User`s guide

3
Introduction
Measuring absolute group delay
through frequency translating
devices such as downconverters
or receivers is difficult at best,
particularly at microwave
frequencies. Vector network
analyzers are excellent tools for
measuring group delay relative
to a “golden device”, but they
are difficult to use for absolute
measurements. For microwave
devices without access to
internal local oscillators (LOs),
the problem is especially
difficult as vector network
analyzers cannot be used at all.
For these cases, a modulation
technique must be used, which
often involves a custom solution
with external mixers, filters,
LOs, and discriminators or
detectors — in other words,
complex and expensive.
Customers have been asking for
a better solution for a long time.
The HP 71500A microwave
transition analyzer coupled with
a downloadable Instrument
BASIC (IBASIC) personality
offers a cost-effective and simple
solution for microwave group
delay measurements of
frequency translating devices.
Product Description
The group delay personality is a
downloadable IBASIC
application program that runs
internally in the HP 71500A.
It is targeted for measurements
of microwave frequency
converters and receivers with
internal LOs, or any microwave
frequency converter where
absolute group delay is desired.
It is intended for measuring
sub-systems or systems where
the source and the HP 71500A
can be located near one another.
It is not intended for end-to-end
measurements separated by a
long-distance, due to the need
for the HP 71500A to measure
both the RF and the baseband
modulation source.
The program uses the modulation
delay technique for measuring
group delay, using either AM
or FM. The modulated RF
carrier is measured in channel
1, and the baseband modulation
signal is measured in channel 2.
Why Measure Group
Delay?
Ideal networks pass modulated
signals containing information
without inducing any additional
distortion. They are
characterized by both flat
magnitude response and linear
(constant slope) phase shift
versus frequency. Since group
delay is the derivative of phase
with respect to frequency, this
implies a flat group delay
response. All of the frequency
components of the signals
passing through the device-
under-test (DUT) will
experience the same amount of
time delay. The real world,
however, is composed of non-ideal
networks, which cause
distortion due to amplitude and
phase nonlinearities. Group
delay is one measure of phase
nonlinearity. It is particularly
important to characterize group
delay for systems where some
type of phase modulation is
used, such as FM, QPSK, or
64QAM.
Group Delay
Techniques
Group delay is defined
mathematically as the negative
of the derivative of a network’s
phase versus frequency
response:
Group delay t
d
=–––
d
Ø
dw
(t
d
in seconds, Ø in radians,
w in radians/second)
or
t
d
=–––
1
360
x
dØ
––
df
(t
d
in seconds, Ø in degrees,
f in hertz)
There are two basic techniques
for measuring group delay of a
component or sub-system.
Direct Phase
For linear, non-frequency
translating devices, the most
commonly used technique is the
direct phase method. A vector
network analyzer is used to
measure the phase response of
a DUT, and a numerical
differentiation is done to derive
group delay. The derivative is
approximated by using ,
where delta phase is the phase
difference between adjacent
trace points, and delta
frequency is the frequency step
size (total frequency span
divided by the number of trace
points). Since delta frequency is
a nonzero quantity, there is
averaging occurring over this
frequency interval. This interval
is known as the measurement
aperture. Larger apertures give
less group delay resolution
(more averaging occurs), but
less trace noise. Conversely, a
smaller aperture implies more
resolution at the expense of a
noisier measurement.
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