As discussed above, signal generators and amplifiers often create noise and spectral impurities that can adversely affect distortion measurements. Harmonic distortion measurements are no different. Bandpass filtering just prior to the device under test is essential. Before harmonic distortion measurements can be performed, ensure that the distortion of the source is at least 10 dB better than the expected distortion of the device under test. Do not overdrive the spectrum analyzer or ADC, because doing so will also degrade the distortion measurement. As with the IMD/OIP3 measurement, the span widths and resolution bandwidths must be narrowed to lower the effective noise floor and get the most out of the equipment on hand. Averaging is useful to help identify the low-level signals of interest. Figure 3 shows a typical harmonic distortion setup. The critical element in the setup is the bandpass filter, which should have rejection of at least 30 dB.

The plot in Figure 4 shows the third-harmonic distortion of the high-performance amplifier described later. The distortion is only about 10 dB above the noise floor. Having the signal of interest this close to the noise floor makes the measurement difficult and the setup critical. This measurement was made with the fundamental IF at 100 MHz. The third-harmonic product was measured at -91 dBc.

Figure 5 shows the setup used to measure the FFT for examining overall ADC and ADC/driver performance. Figure 6 was produced using Analog Devices' ADC analyzer software, which uses an ADC to make the FFT measurement. The ADC analyzer also provides many other system performance metrics, such as the signal-to-noise ratio (SNR) and spurious-free dynamic range (SFDR), which are of interest when driving amplifiers and ADCs. The measurement is performed in the time domain, greatly simplifying the task. In addition to ease of measurement, it provides real world performance measures of the amplifier and ADC pairing chosen for a particular application. This combination can thus give the designer an accurate measure of performance prior to committing to hardware, while also speeding the design cycle significantly by starting with the actual measurement hardware. While single-tone harmonic distortion is an effective way to determine driver amplifier performance, two tones provide a more realistic performance measure in today's modern digital-IF signal chains (Figure 7).

While measuring the performance of an ADC driver amplifier by itself is necessary, it is ultimately the combined performance of driver and ADC that must fulfill the system's requirements. Fast Fourier transform (FFT)-based ADC data analysis software provides an easy and powerful tool for evaluating a signal chain. Figure 6 represents the broadband performance of the AD8352/AD9445 combination at 190 MHz with a 105 MHz sampling rate.

Leveraging the basic broadband performance, narrowband optimization may be employed to further enhance the overall circuit performance for narrower bandwidths.

The AD8352 has a forgiving harmonic vs. frequency slope. This unique ability, along with its ability to further enhance its harmonic distortion characteristics, makes it an ideal subject for measuring harmonic distortion. The device tuning is accomplished via capacitor CD and resistor RD, its external distortion tuning components. Figure 8 shows these components along with the circuit topology for the narrowband optimization of the amplifier for driving the AD9445 ADC for distortion evaluation. Each IF frequency can thus be optimized to further enhance the distortion performance at a given frequency of interest. By lowering the third-order harmonic distortion, better SFDR levels can be achieved. Note that while improved in-band performance can be achieved, it may be at the expense of out-of-band third-order products, which must be evaluated during frequency planning Figure 9 shows the enhanced narrowband performance tune. Such low third-order harmonic products as measured here by the ADC analyzer can also be measured discretely with a spectrum analyzer. Care needs to be taken to make sure the input levels are clean and the output levels are correctly matched to the spectrum analyzer.

As communication transceivers move toward higher frequencies, higher speeds, and wider bandwidths (including higher resolution), the ADC driver amplifier plays an increasingly important role on the overall capability of the system. This article provides a brief description of various methods used to evaluate critical specifications of high-performance amplifiers that are intended for use in modern high-performance communications systems. The amplifier used for evaluation in this article was chosen due to its high OIP3 and its simultaneous low harmonic generation at high frequencies. This level of performance is what modern ADC drivers must deliver, and it indicates the level of difficulty in measuring these parameters today and into the foreseeable future.

Eamon Nash is applications engineering manager for RF standard products at Analog Devices. He holds a Bachelor of Engineering degree in Electronics from the University of Limerick, Ireland.

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