5-8
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 indic a te tha t 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 si gna ls.
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 fre q uency 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:
• using step sweep mode
• decreasing the sweep rate
• decreasing the time delay (∆T)