Conformity Magazine - December 2007 - (Page 28) The Role of Instrument Selection Most modern scanning receivers, spectrum analyzers and traditional EMI receivers are super-heterodyne receivers using one or multiple stages to convert the frequency of the RF input signal to a fixed IF. This is achieved by mixing the unknown signal with a local oscillator (LO) signal in a mixing stage. Since a mixer is a non-linear device, its output includes not only the two original signals at the input but also their harmonics and the sums and differences of the input signals and their harmonics. If any of the mixed signals fall within the passband of the IF filter, it is further processed at the IF and finally displayed. After the filtering, the signal is amplified by either a logarithmic or linear amplifier, rectified by the envelope detector, possibly filtered by a low-pass filter (“video filter”) and finally graphically or numerically displayed. EMI receivers as well as spectrum analyzers convert the IF signal to a video signal using an envelope detector. These signals have a frequency range from zero (dc) to some upper frequency which is determined by the detection circuit elements. In its simplest form, an envelope detector consists of a diode followed by a parallel RC combination, as shown in Figure 3 (top). The output of the IF chain is applied to the detector. The time constants of the detector are chosen, such that the voltage across the capacitor equals the peak value of the IF signal at all times, which requires a fast charge and slow discharge time. In case the preceding resolution bandwidth of the receiver has only one spectral line in its passband (meaning, a CW signal is being measured), the IF signal is a steady sine wave with a constant peak amplitude. The output of the envelope detector will be a constant dc voltage without any variation for the detector to follow, as depicted in Figure 3 (top). However, often times there is more than one signal in the IF filter passband. For instance, in case of two sine waves, as shown in Figure 3 (bottom), these interact to create a beat note, and the envelope of the IF signal varies according to the phase change between the two sine waves. The maximum rate at which the envelope of the IF signals can change is determined by the resolution bandwidth. Since IF filters of receivers are not rectangular, the charge time of the detector needs to be a fraction of the reciprocal of the IF bandwidth (e.g., one-tenth) to obtain the envelope of the IF signal. Specific instrument parameters like the selected detector, resolution bandwidth and sweep time do have an impact on the displayed measurement result, dependent on the characteristics of the signal to be measured. Therefore, they can be used to determine if a signal is broadband or narrowband. Figure 1: Generic definition of narrowband and broadband signals Visit: www.elgar.com • Call: 858.458.0223 28 CONFORMITY DECEMBER 2007 Figure 2: Two different types of narrowband signals http://www.elgar.com http://www.elgar.com
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