiters with ESD provide engineers with an inexpensive way to protect their measurement system investments.
Optimizing RF Circuit Design With Active Probing
An active differential probe provides an effective way for RF circuit designers to optimize and troubleshoot their circuit designs. It can be used to measure a designs harmonics, power levels and frequency, as well as its modulation, to identify faulty or problem areas. When used with a high dynamic range signal/spectrum analyzer, the probes high sensitivity and low distortion levels enable even the smallest signals to be detected, providing engineers with greater insight into the DUTs spurious response. An active differential probe can also be used with a signal source analyzer to measure the phase noise and jitter of a clock in a high-speed digital board design.
Measuring Power With An RF Detector
An RF detector is the ideal accessory for measuring power. As an example, consider the block diagram of a power measurement shown in Figure 3 on this page. Here, the absolute power is measured by characterizing the detector with a power sensor and meter using a reliable source. The detector is connected to a source, which is swept across the frequency or power level of interest. The output voltage measured from the detector is then collected and plotted. Using the slope of the resulting graph, the power detected is calculated based on the measured voltage. This method can also be used beyond the square-law region, where variations are repeatable and predictable for each diode family. Computer-controlled compensation compensates for any nonlinearity.
A detector can also be used in envelope or video detection, a prime example of which is a radar system. In this application, the detector receives and processes recovered signals, sending them on to test instrumentation where they can be displayed. The detectors output signal serves as an indicator of the targets range.
The Bottom Line
Use of low-quality RF and microwave test accessories can result in inefficient, unreliable test systems. Selecting the right accessories, however, ensures test system optimization and minimizes measurement uncertainty. The best way to achieve this goal is by purchasing accessories from a premium vendor with unmatched reliability and repeatability. The accessories should also have a proven long operating life and require minimal maintenance throughout their lifetime. Additionally, the right accessory must be used for the right application in question, whether its enabling reliable signal conditioning or protecting the engineers investment. By following these tips, todays engineers can simplify their test setups and maximize their equipments full potential to ensure the best possible measurement results.
About the author
Theng Theng Quek is a business development manager in Agilent Technologies Component Test Division. She joined Agilent as a product marketing engineer in 2006. Prior to that, she was an R&D engineer for Motorola. Theng Theng holds a bachelors degree in engineering from Multimedia University of Malaysia with a major in microwave and communications.