Specifications

Termination
At high frequencies, all signals
must be properly terminated.
With improper termination
(open end or mismatched
impedance), signals can be
reflected from the termination
point back towards the source.
This will result in an elevated
VSWR and can even damage
the signal source. Coaxial cables
come is various characteristic
impedances (typically 50 or
75 ohms), and the cable needs
to be terminated in the same
impedance. Termination usually
takes place at the measurement
instrument or DUT. However,
when a source instrument is
switched out of the active
path, it should be switched
to a properly terminated load.
This can be accomplished by
purchasing multi-port switches
that terminate inactive ports.
Although a good test procedure
always turns the source OFF
before switching, sometimes
that is not possible, so the active
source needs to be terminated.
Many switch manufacturers
orient their literature towards
Frequency, Insertion Loss,
Isolation, and VSWR. However,
Repeatability and Life can be
absolutely critical to the quality
of the test system. It may be
necessary to contact vendors
and have them explain the
change in insertion loss over
the duration of the lifetime
use of the switch.
Isolation, Repeatability, and
Life significantly affect the
cost of an RF/Microwave switch,
and they also greatly affect the
signal integrity of the test sys-
tem. Manufacturing quality of
these three parameters strongly
correlates to the integrity of
the remaining parameters.
Tips in Building an
RF/Microwave Test System
Where do you start in building
a test system? The following
guidelines are not comprehen-
sive for all test systems, but
they do offer a good foundation
from which you can build your
test system plan.
1. Understand and document the
test system requirements
relative to testing the DUT.
The system requirements
should include how the
operator interacts with the
test system. This will drive
the equipment needs, sys-
tem configuration, and the
requirements for diagnostic
tools.
2. Choose stimulus and response
equipment that exceed your
measurement requirements,
since there will always be
some degradation of signal
between the instruments and
the DUT via the switching
subsystem.
3. Choose switches, attenuators,
and connectors that minimize
signal degradation and exceed
the frequency requirements
of your system. If switches
need to be positioned close
to the DUT, you will need to
control the relay coils with
a dedicated Switch Driver,
general purpose relays, or
digital outputs. You may also
need switches that create
a straight-through path that
bypasses attenuators or
amplifiers in order to cali-
brate and verify your system.
4. Choose coax cabling with
connectors that minimize
insertion loss in the system.
Does the system need to be
re-configured? If so, you may
need to use flexible coax
cabling. Achieving low
insertion loss at high fre-
quencies while maintaining a
flexible cable setup can be
challenging and expensive.
Choose your cable vendor
carefully, since the quality of
the construction of the cable
will significantly affect over-
all system performance.
5. Delay lines and phase
matching between signal
paths demand very tight
dimensional tolerances. You
may need to use semi-rigid
cable to achieve those
requirements. This will likely
require the use of external,
discrete switches.
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