Conformity Magazine - December 2007 - (Page 31) of the signal to be measured. For example, a video bandwidth setting of less than 100 Hz will result in the display of the average value of the signal depicted in Figure 4. It should be noted that the instrument is to be used in linear display mode in order to obtain the average value of the signal under investigation. The proper video bandwidth setting can be easily determined empirically by reducing the video bandwidth stepby-step and observing the resultant amplitude change. If further reductions in video bandwidth do not cause further reductions in measured amplitude, the proper video bandwidth for making an average measurement has been found. The comparison of peak and average detected signal amplitudes, allows the conclusive determination of signal characteristics without changing the resolution bandwidth. This method can also be automated easily and thus allow further automation of the overall compliance measurement process. Sweeptime Test The presence of broadband signals is easily noticeable when a measurement is performed with a scanning receiver or spectrum analyzer. Moving responses can be observed on the instrument display; their actual location and number are dependent on the relationship of the pulse period and the sweeptime setting of the instrument. Figure 6 (top graph), shows how a scanning receiver or spectrum analyzer intercepts an impulsive signal when a slow, single sweep and peak detection is used. The impulse envelope is depicted on the vertical frequency axis, and the occurrences of the impulse are indicated by vertical frequency lines spaced along the time axis. The impulse of the period TP, is detected only half way through the receiver sweep. The measured amplitude at the detection instant is determined by the envelope of the pulse spectrum, as traced out by the IF bandwidth, and represents the impulse response of the receiver to the input signal. The bottom graph of Figure 6 represents the scanning receiver’s display, showing responses only at the detection instances. It is important to note that the pulse repetition frequency (PRF) cannot be determined directly from the display by measuring the frequency difference between two responses with marker functions, since a broadband signal is measured. The receiver’s IF bandwidth is much wider than the PRF; thus, the displayed responses are individual input pulses separated by the pulse period and the frequency, and may be calculated from the sweep time of the receiver. The correct interpretation of the measurement result is difficult without prior knowledge of the presence of a broadband signal. After a single sweep, it is not obvious that the displayed responses are due to an impulse and not caused by individual sinusoidal signals or some type of modulation. However, a narrower measurement span and longer sweep time will lead to more intercepted pulses; hence, the wellrecognized sin(x)/x envelope shape will be traced out, and the impulsive signal will be easily identified. Broadband signals are displayed as time domain responses with amplitudes that are proportional to the envelope of the spectrum. With the instrument tuned to a particular frequency at a point in time, the spectral lines contained within the impulse bandwidth [1] around the tuning frequency, will add periodically at a rate corresponding to the signal PRF. As the analyzer is tuned to a different frequency, the maximum pulse amplitude will change in relation to the change in the envelope of the pulse spectrum. http://www.nec-tokin.com/english/ http://www.nec-tokin.com/english/
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