Question
I have a cable of around 120 m with a velocity factor of 0.7 and a ZVL3.
The length of the cable should be measured.
Which settings should I use?
Which settings should I use for the Transform Time Domain mode?
Answer
When using the ZVL you can measure the length of a cable in the Transform Time Domain mode.
You can make different settings for the Transform Time Domain mode which will have an influence
- on the resolution of the length measurement and
- on the distance you can measure.
These two parameters counteract each other. If you set up a measurement for high resolution, you can
only measure short cables. If you set up a measurement for long cables, this will reduce the resolution.
The resolution is determined by the frequency span. For example, a span of 3 GHz means a resolution of
2/(3 GHz) = 666 ps, which corresponds to an electrical length of 20 cm. In the Low Pass mode (see manual
for detailed explanation), you can double the span, which results in an electrical length of 10 cm.
The distance you can measure depends on the frequency step size (frequency span / sweep points - 1).
The reciprocal of the frequency step size is the ambiguous range. The reflected signal is repeated after this time.
A stop frequency of 3 GHz, a start frequency of 9 kHz and 4001 sweep points yield a step size of 750 kHz,
which corresponds to an electrical length of around 400 m. Because the wave ran two times through the cable
(forward and back), the result has to be divided by 2, which corresponds to an electrical cable length of 200 m.
What is the best setting for a 120 m cable with a velocity factor of 0.7?
The electrical length is 120 m/0.7 = 171.4 m.
This means that the frequency step size must be (3e8m/s)/(2*175m) = 850 kHz. When using the Low Pass
mode, this is also the start frequency.
With an ZVL3, the following applies: 3 GHz/850 kHz = 3529, which means that the number of sweep
points must be 3529.
Make the following settings:
- Preset
- Select S11
- Set the velocity factor to 0.7 in the Mechanical Length menu
- Set the number of sweep points to 3529
- Set the start frequency to 850 kHz
- Switch on the Transform Time Domain mode
- Change the Time Domain Stimulus Axis to Distance
- Change the Stop Distance to 122 m
- Select the Low Pass Impulse in Define Transform. Adjust the Low Pass Settings by keeping the 'Stop Frequency
and Number of Points' and push the Extrapolate button in the same menu once to obtain the DC value
- Perform a full one-port calibration on port 1
- Connect the cable and measure the length of the cable
