S
SCALE FIDELITY
Dynamic accuracy
SCALAR NETWORK ANALYSIS
Measurement technique used to characterize the transmission and reflection behavior of single or multiport networks. The magnitudes of the transmission factor and the reflection coefficient are measured.
SELECTIVE DETECTION
Method used in scalar network analysis which uses a selective measuring receiver tuned to the measurement frequency. This method is mostly used with spectral analyzers equipped with a tracking generator.
Advantages over broadband detection:
- High dynamic range of more than 100 dB
- Insensitive to tracking generator harmonics.
SENSITIVITY
The noise floor of the analyzer determines the smallest level which can still be measured for a given resolution bandwidth. Signal levels below the noise floor cannot be measured, and signal levels just above the noise floor will be measured inaccurately. The sensitivity indicates the signal level required for a display level 3 dB above the average noise level. The relationship between sensitivity, noise figure and equivalent noise bandwidth is given by:
S/dBm = Ltherm/dBm + 10 log (NBW/Hz) + F/dB + 3
where:
S = sensitivity
Ltherm = thermal noise floor (–174 dBm)
NBW = noise bandwidth
F/dB = noise figure in dB
SETTLING ERROR
An amplitude error caused by sweeping the set frequency range too rapidly for the selection filter to settle properly (sweep time). In addition, a frequency error occurs, when the SWT has been much too short. In the automatic mode the parameters are set in such a way that this error is not recognizable.
Amplitude and frequency are measured incorrectly with too low SWT
SHAPE FACTOR
Defines the selectivity of a filter. Usually the ratio of the 60 dB to the 3 dB (sometimes 6 dB) bandwidth (RBW) of the selection filter is used here. The shape factor indicates how well the spectrum analyzer can discriminate between closely spaced signals of different amplitudes.
The shape factor characterizes the selection filter
SHORT-TERM STABILITY
Variations in the frequency and amplitude of the local oscillator signal measured over short periods of time of a few seconds or less. These variations may be interpreted as modulations. The short-term stability governs the ability of the analyzer to discriminate between closely spaced signals of different amplitudes and affects the reproducibility of measurements made using „narrow” resolution bandwidths. The short-term stability is expressed in terms of residual FM, residual AM and single sideband noise.
SIDEBAND NOISE
Single sideband noise
SIGNAL TRACK
Automatic adjustment of the center frequency to ensure that the displayed signal is centeredon the screen (i.e. center of sweep range) after each sweep.
SINGLE SIDEBAND NOISE
Describes the short term stability of the local oscillator. Noise sidebands are modulated onto the carrier as a result of nonlinear effects in the LO. The power of these sidebands decreases as the spacing from the carrier increases. A distinction is made between amplitude noise (random variation in level stability) and phase noise (random variation in frequency/phase stability).
Phase noise is the dominant factor, and is specified as the absolute power level of a sideband offset from the carrier by a frequency foff referred to a measurement bandwidth of 1Hz and the carrier power level. It is expressed in dBc/Hz (dB). As single sideband noise is converted into the IF level, it can be used as a measure of the ability of the analyzer to discriminate between closely spaced signals of different amplitudes.
SPAN
Also called frequency span. The frequency range swept by the analyzer during a measurement. The span can usually be set either directly via the Span parameter or indirectly via the parameters „Start frequency“ and „Stop frequency“. When setting the span, care should be taken to select a value which is appropriate for the measurement task. If the span is too large, too long sweep times might result (in particular for small resolution/video bandwidths), while too small a span might prevent the detection of spectral portions which then lie outside the monitored range.
SPECTRAL PURITY
Describes the degree to which the local oscillator signal is free of !spurious products, and hence indicating the accuracy of the displayed input signal. The ratio of the peak carrier level to the spurious responses (intrinsic noise) and single sideband noise is usually quoted.
SPECTRUM ANALYSIS
Measurement technique used for displaying the variation of amplitude and/or power level of a signal over the frequency.
SPURIOUS RESPONSES
Refers to signals occurring within the analyzer and which are displayed, but which are not part of the signal to be measured. Such spurious responses are visible in spectrum analysis as spectral lines at the fundamental or harmonics of the spurious signals, or as sidebands on the test signal. Among others, spurious responses are caused by:
In network analysis, the most important spurious response is crosstalk.
STABILITY
Drift, short term stability
STANDARDS
Standard devices used as references. In spectrum analysis, usually a fixed frequency calibration source; in network analysis, a standard short-circuit and a standard open-circuit used for reflection measurements or a standard thru-connection for transmission measurements.
STANDING WAVE RATIO (SWR)
Sometimes called voltage standing wave ratio. Describes the deviation of an impedance from the characteristic impedance Z. The SWR is calculated from the reflection coefficient r.
SWR = (1 + |r|) / (1 – |r|)
SWEEP TIME
The time taken for the analyzer to sweep the entire frequency span set. The sweep time must be chosen to match the resolution bandwidth, video bandwidth and span, since the IF filters have a finite response time. In spectrum analysis, the following basic relationship should be observed (automatic mode):
SWT > K [SPAN/(RBW)2], where K ~ 2
VBW % RBW
SYNTHESIZER
A signal generator which “synthesizes” the output frequency by digital manipulation of a single timebase. The usual operations are multiplication, division or mixing (heterodyning). The advantage of a synthesizer is that each settable frequency has the same accuracy and stability as the timebase. Thus the frequency-related measurement results, e.g. of the markers, will be much more accurate than with units without synthesizer.
Modern spectral analyzers use a synthesizer as local oscillator to increase frequency accuracy.
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