D
DECIBEL, (dB)
If two signals have the power level P1 and P2, their level difference is 10 x log(P2/P1) dB. An ideal way to express amplification and attenuation is just to offset the power levels P1 and P2 at the input and output of a quadripole against one another.
Note: If you calculate with voltages (U1 and U2) and not with power levels, the level difference is 20 x log(U2/U1).
It is also possible to indicate absolute levels comparing the actual level to a reference level. Variants on the decibel (dB) measure used in spectrum and network analysis are:
- dB
spectrum analysis: absolute voltage level referred to 775 mV
network analysis: level difference (gain/loss) referred to the reference level (0 dB = 775 mV)
- dBµV
absolute voltage level referred to 1 µV (0 dBµV = 1 µV)
- dBm
absolute power level referred to 1 mW (0 dBm = 1 mW)
- dBc
level difference referred to carrier level (c)
- dBc/Hz
noise level difference referred to carrier level calculated for a measurement bandwidth of 1 Hz.
DIRECTIONAL BRIDGE
Circuit based on the principle of the Wheatstone Bridge; used for measurement of reflection coefficient. Can also be used for low-reactive injection of signals.
DIRECTIVITY
A measure for the performance of the directional bridge or directional coupler. It specifies its ability to separate the forward and backward (reflected) components of a signal. A finite directivity, D, gives the following measurement error for the reflection coefficient r:
|rmeasured| = |r ± D|
DISCRETE SPURIOUS RESPONSES
Spurious products
DISTANCE TO FAULT (DTF)
DTF measurements are a performance verification and failure analysis tool for any kind of transmission line or antenna. It uses the Frequency Domain Reflectometry. DTF measurements indicate either the return loss or the standing wave ratio versus distance. The velocity factor of the cable measured must be known. The following formula shows the mathematic coherence:
Dmax = (f * c * V ) / 2
where Dmax = maximum distance , c = speed of light, f = frequency of the standing wave, V = velocity factor
The maximum distance to be measured should also be known in advance. The entered distance affects the frequency range in which the measurement is performed; the formula below shows the relationship between the maximum distance measured and the frequency span being used.
D = (FACTOR * Vrel * Xpoints) / (F2 - F1)
where D = distance to fault, FACTOR = system specific factor, Vrel = relative propagation velocity, Xpoints = number of frequency data points, F2 = stop frequency , F1 = start frequency
The return loss measurement (in dB) shows
a problem in a distance of 21.45 m.
DRIFT
Relative local oscillator (LO) frequency change measured over a fixed period of from one hour to one year. Caused by thermal response and aging effects. Drift influences frequency accuracy and reproducibility at narrow resolution bandwidths. Innovative designs such as the Willtek 9101 Handheld Spectrum Analyzer with LO synthesizer and digital IF reduce the drift into the ppm range.
DRIVE LEVEL
Mixer level
DYNAMIC ACCURACY
Also known as amplitude accuracy, scale fidelity; a measure of the accuracy of measured levels which are not equal to the reference level. Inaccuracies are caused by nonlinear behavior of variious analyzer circuits, in particular:
- Compression effects in the mixer and IF stages
- Errors in the logarithmic amplifier characteristic
- Detector nonlinearities
- Calibration uncertainty
Innovative designs with digital IF eliminate errors in logarithmic amplifier characteristic and detector linearity and part of calibration uncertainty as well.
DYNAMIC RANGE
Network analysis: The greatest difference in level which can be measured to a given degree of accuracy; normally the level difference between the analyzer noise floor and the onset of compression.
Spectrum analysis: The greatest difference in level between two signals applied simultaneously to the analyzer input which can be measured to a given degree of accuracy. Three main interpretations of the term are distinguished, depending on the source of the measurement error; they cannot be interconverted.
The following diagram illustrates the relationships between the dynamic ranges:
The three types of dynamic range are
- Intermodulation-free dynamic range (A)
This describes the maximum difference between the noise floor and the level at which the intermodulation of the analyzer just do not exceed the level of the noise floor. Other spurious responses than intermodulation are ignored.
- Interference-free dynamic range (B)
This describes the maximum difference between the noise floor and the level at which the spurious responses of the analyzer (intermodulation, distortion) just do not exceed the level of the noise floor.
- 1 dB compression dynamic range (C)
This describes the maximum level difference between the noise floor and the level at which the measurement error of the analyzer due to compression is 1 dB. The spurious responses of the analyzer are ignored.
There is an optimum analyzer drive level (mixer level) for each set of conditions which maximizes each type of dynamic range.
The automatic setting facilities of modern analyzers are normally programmed to give the best possible interference-free dynamic range. To achieve optimum conditions for the other types of dynamic range, the analyzer may be set manually.
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