Vector Network Analyzers Using CMT VNA Software

 

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The Analyzer measures parameters of the DUT in the frequency domain. Time domain transformation is a function of mathematical transformation of the measured parameters in order to obtain the time domain representation.

Time domain function simulates time-domain reflectometry. The meaning of which is to influence the DUT with a pulsed or step signal, followed by the analysis of the reflected signal. The magnitude, duration, and shape of the reflected signal determine the nature of the impedance variation in the DUT. The Analyzer does not affect the DUT either in pulses or steps. Instead, a Chirp-Z transform algorithm is used to calculate time information from the frequency measurements. The Chirp-Z transform is a generalization of the Fourier transform that allows to set arbitrary transform start and stop values.

Transformation Types

The time domain function supports the following transformation types:

•Bandpass mode simulates the response of the bandpass network to the impulse.

•Lowpass impulse mode simulates the response of the lowpass network to the impulse.

•Lowpass step mode simulates the response of the lowpass network to the unit step function.

The time domain resolution in the lowpass mode is twice as high as in the bandpass mode. The bandpass mode determines the distance to the discontinuity but does not provide information about the nature of the discontinuity. The lowpass mode determines the distance to the discontinuity and provides information about the nature of the discontinuity (open or short circuit, for example). The lowpass step mode is useful for the impedance measurement along the distance.

Bandpass mode is applied to the DUTs that do not operate with DC current such as band pass filters. The frequency settings in the bandpass mode can be arbitrary.

Lowpass mode is applied to the DUTs that operate with DC current such as cables.

The frequency settings in the lowpass mode is required to be a harmonic frequency grid, where the frequency value at each frequency point is an integer multiple of the start frequency. The Analyzer can set the harmonic frequency grid from the current frequency settings with one click.

The value of the DUT response at DC is required to be known in the lowpass mode. The DC value cannot be measured directly by the Analyzer. The Analyzer offers two options: the DC value is automatically extrapolated or manually set. The last option is used when the DUT response at DC is well known, for example, for a low loss cable the DC value is:

•"1" for open-ended cable.

•"-1" for a short-circuited cable.

•"0" for a cable terminated with a matched load.

Transformation Unambiguity Range

The time domain response is a periodic function due to the discrete nature of the frequency response. The time domain unambiguity range is determined by the step in the frequency domain:

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Windowing

The time domain response has a ringing due to the finite nature of the frequency response. To reduce the ringing the windowing is applied to the frequency response. The time domain transformation function applies the Kaiser window function. The window function selection is a tradeoff between the ringing reducing and the time domain resolution.

The Kaiser window is defined by the β parameter, which smoothly fine-tunes the window shape from minimum (rectangular) to maximum. The user can fine-tune the window shape, or select one of the three pre-programmed windows:

•Minimum (rectangular)

•Normal

•Maximum

Pre-programmed window types

X-axis Representation

The X-axis units can be set in seconds or distance units (meters or feet). When the distance units are selected, the velocity factor is used to compute the distance from time. The velocity factor setting is located in the cable correction function (See Cable Correction Function).

The two types of reflection can be selected: round trip or one way. The round trip setting shows the total time or distance that the signal travels in both directions along the DUT. The one-way setting shows the time or distance the signal travels in one direction along the DUT.

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The time domain transformation is applied for separate traces of a channel. The trace to which the function is applied must be preselected as active (See Selection of Active Trace/Channel).

Time Domain Transformation Activation

 

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Time Domain Transformation Span

To define the span of time domain representation, its start and stop, or center and span values can be set.

 

 

 

 

Time Domain Transformation Type

 

Time Domain Transformation Window Shape Setting

 

 

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Cable Correction Settings

When the length units are selected the velocity factor setting of the Cable correction function affects the X-axis scale. See Cable Correction Function.

Lowpass Mode Settings

If lowpass mode is used, the frequency range must be set to a harmonic grid. The frequency values in measurement points are integer multiples of the start frequency.

In lowpass mode, the value of the DUT response at DC is extrapolated from the first few frequency points, or manually set. Set the DC value manually if the response of DUT is well known. For example, if the DUT is a cable then DC value is:

•"1" for open-ended cable.

•"-1" for a short-circuited cable.

•"0" for a cable terminated with a matched load.

 

 

 

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