High-power RF signal generators have the edge

In RF testing, an essential attribute of every signal generator is the maximum output power it can supply to a device under test (DUT) while maintaining spectral purity and level accuracy.

The ability to deliver a pure, accurate signal at +25 dBm or greater not only ensures improved measurement accuracy but also enables testing of extreme or unusual operating conditions.

These capabilities can simplify the testing of high-power amplifiers, overcome losses within automated test equipment (ATE) systems and address the attenuation of signals within long cable runs.

Ultimately, the benefits of using a signal generator with high output power include reduced cost, size and weight of the resulting test system or configuration.

Travelling wave tube (TWT) microwave amplifiers are a classic example: they produce more than 100 W or +50 dBm of power and require input levels of +25 dBm or greater. Unfortunately, most of today’s microwave signal generators don’t provide levelled outputs with that much power.

As a result, the only option is to connect the signal generator to an external microwave preamplifier and the support equipment necessary to monitor, level and calibrate the signal delivered to the TWT.

Figure 1 shows a commonly used configuration for testing high-power amplifiers. The signal generator output, which is typically +10 to +23 dBm, is fed into a preamp with 30 to 35 dB of gain.

Figure 1: Example configuration for testing high-power amplifiers.

The preamp can be either a broadband device or one that matches the frequency range of the amplifier under test (AUT).

To keep the input power as flat as possible, the preamp output is fed into a levelling coupler, which provides a proportional sampling point that is fed into the signal generator’s automatic level control (ALC).

This type of dynamic mismatch correction is necessary due to the high likelihood of variation in the frequency response of the preamp.

To adequately characterise the AUT gain, the coupler’s output signal must have a level accuracy of at least ±0.5 dB at the amplifier input.

The amplifier drives a load that is usually water or oil cooled to handle the high-power output of the AUT. As with the coupler, the output of the load is proportional to the AUT output, bringing the signal into a range that is easily measured with commercial power sensors and power meters.

Unfortunately, this widely used approach has three noteworthy shortcomings: the configuration is somewhat complicated; it requires costly test accessories capable of handling high power; and its overall accuracy depends on the precision of every element within the system.

Cost and complexity will increase if multiple narrowband preamps are needed to maintain proper input power across the AUT frequency range.

A signal generator with high output power enables the simpler test configuration shown in Figure 2. If the signal generator is capable of providing at least +25 dBm of output power, then the preamp and external levelling coupler can be removed. This reduces system cost and also improves system performance.

Figure 2: Simplified configuration for amplifier testing with high-power signal generator.

Optimising measurement accuracy

The ability to achieve level accuracy that provides meaningful measurement results depends on external levelling techniques that correct for mismatches and frequency-response variations.

In most cases, a combination of static and dynamic levelling is the most effective approach.

Static levelling relies on an array of user-created calibration factors that ensure the delivery of flat power at an interface beyond the signal generator’s RF output connector.

The calibration array can be generated by a power meter under the control of the signal generator: the two instruments step through a list of key frequencies, generating and measuring the associated power levels.

The signal generator interpolates values for in-between frequencies.

External dynamic levelling uses either a power meter or a crystal diode detector. The key difference is a trade-off between speed and accuracy.

The power meter-based approach is more accurate while the diode-based approach is faster.

Working in concert, the static flatness correction removes frequency response errors, including those from the coupler and diode detector; the dynamic levelling will correct for mismatches that vary with frequency and signal level.

To maintain the desired ±0.5 dB level accuracy across the AUT frequency range, the signal generator can obtain correction factors from the power meter. For greater level accuracy, the external levelling coupler can be added to this configuration.

This makes it possible to accurately measure AUT input power and enables external power levelling for the signal generator (Figure 3).

Figure 3: Simplified configuration with high-power signal generator and external dynamic levelling.

This retains some of the cost and complexity of the original configuration (Figure 1); however, it eliminates the cost of either one broadband preamp or multiple narrowband preamps.

A typical ATE system accumulates signal-power losses throughout a variety of system elements: cabling; switches; and the passive couplers, combiners, isolators and so on, that enable signal sharing.

The availability of greater power from the signal generator can overcome these losses and thereby ensure greater measurement accuracy.

Extra power also makes it possible to insert filters and signal monitors, improving overall measurement quality.

The decision to include a high-power signal generator brings wide-bandwidth and relatively low-noise amplification of stimulus signals to the ATE system.

Ultimately, the use of such a source reduces system cost by eliminating narrowband amplifiers and the associated switching systems.

Protecting the DUT

To prevent damage to unique or high-value DUTs such as satellite systems and components, the high-power signal generator should include a power clamp feature with a response time of 30 µs or faster.

It should also allow the user to set minimum and maximum allowable power levels.

The most failsafe approach is to implement this capability in hardware rather than software: if the instrument is inadvertently reset, the hardware implementation ensures that it will not return to a state of maximum power.

Linking the power clamp feature to external or internal power levelling provides additional protection.

When testing antennas or satellite subsystems the signal source may reside a significant distance from the DUT. On an antenna test range, for example, the transmit antenna may be placed on a tower that is 4.5 to 24 m tall (Figure 4).

Figure 4: A high-power signal generator can overcome cable attenuation in antenna testing.

Satellite subsystems may be placed into a thermal/vacuum chamber but the test system will be outside the chamber, which may be quite large and the access ports may be far above the floor in a high-bay building.

In such cases, the most common solution is to use long runs of coaxial cable. However, these cause considerable losses in RF power and, of course, these losses increase at higher frequencies.

Although the attenuation can vary widely depending on cable quality, typical values for 30 m of coax are 45 dB at 12 GHz and 70 dB at 20 GHz.

One solution is the Heliax type of coaxial cable, which is inherently low-loss; however, it can be difficult to work with because it is rigid and not meant for the frequent movement and reconfiguration of a typical test environment.

An alternative solution is additional amplification within the signal source and the preferred approach is to use cascaded power amplifiers to increase the output power. With the extra output power integrated into the source, there will be savings in cost, space and weight.

A signal generator with high output power provides multiple advantages, ranging from simplified test configurations to reduced size, weight and cost of a test system. The ability to deliver a pure, accurate signal at +25 dBm or greater can ensure improved measurement accuracy and also enable testing of extreme or unusual operating conditions.

By John S Hansen, senior applications engineer, Agilent Technologies