User`s guide

9
points, so set the start, stop,
and step frequencies
accordingly.
Care should be taken when
measuring frequency-inverting
devices, where the IF frequency
sweeps downward in frequency
as the RF is swept upward. In
this case, the reference and test
sweeps as measured at the HP
71500A input should both be in
the same direction (i.e., either
both upwards or both
downwards in frequency). This
means that the RF sweep
direction will reverse from the
reference to the test sweep.
What frequency span to use?
When performing a
normalization on a frequency
translating device, there are
two choices for the frequency
range to use for the reference
sweep: either the frequency
span corresponding to the
DUT’s input, or to its output. In
both cases, either the input
cable or the output cable will be
measured during the reference
sweep at a different frequency
than during the test sweep.
This will not be a problem if
high quality interconnect cables
of the appropriate type for the
frequency range of the
measurement are used. Cables
such as these will have flat
group delay over the full RF and
IF frequency range. Their
contribution to absolute delay
will then be removed by the
normalization process.
When using good cables, the
choice of which frequency span
to use for the reference trace is
simple. For relative
measurements, use the
frequency span corresponding to
the DUT output for establishing
the reference trace. This will
remove any measurement
irregularities due to the HP
71500A (these are more
significant at lower frequencies,
especially below 300 MHz).
For absolute measurements, the
first choice for establishing the
reference trace is to use the
span corresponding to the DUT
input. This ensures that the
delay contribution of the
modulator will be removed. If
the modulator delay is
independent of carrier
frequency, then the
normalization could be done at
the span corresponding to the
DUT output, which has the
advantage of removing any
irregularities due to the HP
71500A. The delay of some FM
modulators is also a function
of the modulation frequency
(this is true for the FM
modulator in an HP 8360 series
synthesizer). If the modulation
frequency is changed after a
normalization has been done,
a new reference sweep is
required.
Normalization
Trace normalization is desirable
for relative measurements, and
required for absolute delay
measurements. For obtaining
the best relative accuracy,
performing a normalization
removes the group delay
contributions of the
interconnect cables and the test
equipment. An absolute group
delay measurement can only be
done by first establishing a
reference, which is the group
delay of the test setup without
the DUT in place. In general,
normalization improves
measurement accuracy
considerably, but is not as good
as what can be achieved using
calibration standards and 12-
term error correction as done in
standard vector network
analysis.
How to do normalization
Normalization is done by doing
a measurement without the
DUT, and then pressing the
NORMALZ key under the
Process menu to store the
reference trace. The
measurement configuration
must be set up for a non-
frequency translating
measurement, by setting the LO
frequency to 0 Hz and the RF
and LO multipliers (N and M) to
one, under the Config menu.
Next, the DUT is inserted and
another measurement is done,
this time setting the correct LO
frequency and multipliers that
correspond to the frequency
translation of the DUT (see step
3 on page 14 about the Config
menu for a more detailed
description of setting up a
measurement). The reference
and test measurements must
have the same number of