Managing RF Signals in Test Systems

Minimizing reflections and signal losses in test systems associated with the design, manufacture, and production of RF products can be a complex challenge. In addition to issues such as cable and interconnect quality, engineers must consider conductor length, physical layout, and other aspects of system design that have little effect on DC circuits, yet are fundamental to the satisfactory operation of high-frequency RF systems. Understanding and dealing with the mechanisms of impedance matching and signal loss are crucial in designing test systems for wireless products.

Basic Electrical Properties

Several electrical properties must be considered when developing an RF test system. The main parameters include system bandwidth, insertion loss, isolation, power-handling capability, and voltage standing wave ratio (VSWR).

Power losses primarily are a function of the resistive and impedance mismatch losses through a circuit path. Mismatch usually is the largest contributor to power measurement uncertainty and can be calculated from the magnitudes of the reflection coefficients of the source and load as

Uncertainty = 20 log (1 ± G_S G_L) dB (1)

where G_S = the reflection coefficient of the source
G_L= the reflection coefficient of the load

RF sources, loads, and signal paths all have characteristic impedances that must be perfectly matched or energy will be reflected back through the system. Mismatch is quantified as either return loss or VSWR.

The reflection coefficient can be determined from the Return Loss by

Similarly, the reflection coefficient can be calculated from the VSWR by

RF Signal Conditioning and Switching Application Example

Test systems often must simulate actual application environments using complex signal paths that contain switches, other passive components, and active components. Characterizing how well a cellular phone can reject multipath interference, noise, and other RF signals is one example of such a production test.

Simulating the mechanism of multipath interference in a test system is a complex procedure because it requires that noise and time-delayed signals reach the phone under test with appropriate power levels and phase relationships. Figure 1 (see the December 2001 issue of Evaluation Engineering) shows a typical system for testing mobile phones.

For mobile-phone receiver testing, the output of the mobile-station test set can be switched to two types of paths:

• A path through instrumentation that simulates multipath fading and noise interference.
• Paths that can switch in gain to simulate varying distances between the mobile phone and the base station.

For phone-transmission testing, the output of the phone is directed to the mobile-station test set through either an attenuated or unattenuated path. For monitoring purposes, the output of the phone or the input to the phone can be connected to a spectrum analyzer.

Understanding and Managing Power Loss

Before attempting to perform any meaningful testing or calibration, it’s critical to quantify the power losses through the RF test system. Some first-time users of RF test systems may expect that such systems will be completely invisible, implying perfect matching, zero insertion loss, and insensitivity to frequency-dependent effects.

However, switches, circulators, isolators, couplers, and attenuators present many avenues for slight impedance mismatches and power loss. Table 1 (see below) lists typical specifications for these components at 2 GHz. In general, matching and VSWR become worse as frequency increases.

Table 1. Typical System Component Characteristics at 2 GHz

• Switches are used to route signals to different parts of a system. Typical switch types include electromechanical and solid-state. Generally, switch performance is judged in terms of bandwidth, insertion loss, and VSWR.

Electromechanical switches typically offer wider bandwidth, lower insertion loss, and lower VSWR specifications than solid-state switches. However, electromechanical switches have a shorter life cycle—typically 2 million switching cycles as opposed to 10 million for solid-state switches. Electromechanical switches also require switching times on the order of 25 ms, as compared to 25 ns for solid-state switches.

• Directional couplers extract a specific amount of RF energy from a wave traveling in one direction through a transmission line. Directivity is the difference between the coupled port’s output with power flowing in the forward direction and its output with power flowing in the reverse direction.

Often, the insertion-loss specification of a coupler does not include the loss represented by coupled power. This power loss also must be included when estimating total system-insertion loss.

For example, the insertion loss of a directional coupler might be rated at 0.1 dB maximum, but the insertion loss from the coupling of 10 dB of the power theoretically would be 0.46 dB. As a result, 0.56 dB of insertion loss would have to be budgeted for the coupler.

• n-way strip-line power dividers/combiners divide an input into n separate paths or combine n inputs into one output with a specified amount of isolation between inputs. These devices are band limited. Often, the insertion loss specification of dividers/combiners does not include the split loss. The insertion loss from the power division usually has to be added to the insertion-loss specification.

A typical two-way divider might have an insertion-loss specification of 0.4 dB. The insertion loss that results from dividing the power in half is 3 dB. As a result, 3.4 dB of insertion loss would have to be budgeted for this power divider when dividing the power and only 0.4 dB when combining the power.

