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Optimizing Measurement Results
Making Accurate Measurements of Electrically Long Devices
Making Accurate Measurements of Electrically Long Devices
A device with a long electrical delay, such as a long length of cable, a SAW filter, or normal devices
measured over wide sweeps with very fast rates presents some unusual measurement problems to a
network analyzer operating in swept frequency mode. Often the measured response is dependent on the
analyzer's sweep time, and incorrect data may be obtained. At faster sweep rates, the magnitude of the
response may seem to drop and look distorted, while at slower sweep rates it looks correct. The results may
indicate that a cable has more loss than it truly does, or that a filter has some unusual ripple in the passband
which is not really there.
This section describes the cause of this behavior, and how to accurately measure these electrically long
devices.
The Cause of Measurement Problems
When using a vector network analyzer to measure a device that has a long electrical delay (T), the device's
time delay causes a frequency shift between its input and output signals. The frequency shift, F, equals the
product of the sweep rate and the time delay:
F = dF/dt T
Since frequency is changing with time as the analyzer sweeps, the time delay of the DUT causes a frequency
offset between its input and output. In the analyzer receiver, the test and reference input signals will differ in
frequency by F. Because the test signal frequency is slightly different than the receiver frequency, the
analyzer will err in measuring its magnitude or phase. The faster the analyzer's sweep rate, the larger F
becomes, and the larger the error in the test channel.
The analyzer does not sweep at a constant rate. The frequency range is covered in several bands, and the
sweep rate may be different in each band. So if an operator sets up a broadband sweep with the minimum
sweep time, the error in measuring a long device will be different in each band, and the data will be
discontinuous at each band edge. This can produce confusing results which make it difficult to determine
the true response of the device.
To Improve Measurement Results
To reduce the error in these measurements, the frequency shift, F, must be reduced. F can be reduced by
using the following methods:
• decreasing the sweep rate
• decreasing the time delay (T)
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