Routing electrical and even optical signals from one source to another requires
an overall understanding of the application, signals being switched and finally
an understanding of how to select a switching solution that helps maintain signal
integrity at a reasonable cost. Some of the main Issues to consider are:
Types of Relays used in Test and Measurement
Dry Reed Relays
- Types of relays
- Types of relay topologies
- Selecting appropriate switching configurations to minimize size and cost while maintaining
- Communicating with the switching system to maximize overall system throughput
- Cabling and wiring to/from one source to another
- The ability to easily configure and expand the switching system for several applications
Contacts are made from ferromagnetic material (reeds). These contacts are encapsulated
in glass. An energizing coil is wrapped around the glass and an EMF brings the two
reeds together, closing the contacts.
- Hermetically sealed, reduces oxidation build-up
- High Isolation (about 1014 Ω)
Mercury Wetted Reed Relays
- EMFs effect adjacent relays – requires shielding of relays in high-density applications.
- Inconsistent contact resistances
Similar in construction and operation to dry reed relays except that a small amount
of mercury is added inside the glass tube to provide more consistent contact resistances.
- More power handling than a dry reed
- Consistent and low contact resistances
Solid State Relays
- Position sensitive
- Mercury is a sensitive material
- Expensive relays
In a solid state relay an opto isolator is typically used to control FETs, SCRs
or triac types of solid state devices.
• Offers long mean time between failures (MTBFs)
• Very fast switching times (typically in microseconds) Mercury Wetted Reed Relays
• Allows for high density signal switching
• Degrades signal integrity
• Typically not bi-directional
• Not useful for high voltage swings
• Typically application specific
An electromechanical relay uses an armature to bring two electrical contacts (i.e.,
gold over silver) together. This method provides consistent contact resistances.
There are many different types of armature controlled electromechanical relays such
as contactor for high power, bifurcated contacts for better general purpose switching,
coaxial switches for microwave applications.
- Ideal for general purpose switching
- Good consistent contact resistance
- Many variations available
- Allow switching of high power
- Allows for RF/microwave switching
• Not ideal for low thermal, very low voltage switching.
Switch Card Types
The types of relays used determines the application of the switch card. The types
of switch cards can be divided into the following categories:
Power switching cards that use high power electro- mechanical relays that can handle
switching signals above 2 A - the SMP2000 series
General purpose switching cards designed for low-level and low-frequency applications
(less than 2 A, and typically less than 20 MHz) - the SMP3000, SMP4000, SMP5000
RF switching cards that are controlled impedance (typically 50 Ω or 75 Ω). These
modules are typically designed to handle RF signals under 1 GHz - the SMP6000 series
Microwave switching cards designed for switching frequencies between 1 GHz and up
to 40 GHz - the SMP7000 series
Each switch card provided by a switching system manufacturer will typically consist
of relays that have one of the following topologies:
A SPST relay simply opens or closes a signal path. This relay is typically normally
open when power is removed, and the relay needs to be energized in order to close
the signal path. A normally open relay is sometimes also referred to as a Form A
relay. A normally closed SPST (i.e., closed when no power is applied) is referred
to as a Form B relay. A DPST relay is in essence two SPST relays, but energized
from the same coil. A SPDT relay has one normally open and one normally closed path,
and a DPDT relay is in essence two SPDT relays which are energized from the same
coil. A SPDT relay is also referred to as a Form C relay.
A typical automated switching system consists of several switch cards/configurations
combined together to achieve the final result. The flexibility, modularity and mixture
of different switch cards/configurations from a manufacturer play an important role
in selecting the correct switching system for a particular application. Care needs
to be applied to how these building blocks are used and combined since a bad configuration
can effect the size, cost and signal integrity of the system. The main types of
switch configurations are tree, multiplexer. and matrix.
The tree only allows one channel to be selected at a time, whereas the multiplexer
can allow all channels to be selected simultaneously. The multiplexer configuration,
however, has limited bandwidths due to unterminated stubs hanging onto the selected
channel. The tree configuration is typically used in RF applications, whereas the
multiplexer is used for general purpose switching.
The matrix is the most flexible type of switch topology, since any input can be
connected to any output. However, it is also the most expensive, and effects overall
signal integrity more than other configurations.
A low frequency matrix has large unterminated stubs, where as a matrix designed
for RF applications has no unterminated stub effects. Although the matrix is the
signals - the SMP8000 series
All SMIPII™ switch modules come with a GUI, and both IVI and VXIplug&play drivers.
Complete system GUIs can also be provided by request.
