User`s guide

20
Appendix A
Comparison to Other
Instruments
This discussion is meant to
clarify where the HP 71500A
fits in compared to other group
delay measurement solutions.
Vector Network Analyzers
The most common tool for
measuring group delay of linear
devices is the vector network
analyzer (VNA). VNAs utilize
the direct phase measurement
technique. They are very fast
and accurate. A VNA by itself
can only measure non-frequency
translating devices. For
frequency translating
components, an external mixer
or extra DUT must be used so
that the frequency at the
reference and test channels is
the same. However, in order to
make a group delay
measurement, the LO for the
external mixer must be phase
locked to the LO used within
the DUT. This can be
accomplished by either providing
or having access to the internal
LO (or its frequency reference).
This prevents VNAs from
measuring components where
the LO is embedded with no
external access.
The VNA cannot directly
measure absolute group delay
either. Absolute delay must be
inferred from several
measurements involving
swapping of mixers, or an
assumption that the mixer
contributes little delay. The HP
71500A can measure absolute
delay directly, with a relatively
simple normalization routine.
Trade-offs
Although the HP 71500A can
make these absolute and
relative group delay
measurements, there is a trade-
off in performance compared to
using a vector network analyzer
— reduced speed and accuracy.
Measurement speed is slower
for two reasons. The first is that
a stepped CW approach is
required, which is slower than
using a continuous sweep. The
second reason is that the noise
reduction techniques needed for
high accuracy measurements
take a fair amount of time (FFT
zoom, trace averaging, or both).
An example of speed for a
typical measurement using FFT
zoom and no averaging, is
about 2 minutes for a 32 point
sweep (using a zoom factor of
16). For absolute delay
measurements, there may be an
overall measurement speed
improvement because of a
simpler calibration routine —
no external mixer calibrations
are needed.
The second disadvantage is that
measurement accuracy is likely
to be less than what can be
achieved using a VNA due to lack
of full vector error correction,
and linearity limitations within
the HP 71500A. An HP 8510C
network analyzer can achieve
relative group delay accuracies
below 100 ps for linear devices,
or for measurements relative
to a golden standard.
Scalar Network Analyzers
Some scalar network analyzers
(SNAs) can be configured to
measure AM group delay,
providing fast sweeps and
medium accuracy for relatively
low cost. Their primary target is
for high volume manufacturing
test, where throughput is more
important than accuracy. SNAs
use the AM envelope technique,
using broadband diode detectors
to strip the AM from the RF
carrier. As such, they can
measure both linear and
frequency translating devices
with internal LOs. The lack of
capability to provide FM group
delay precludes their use for
some applications where AM
can’t be used. Although slower
and more expensive, the MTA’s
main advantages are providing
FM group delay and better
accuracy.
Microwave Link Analyzers
The microwave link analyzer
(MLA) provides fast and
accurate FM group delay
measurements, and also makes
other important measurements
on analog microwave radio links.
For loop-back testing (testing a
link over a short distance), one
MLA is used. For end-to-end
link measurements, two MLAs
are used. MLAs are used directly
only for IF-IF measurements
(non-frequency translating
links). For measuring frequency
translating links, (RF-IF or
IF-RF measurements),
the system must provide the
necessary converters and access
to IF ports. For measuring
subassemblies like frequency
converters, extra components
such as mixers, filters, and LOs
would be needed, thereby greatly