The Challenges of Precision Analog Modulation Measurement

In today’s digital world, many established analog techniques are being replaced by modern digital alternatives. Digital modulation is now commonplace, particularly in mobile communications, but traditional analog amplitude and frequency modulation are still in widespread use. Laboratories performing RF calibration report that analog modulation meters and analyzers are a part of their workload that cannot be ignored. When a new RF calibration source was designed, precision analog modulation was included to address this workload. This paper describes the digital signal processing based techniques used to measure its modulated outputs, and explores the challenges in assessing modulation measurement uncertainties and validating the results obtained. Introduction The need to measure analog modulation has existed since the invention of radio communications and still remains in today’s predominantly digital world. For example, measuring analog modulation is commonplace in the broadcast and civil aviation industries. Making accurate analog modulation measurements is a requirement in many applications. Analog modulation features are found in the majority of signal generators, often alongside digital modulation in the more sophisticated instruments. Consequently, one might expect obtaining traceability for modulation measurements at low levels of uncertainty would be easy. In practice, it is not. The oldest and simplest modulation technique, amplitude modulation (AM), is the most difficult to measure accurately. A number of techniques can be employed, providing uncertainties for modulation index (AM depth) ranging from a few percent down to just below 0.5% at best. Frequency Modulation (FM, developed in the 1930s), is theoretically more complicated but does have the advantage of a mathematical relationship (the Bessel function) that can be exploited to provide an extremely precise determination of modulation index. However, even when this relationship is exploited, the best uncertainties achieved in practice are often a few tenths of a percent. During the development of a new RF source instrument intended for calibration applications with design goals for analog modulation accuracy of better than 0.1% and distortion <0.05% (-66dB), it became rapidly evident that the traditional modulation measurement techniques would not provide the desired levels of uncertainty. The instrument provides amplitude, frequency, and phase modulation from an internal source. Operation with an external modulation source is included, but is not the primary mode of operation. In summary, there are two key aspects to the measurement requirements: • Accuracy of the modulation – to determine the accuracy of the AM depth or Frequency/Phase modulation deviation and the accuracy of the modulation rate. • Quality of the modulation – to determine the amount of distortion present on the modulation and the amount of incidental modulation produced (for example, unwanted AM produced when generating FM). Determining the amount of unwanted (residual) modulation present on the unmodulated output signal is also important. The techniques used for measuring low levels of intentional modulation can also be applied to measuring residuals. A measurement technique based on digital signal processing is described, used for the following modulation characteristics: • AM depth accuracy • AM Distortion • FM deviation accuracy • FM Distortion In practice, the same techniques are used to measure the residual and incidental modulation characteristics, but detailed discussion is beyond the scope of this particular paper. Digital Measurement Demodulator The technique chosen employs one of the new high performance spectrum analyser/measuring receiver based instruments featuring a measurement demodulator. The digital signal processing in the spectrum analyzer, used in the analyzer mode for digital IF filters, is also ideally suited for demodulating FM or AM signals. The block diagram of the analyzer signal processing below (Figure 1) shows the analyzer’s hardware from the IF to the processor. The IF filter is the resolution filter of the spectrum analyzer, with a bandwidth range from 300 kHz to 10 MHz. The A/D converter samples the IF (20.4 MHz) at 32 MHz. Lowpass filtering and reduction of the sampling rate follow the downconversion to the complex baseband. The decimation depends on the selected demodulation bandwidth (DBW) setting. The DBW setting is not a 3dB bandwidth but is the range of frequencies for which amplitude and phase errors are negligible. The output sampling rate is set in powers of 2 between 15.625 kHz and 32 MHz. Useless oversampling at narrow bandwidths is avoided, saving computing time and increasing the maximum recording time. By sampling (digitization) at the IF and digital downconversion to the baseband (I/Q), the demodulator achieves maximum accuracy and temperature stability. The accuracy and stability is maximized by minimizing the analog circuitry prior to the IF analog to digital converter. For this reason only the 10MHz IF filter setting is selected when using the analyzer for modulation measurements. There is no evidence of commonplace errors of analog downconversion and demodulation like AM/FM conversion, deviation error, frequency response or frequency drift at DC coupling. Only the characteristics of the analog IF filter ahead of the A/D converter need to be taken into consideration. The software demodulator runs on the main processor of the analyzer. The demodulation process is shown in Figure 2, the block diagram of the software demodulator. All calculations are performed simultaneously with the same I/Q data set. Measurements results can be displayed as time domain or frequency domain traces, numeric data, and results are also available via the analyzer’s remote interface (GPIB). Figure 1. The R&S FSMR Measuring Receiver/Spectrum Analyzer and its IF to processor hardware.