Strategy for traceability of complex modulated signals using RF waveform metrology David A Humphreys, Matthew R Harper, Lindsay K J McInnes and James Miall National Physical Laboratory, Hampton Road, Teddington, Middlesex, TW11 0LW, UK.
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Abstract RF Waveform metrology provides a traceable link between the CW RF primary standards, the primary electrical risetime standards and the complex RF test-equipment used in the wireless communications industry. The advantages and limitations of different instrument types are compared. Examples are shown of modulation waveform uncertainty calculation and choice of RF filter for pulsed RF transition duration.
1. Introduction Despite the rapid growth of simulation and modeling work in RF design there will always be an eventual requirement for measurement and validation within the design process. The relatively high cost associated with such measurements means that there is always a drive to simplify this overhead. Confidence in the results is important as it allows the number of measurements to be minimized. National Measurement Institutes disseminate RF standards to industry through CW quantities, such as impedance (traceable to dimensional measurements/frequency) and RF power (traceable to voltage and current), driven by the need to provide generic standards that are not tailored to advantage a particular customer. In the past, it has been left to the industry customer to link these CW standards to their own application. Taking wireless communications systems as an example, the modulation formats have become increasingly complex and so the gulf is increasing between the industry need and the simple CW standards. The ability to make some of these complex measures explicitly traceable through waveform metrology gives confidence in the results to both the equipment manufacturer and to the user. Compliance with ISO17025 is a driver for the demand for traceability for wireless communication systems.
2. Waveform metrology The first work on waveform metrology at NPL was driven by the need to provide traceable calibration of Digital Sampling Oscilloscopes (DSO) [1]. Electro-optic sampling, (EOS) coupled with deconvolution and de-embedding techniques, provides the traceability for calibration of these instruments. The starting point for dissemination of traceability for waveform measurements is a well-characterized electrical pulse generator. At NPL we use a photoconductor or photodiode and a modelocked Titanium: Sapphire laser (Ti:S) to give electrical pulses, on a coplanar transmission line, of 10 ns. In many RF applications the required bandwidth is much smaller, typically less than the minimum achievable frequency spacing set by the laser repetition rate. A swept-frequency technique can be used to interpolate between the frequency points [2]. As higher frequencies and wider bandwidths are used, the phase linking over the measurement system bandwidth will become more important. Communications test equipment is designed to accept and demodulate RF signals with a specific modulation format. The output from the test equipment may be a single figure or pass/fail test. This test equipment cannot interpret
the well-defined step or impulse signal measured by the EOS or DSO, traceability must be achieved through more generalized instrumentation that can be used to measure both simple and complex waveforms [3] . Modulated Wideband RF Real-time sources The key criteria are: Power meters Spectrum Analyzers Dedicated 1. Explicit off-line analysis of the measured Dissemination test waveform – this allows validation by a 3rd method Parametric equipment Develop and test party such as an accreditation body. measures with for native 2. The test equipment must not be uncertainties environment WCDMA vendor/feature specific. OFDM EVM UWB 3. The process must not compromise the GSM Simple equipment manufacturer’s intellectual Peak power/ Modulation Other other (AM) formats property through a need to make important Modulation Modulation format design details public. Nonlinear waveform with 4. The system impairments must be testable. subsystem uncertainties
RF waveform metrology has been used to traceably calibrate wideband RF power sensors (2nd generation RF communications, e.g. GSM) where the active element in the receiver is an RF detector diode [4]. More recently we have applied the techniques to WCDMA EVM [3], precision measurement of amplitude modulations [5] and for diagnosing waveform errors. The relationship linking the primary standards to the instruments used in industry is shown in Fig. 1.
3. Instrumentation
development Digital oscilloscope
Key Future work Existing
Sampling oscilloscope Impairment model
Deconvolution and signal processing
RF Waveform metrology
work Numerical methods Test equipment Primary
RF power
Standards
Electro-optic Sampling
RF Impedance Frequency reference
Primary standard traceability
Figure 1 Stages in the RF Waveform metrology process to link the primary standards to the complex modulation format RF test equipment used in industry
DSO and real-time digital oscilloscopes (RTO) can measure both the baseband signals (step or impulse) and modulated RF signals, making them ideal candidates to validate more complex source instruments. A RTO acquires the full RF trace, which must be mathematically demodulated. We have developed a model of the RTO impairments that helps to minimize uncertainties through the choice of RF signal frequencies. Certain designs of DSO with multiple inputs trigger all their sampling gates nearly simultaneously (
