R
REFERENCE
Value to which all level measurements are referred. A measurement at the reference level has maximum accuracy, since the scaling error (dynamic accuracy) disappears at this point.
REFLECTION COEFFICIENT
A measure of the deviation of an impedance Z from the specified characteristic impedance Z. Can be expressed as a linear (r) or logarithmic (~) quantity. Z û Z,
r = Re(r) + j x Im(r) = Z + Z,
RESIDUAL FM
Measure of the short term stability of the displayed frequency as determined by the instability of the local oscillator. Expressed in terms of peak-to-peak frequency deviation. The residual FM determines the minimum resolution bandwidth which can be used, since at smaller bandwidths, the residual FM becomes visible.
RESIDUAL RESPONSES
Spurious responses of the analyzer in the absence of an input signal, usually given for the input attenuator setting of 0 dB with the input terminated with the characteristic impedance.
RESOLUTION BANDWIDTH RBW
Normally the 3 dB (occasionally the 6 dB) bandwidth of the IF filter used for selecting the signal to be measured. The RBW describes the ability of the spectrum analyzer to discriminate between adjacent signals of similar amplitude. Only signals spaced at a frequency of more than the RBW can be discriminated from one another.
For measurements on signals with closely spaced components, such as two-tone signals or sideband noise, an analyzer with a narrow RBW is required. For measurement of broadband signals such as TV carriers or pulse spectra, a wide RBW is necessary. The RBW indirectly affects the
as it determines the equivalent noise bandwidth of the analyzer.
As described above the correct selection of resolution bandwidth may have a critical influence on the measurement results.
Narrow filters:
- increase the measurement sensitivity of the receiver by reducing noise power.
- allow signals with a small frequency spacing to be separately displayed on the screen.
- increase the sweep time due to significantly longer settling times as compared with broadband filters.
(A reduction of the resolution bandwidth by a factor of 3 of the original bandwidth will increase the sweep time by a factor of 32, i.e. by a factor of 9 against a 3 times larger bandwidth).
- are hardly suited for signals with rapidly changing amplitudes (e.g. radar, TV) and might lead to significant amplitude errors with such signals.
High resolution bandwidth = 300 kHz
Low resolution bandwidth = 30 kHz
Broadband filters:
- reduce the measurement sensitivity of the analyzer by increasing the detected noise power of the analyzer.
- do not allow signals with a small frequency spacing to be separately displayed on the screen.
- reduce the sweep time due to the significantly shorter settling times as compared with narrow filters.
- are necessary for signals with rapidly changing amplitudes (e.g. radar, TV) in order to prevent the amplitude errors caused by narrow filters with their possibly too long settling times.
In general it can be said that CW signals, i.e. signals whose amplitude or frequency does not or hardly change during the monitoring period, can be measured using narrow filters. Filters need to be sufficiently narrow in particular when measuring signals with a tight frequency spacing (thumb’s rule: filter bandwidth = 1/10 of the frequency spacing) in order to be able to display the separate signals on the screen.
In case of rapidly changing signals (e.g. TV signals) a sufficiently wide resolution bandwidth (and video bandwidth) needs to be selected in order to prevent amplitude errors (settling errors). When measuring unknown signals the resolution bandwidth and the video bandwidth should be changed in steps and the effects on the measurement results monitored.
Any “drop” in signal amplitudes might indicate the selection of too narrow bandwidths.
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