Making Accurate Measurements Of Electrically Long Devices; The Cause Of Measurement Problems; To Improve Measurement Results; Decreasing The Sweep Rate - Agilent Technologies 8719D User Manual

Making Accurate Measurements of Electrically Long Devices

A device with a long electrical delay, such as a long length of cable or a SAW flher, presents
some unusual measurement problems to a network analyzer operating in swept sweep mode.
the measured response is dependent on the analyzer's sweep time, and incorrect data
Often
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 isn't really there.
This section describes the cause of this behavior, and how to accurately measure these
electrically long devices.
When using a vector network analyzer to measure a device that has a long electrical delay
(AT), the device's time delay causes a frequency shift between its input and output signals. The
frequency shift, AF, equals the product of the sweep rate and the time delay:
AF= dF/dt * AT
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 AF'. 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 AP becomes, and the larger the error
in the test channel.
The HP 8719D/20D/22D network analyzers do 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.
lb reduce the error in these measurements, the frequency shift, AF, must be reduced. AP can
be reduced by using the following three methods:
decreasing the time delay (AT)
n
using stepped sweep mode
n

Decreasing the Sweep Rate

The sweep rate can be decreased by increasing the~,,,anaIyzer's sweep time. lb increase the
step m @'J keys, or the front panel keypad enter in the appropriate sweep time.
Selection of the appropriate sweep time depends on the device being measured; the longer the
electrical delay of the device under test, the slower the sweep rate must be. A good way to
tell when the sweep rate is slow enough is to put the vector network analyzer into a stepped
frequency mode of sweeping, and compare the data. In this mode, the vector network analyzer
does not sweep the frequency, but steps to each frequency point, stops, makes a measurement,
then goes on to the next point. Because errors do not occur in the stepped-frequency mode, it
Optimizing Measurement Results 5.51