Specifications
6. Choose a switching sub-system
that is easy to configure, verify,
and provides the capacity to
expand your test system in the
future. How you control external
switches is a key factor, since you
may require position indication
that gives the actual state of
the switch (vs. the programmed
state of the switch).
7. Once your system is assembled,
make detailed measurements
with the DUT in place for every
path and understand such per-
formance factors as impedance
matching, phase delay, and
insertion loss.
8. Software will play a key role in
controlling your signal path. A
switch executive can provide
you with mutual exclusion of
switches, so you do not inadver-
tently drive two sources togeth-
er. This can be done within the
switch subsystem or through a
software driver supplied with
the switch system.
It is recommended that you
keep signal paths as short
as possible to improve signal
integrity. Long signal paths
contribute to rising edge and
phase delay, because those paths
consist of lead resistance and
capacitance. Phase delay
distortions produce ringing,
overshoot, and undershoot. In
some systems, phase matching
is required, and signal paths
must match impedance and
length. High speed digital signals
exhibit RF behavior in real world
devices and are directly affected
by edge and phase delays.
System accuracy is also affected
by the number of components
(switches, attenuators, amplifiers,
RF couplers, dividers) and
connector adapters between
the source and the DUT. For
increased system accuracy in
the presence of strong adjacent
signals flowing in the test
system, it is sometimes neces-
sary to add cascaded switches,
route signals carefully, and
provide shielding around
sensitive signal paths.
Key Factors in Selecting
RF/Microwave Switches
There are several key factors
a switch should provide for
an RF/Microwave test system.
The parameters covered in
the previous sections will now
be used to identify the right
type of switch:
1. The insertion loss, VSWR,
and isolation at the desired
frequencies must not compro-
mise the measurement. Signal
degradation through the switch
can be a minimized by using
high quality switches configured
with proper connectors.
2.Long life and high repeatability
between switch cycles will
improve test system integrity.
3.The switch should provide
position feedback, so you can
determine the actual path of
the signal instead of only the
programmed state.
When selecting RF/Microwave
switches, be aware that the most
common and readily available
switches often present a mini-
mum viable solution. The cost of
the switch goes up considerably
when adding options to achieve
the desired level of repeatability,
performance, and usability. A
test system that gives varying
results over time or is difficult
to troubleshoot is often an
indication of lower quality
switches and cables.
Minimum viable RF/Microwave
switches typically have the
following features:
• Switches are Normally Open
(Failsafe) and driven by a
single coil. These coils are
continuously energized and
require 100 mA-300 mA
(24 V) of constant current
per coil. When power is
removed, the switch opens.
• Switches have solder-lugs
or bare wires rather than
easy to use D-sub socket
connectors.
• No position feedback is
included to electrically
determine the state of the
switch
• Bandwidth of switch is lower
• Isolation is typically 50 – 80 dB
• Switches are only specified
for 1 million cycles and don’t
include repeatability specs
• Switches provide no
protection diodes when
driving relay coils
• Switches are unterminated
10