How to Minimize the Measurement Uncertainty in RF Signal Generation

Introduction

Test equipment characterizes design performance and verifies devices under test function as expected. To be effective, test equipment must outperform the device under test. Minimizing measurement uncertainty attributed to test equipment requires careful consideration of all components in the testbed.

This application note helps you minimize measurement uncertainty attributed to test stimuli in a test system.

Discover how to achieve the best signal quality for vector modulated signals using RF vector signal generators.

Vector Signal Generators

A great deal of systems tests has shifted to vector signal generators (VSG), driven by the popularity of digital modulation schemes in today’s communications systems. VSGs, equipped with dual arbitrary waveform generators (AWG), generate complex baseband I/Q waveforms.

Dual AWGs control the playback sequence of waveform segments, which are downloaded into the random-access memory (RAM) in the internal baseband generator. The baseband generator output I/Q signals travel to the I/Q modulator and upconvert to an intermediate frequency (IF) and RF. The RF output section includes amplifiers, attenuators, and an automatic leveling control (ALC) circuit to maintain precise control of the output level. In the next section, we explore common sources of error from these sub-systems and how to overcome these problems.

Waveform Phase Discontinuity

Arbitrary waveform generators are frequently used to repeatedly playback a previously sampled waveform. A side effect of waveform playback is spectral regrowth and distortion, caused by phase discontinuity between the end of a waveform and the start of the next repetition.

For example, the sampled waveform in Figure 2 does not cover the entire period of the sinewave. When playing back the waveform repeatedly, a phase discontinuity appears at the transition point between the end of the waveform and the beginning of the next retransmission. This phase discontinuity results in periodic spectral regrowth and distortion. Figure 3 shows the impact with and without phase discontinuity.

Avoiding Phase Discontinuities

Simulating an integer number of periods when creating your waveform segment can avoid phase discontinuities for periodic waveforms shown at the top of Figure 4.

For time division multiple access (TDMA) or pulsed periodic waveforms, resolve phase discontinuity by adding off-time at the beginning of the waveform, and by subtracting an equivalent amount of off-time from the end of the waveform as shown in the button of Figure 4. Making this adjustment can avoid spectral regrowth.

Interpolation Overshoot

Dual AWGs provide flexibility when generating modern complex modulation signals. You can simulate your design and create the waveform files on a PC, then use the AWGs to convert the waveform files into analog signals. However, you must be cautious about introducing unexpected errors due to interpolation overshoot during digital-to-analog conversion.

Digital-to-Analog Conversion Over-Range

Baseband generator uses an interpolation algorithm to resample and reconstruct a waveform. However, interpolation can cause overshoots which result in a digital-to-analog converter (DAC) over-range errors.

For example, if a baseband waveform has a fast-rising edge, the interpolator’s filter overshoot becomes a component of the interpolated baseband waveform as shown in Figure 6. This response causes a ripple or ringing effect at the peak of the rising edge. This ripple overshoots the upper limit of the DAC output range (red line), causing the signal generator to report a DAC over-range error.

Scaling a Waveform to Eliminate DAC Over-Range Error

To avoid the DAC over-range problem, you must scale the I and Q input values, so that any overshoot remains within the DAC range. Scaling reduces the amplitude of the baseband waveform while maintaining its basic shape and characteristics, such as peakto-average power ratio (PAPR) as shown in Figure 7. To achieve the maximum dynamic range, select the largest scaling value that does not result in a DAC over-range error.

Accelerate the Evaluation with Runtime Scaling

New vector signal generators, such as the Keysight MXG N5182B and Keysight EXG N5172B provide runtime waveform scaling so that the tradeoff between distortion performance and dynamic range can be evaluated in real-time. This feature does not impact stored data, and you can apply runtime scaling to either a waveform segment or waveform sequence.

In Figure 8, you can see an example of evaluating distortion performance by adjusting the waveform scaling from 100% to 70%. The scaling reveals a 5-dB improvement in the third-order intermodulation distortion (IMD3).