• A circulator is a multiport device that allows power to travel sequentially from one port to the next port. Each port can be used as an input or output. In an ideal four-port circulator, for example, power input at port 1 would only appear at port 2, power input at port 2 would appear only at port 3, and power input to port 3 would appear only at port 4.

• An isolator is a circulator containing a terminated port. If a four-port circulator had port 4 terminated, power could be injected in ports 4, 1, or 2 and would appear at the next port. Power input to port 3 would be dissipated by the termination on port 4.

• A termination is an RF device that, ideally, completely absorbs all RF energy flowing into it and reflects no energy back to the transmission line. This implies a VSWR = 1.0:1.

• An attenuator, ideally, also reflects no energy, but it reduces the RF power in the path by a specified amount and passes the remainder to an output port. Attenuators can be fixed or adjustable.

• Interconnecting cables sometimes are ignored when designing an RF test system, but they can be a critical element in system performance. In addition to electrical parameters such as characteristic impedance and insulation properties, physical attributes such as diameter, length, conductor and shielding design, and plating can strongly affect bandwidth, loss, and VSWR and the suitability of a given cable in RF applications. Generally, larger diameter cables offer lower insertion loss and higher power-handling capability but decreased bandwidth and flexibility as compared to smaller diameter cables.

Quantifying the Performance Of a System

Estimating system VSWR requires that sources of reflected power be identified and consideration be given to the effects on reflected power of components in the signal path. Figure 3 (see the December 2001 issue of Evaluation Engineering) shows a simple RF path consisting of a combiner, an isolator, an attenuator, and cables.

Table 2 (see below) lists specifications for the components in Figure 3. The reflection coefficients in the table have been determined by using equation 5.

Table 2. Characteristics of Components in Figure 3 @ 2 GHz

To estimate the VSWR at the input of the system, start at the output port. The reflection coefficients of the components in the path are added in an rms sum to yield the VSWR value at the input port. In this case, VSWR is calculated to be 1.389:1. The steps taken to arrive at this conclusion are detailed in Table 3.

Table 3. Input VSWR Estimate

The best-case input-to-output insertion loss would be no worse than the summation of the components’ insertion loss specifications. In this case, that would be four cables at 0.4 dB each, the combiner at 0.4 dB, the isolator at 0.4 dB, and the highest value of attenuation of 3.3 dB, adding up to a maximum of 5.7 dB of insertion loss through the system.

The worst-case output-to-input isolation can be estimated by the lowest value of attenuation of 2.7 dB, 20 dB from the isolator, and 3 dB for the power split in the combiner, adding up to at least 25.7 dB in output-to-input isolation for the system. The input 1-to-input 2 isolation should be no worse than the isolation specification of the combiner, which is 22 dB.

Conclusions and Recommendations

Understanding power losses and how to determine their magnitude are essential for estimating actual circuit performance, factoring the information into the stimulus levels applied to the DUT, and interpreting the results read back by test instrumentation. Considerable power can be lost throughout a system as a result of impedance mismatches and resistive losses.

With the complexity of today’s high-frequency RF test systems, losses can be substantial. Even a simple signal path with only three components can attenuate a signal by 5.7 dB. The mismatch error due to the path’s VSWR of 1.389:1, compared to an ideal VSWR of 1.0:1, adds further uncertainty to the total power delivered through the path.

The more realistic test system shown in Figure 1 can have as many as 10 components, not including cabling, in a single path. Minimizing the number of components in a pathway is essential, and whenever possible, you should use components with the lowest available insertion loss and VSWR.

About the Authors

Robert Green is a senior market development manager at Keithley Instruments. During his 12-year career at Keithley, Mr. Green has been involved in the definition and introduction of digital multimeters and sensitive measurement products, most recently for the wireless industry. He received a B.S. in electrical engineering from Cornell University and an M.S. in electrical engineering from Washington University, St. Louis. 440-498-2473, e-mail: [email protected]
Jeff Slaughter is a Keithley RF/microwave applications engineer. Previously, he has served as a design engineer at Allen Telecom and as a design engineer for Bird Electronic. Mr. Slaughter received a B.S.E.E. from the University of South Florida. 440-498-2745, e-mail: [email protected] Keithley Instruments, 28775 Aurora Rd., Cleveland, OH 44139

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December 2001