Atec Agilent-8703A - Manual (Page 13)

Introduction
Analog and digital device design, testing and trouble
shooting are made easier by using both the distance-time
domain and frequency domain capabilities of the 8703A.
This combination lets the user:
1) Discover if a problem exists.
2) Locate and quantify potential causes of the problem
(i.e. unexpected reflections, attenuations, etc.)
3) Simulate frequency domain and distance-time
domain results with unwanted responses
mathematically removed using the Gating function.
Method
A step or impulse response is simulated by processing
frequency domain data through an inverse Fast Fourier
Transform (FFT). This produces a linear distance-time
response. This is similar to a time domain reflectometer
(TDR) response done with a broadband oscilloscope
and a small signal step or impulse stimulus. Data is
displayed in a parameter-versus-time format for
transmission and reflection parameters.
Features
Measurement range is the maximum distance or time
span that can be displayed given that the test signal stays
within the dynamic range of the 8703A. Range (Ta), also
called “alias free range”, is defined below:
Ta = (N-1) / Freq. Span
where “N” = number of CRT data points
42
. If N = 201 points
and Freq. Span = 20 GHz then Ta = 10 nano- seconds (or
approximately 2 meters in fiber cable with a 1.4 index of
refraction). Longer ranges are achieved by changing the
key parameters.
Measurement range-resolution is a measure of the
8703A’s ability to locate a single response and is defined as:
Tr = (Time Span) / (N-1)
where the “Time Span” is the span of time displayed on the
8703A’s CRT. “N” is the number of display data points
41 .
For example, range-resolution is 0.5 pico- seconds for a
time span of 0.4 nanoseconds and N = 801 in the bandpass
mode. This is approximately 0.1 mm in single-mode fiber.
Response resolution is the smallest distance or time
between two responses, where each response can be
identified. Response resolution is estimated for the three
stimulus types available in the 8703A:
Lowpass step response 42 :
Tr = (0.45 / Fspan) x 1.0 (min.) window factor
2.2 (normal)
3.3 (max.)
Lowpass impulse response 43 :
Tr = (0.6 / Fspan) x 1.0 (min.) window factor
1.6 (normal)
2.4 (max.)
Bandpass impulse response 43 :
Tr = 2 X (0.6 / Fspan) x 1.0 (min.) window factor
1.6 (normal)
2.4 (max.)
Where the “Fspan” is the frequency span of the frequency
domain measurement. For example, if the Fspan is 20 GHz
and a normal window factor is used for the lowpass impulse
mode, then the response-resolution is 48.5 picoseconds
(approximately 5 mm of separation between reflection
responses in fiber cable)
44 .
Window factors control the pulse width or step rise time
used in the inverse Fourier transform. Minimum, normal
and maximum windows are user selected to make trade
offs between time resolution versus overshoot and ringing
in the response.
Distance-time markers can be used to automatically
calculate and display length and location of optical and
electrical responses. The relative velocity factor or
refractive index value used in the marker calculations can
and should be set to match the medium being used.
Gating enables some frequency domain and distance-time
domain test conditions to be isolated and simulated.
For example, unwanted reflection and transmission paths
within a device can affect a device's response. Gating
enables the effect of these unwanted paths to be marked
and mathematically removed in the distance-time domain.
This new simulated response can also be viewed in the
frequency domain while the gating function is active.
In this way the simulated effect of a design change can
be evaluated.
41 Lowpass impulse and step modes have a 201 CRT data point
maximum limit. This does not apply to the bandpass mode.
42 Effective rise time of the 8703A’s step signal is equal to the
response resolution “Tr”.
43 Effective pulse width (full-width-half-maximum) of the 8703A’s
impulse signal is equal to the response resolution “Tr”.
44 Calculated time is for the actual distance traveled. Reflection paths
must be considered to estimate physical locations. The 8703A
automatically calculates the distance traveled for a reflection
measurement and displays the “one way” path length. Multiple
reflections in transmission paths are not automatically accounted for.
13
Distance-time domain
performance summary