Specifications

A disadvantage of continuous
drive switches is they may
change performance due to
power dissipation in the
switch housing. This can lead
to deviations in performance
of your test system.
Each 34945EXT can drive a total
of 2 A continuously when using
an external power supply. When
using an L4445A or 34945A
mainframe module to supply
power to the 34945EXT, the
power from any particular slot
in the mainframe is limited to
24 volts at 100 mA continuous
+ 200 mA pulse for up to 15 ms
(25% duty cycle). The module
uses storage capacitors to
provide short bursts of high
current. This is ideal for
switches or attenuators that
use latching, pulsed relays.
Latching relays use pulse-mode
drive because they use two
coils per switch or attenuator
section. A latching relay switch
requires much lower overall
current, since latching relays
only require a short-duration
pulse (typically 15 ms) of
100 mA to 300 mA. This permits
the switch driver to control
many more switches with a
lower current supply. And,
there is no constant current
through the coils to heat up
the switch housing.
Some vendor switch drivers only
provide digital outputs for dri-
ving the relays. It is up to the
user to distribute the power to
the switches. Additional digital
input and output modules
would also need to be purchased
to read switch position and
display LED status. All of this
is included in the 34945EXT,
and the built-in Web Server
provides a powerful, graphical
interface for controlling and
monitoring switches and
attenuators.
Key RF and Microwave
Test System Specifications
The choice of cables, con-
nectors, and switches will
significantly impact overall
system performance and
reliability. The cables, connec-
tors, and switches create the
signal path from the DUT to
measurement instruments
or other components. Ideally,
there would be no signal
degradation when routing
from one point to another.
For actual test systems, there
is always some degradation of
the signal in passing through
cables, connectors, and switches.
Signal degradation is directly
related to signal frequency
and the quality of switches
and cables.
While poor quality cables
and switches can significantly
affect system performance at
higher frequencies, it is not
enough to have the best quality
switches and cabling. You must
also match source, cable, and
load impedances. Any mis-
match of impedances between
the source and the load will
cause a reflection of the source
signal. A reflected signal adds or
subtracts to the desired signal
causing signal degradation,
and the reflected signal can
possibly damage the signal
source.
Key parameters for custom
RF/Microwave switching
systems are listed below.
This application note will
concentrate more on switch
specifications.
• Frequency range
(bandwidth)
• Insertion loss
• Isolation
• VSWR
•Life
• Repeatability
• Termination
7