Conformity Magazine - December 2007 - (Page 32) A scanning receiver or spectrum analyzer will therefore display a response every 1/PRF seconds with an amplitude proportional to the spectrum envelope at the tuning frequency of the instrument. This phenomenon is used for the discrimination of narrowband and broadband signals. When changing the displayed frequency span on the instrument, the spacing of responses resulting from a broadband signal will not change, since they are a time phenomenon. In case of a narrowband signal, the responses are a frequency phenomenon and a change in span will cause a change in the spacing of the displayed responses. A change in sweeptime, however, will not affect the spacing of narrowband responses but have an impact on the spacing of the broadband responses. Slower sweeptimes will cause the display to show more responses, since more responses will be intercepted during a single sweep. This discrimination method is useful to quickly determine the signal characteristic. However, if a complex spectrum is displayed, it may be difficult to observe the changes in spacing of responses. Tuning Test Some older commercial and military EMC standards propose a tuning test as a method for discrimination between narrowband and broadband signals. This test involves the de-tuning of a receiver by one or two impulse bandwidths to either side of the initial tuning frequency. The initial tuning frequency is to be identical with the frequency of the maximum signal response observed. The observed amplitude change on either side is then compared to a criterion (e.g., 3 dB or 6 dB) to determine if the signal is narrowband or broadband. If the de-tuning results in an amplitude change greater than the criterion, the signal is considered narrowband. Conversely, if the amplitude change on either side of the initial tuning frequency is less than the criterion, the signal is determined to be broadband. This method can provide inconclusive results when the detuning on one side of the maximum response is larger than the criterion, and on the other side a smaller amplitude variation is determined. This situation can occur if a signal spectrum is investigated that is rather complex, which may not allow the exact determination of the frequency at which the maximum response really occurs. Furthermore, this method requires the knowledge of the impulse bandwidth of the instrument, which is not identical to the 3 dB or 6 dB bandwidth of the measuring instrument. Furthermore, this method was initially based on the use of a fixed tuned receiver; as such, this approach is not suitable for automated testing. Summary In the literature and standards, four main methods for the determination of signal characteristics are described. Their main aspects are summarized in the Table 1. Their advantages and limitations have been described, and the peak versus average detector method has been identified as most suitable. This method is also called out by most EMC standards that currently require the determination of signal characteristics as part of the compliance measurement process. Werner Schaefer is a technical leader and quality manager with Cisco Systems, and can be reached at wsemc@cisco.com. Acknowledgement The author would like to thank Mrs. Tori Barling for proof reading this manuscript. References 1. W. Schaefer, “Understanding Impulse Bandwidth Specifications of EMI receivers,” IEEE International Symposion on EMC, 1999, pp. 958-961 W. Schaefer, “Measurement of Impulsive Signals with a Spectrum Analyzer or EMI receivers,” IEEE International Symposion on EMC, 2005, pp. 267 – 271 2. FAST Link www.conformity.com/1250 Discrimination Method Bandwidth Test (par. 3) Peak vs. Average Test (par. 4) Sweeptime Test (par. 5) Tuning Test (par.6) Narrowband No change in amplitude No change in amplitude No change in response spacing Δ amplitude > 3dB (6 dB) Broadband Change in amplitude Change in amplitude Change in response spacing Δ amplitude < 3dB (6 dB) Table 1: Main aspects of signal charateristics 32 Conformity DeCember 2007 http://www.conformity.com/1250
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