Cabling Wiring to/from the Switching System
A switching system is merely an extension of the cabling/wiring from one point to
the next. The choice of cable used is as important in the overall system integrity
as the method of cabling/wiring to the switching system. For example, if twisted
pair shielded cabling is to be used to wire between the unit-under-test (UUT) and
an instrument via the switching system, the switching card employed needs to be
an extension of the twisted shielded pair cable. In such a case a switch module
with a balanced line pair (2 lines/ channel) should be used. This same switch module
should have isolated shielding that would be an extension of the shield on the twisted
pair cable (eg. SMP3001).
A universal switching system also needs to provide the user with the capability
to wire up to the switching system in a variety of methods, allowing the user to
determine the simplest approach for cabling/wiring the switch to the rest of the
system. If the switching system is to be used in an application where the switch
cards will be wired and re-wired several times (i.e. in R&D, engineering or
scientific applications), the user should consider wiring to/from the switch card
using a screw-down terminal block approach. If the switching system is to be used
in an ATE application and wired once for a long period of time, crimp type pins
and connectors are recommended.
The connector kits available on the SMIPII™ family give the user several options
for cabling and wiring to the switching system.
Since signal switching is a very key part of overall system throughput (i.e., scanning
channels and changing points to measure on the UUT quickly), commands need to be
sent and processed quickly by the switching system. Until the advent of VXI over
a decade ago, the majority of programmable switching systems were programmed using
the IEEE-488 bus or RS-232. With the introduction of the VXIbus, several alternative
methods of programming switching systems became available (see technical note: “Application
Programs, Instrument Drivers and VXIplug&play Overview”) allowing test times
to be considerably improved.
There are two types of switching systems available on the VXI platform. Message-based
and register based. A message-based switching system is programmed using message-based
strings, similar to programming an IEEE-488 switch, and up until the introduction
of the SMIPII™ family from VXI Technology, this was the only way to obtain an intelligent
switching system. A register based switching system is programmed by directly accessing
registers (see technical note: "Message-based or Register-based Programming").
Message-based switching is considerably slower than register-based switching and
can add significant time to overall test throughput. For example, even if relays
typically take 2 ms to 5ms to close/open, it takes approximately 1ms to send and
process a character on a message-based switch (Racal series 1260). To send a command
to close three relays (Cl. 1.00,02,04) on a message-based switch takes approximately
13 ms (13 characters). In comparison, writing data to a register takes approximately
Specifications to Consider and What They Mean
The following is a brief definition of specifications found for switching cards.
The specifications defined by a manufacturer are normally only for a switch card
and not the complete switch system, since this would consist of several switch cards
combined with the appropriate cabling and wiring.
• Maximum Switching Voltage is the maximum open circuit voltage which can safely
be switched by the contacts. AC and dc voltage maximums will differ in most cases.
• Maximum Switching Current is the maximum current which can safely be switched
by the contacts. AC and dc current maximums may differ.
• Maximum Switching Power is the upper limit of power which can be switched by the
contacts. Care should be taken not to exceed this value.
• Maximum Carrying Current is the maximum current that can safely pass after closing
or prior to opening the contacts.
• Path Resistance is the resistance of the worst-case signal path and includes relay
contact resistance, trace resistance, and connector resistance.
• Breakdown Voltage is the maximum voltage that can be tolerated by the relay without
damage for a specified period of time.
• Relay Settling Time is the time it takes the relay to close or open and settle
from all bounce.
• Mechanical Life is the minimum number of times the relay can be operated under
normal conditions with no load on the contacts.
• Electrical Life is the minimum number of times the relay can be operated under
nominal conditions with a specific load being switched by the contacts.
• Isolation : High frequency signals leak through the stray capacitance across contacts.
This is called isolation and is specified in dB to express the magnitude of the
leak signal. The larger the dB the better the isolation.
Isolation will typically be expressed from input to output of the worst case channel,
with the relay open. The isolation specification is important to consider since
it determines the amount of noise that is induced to the output of a channel from
the input of that channel when the channel is open.
• Crosstalk is used to express the stay capacitance between adjacent channels on
a switch card and is specified in dB. The larger the dB, the better. The crosstalk
specification is important to consider since it determines the amount of noise that
is induced to the signal switched from an adjacent channel.
• Insertion Loss. At high frequencies signal disturbance occurs from self-induction,
path resistance, dielectric loss, as well as from reflection due to impedance mispatch
This type of loss is called insertion loss is expressed in dB, and refers to the
magnitude of the loss of the input signal on the output. The lower the magnitude
of the insertion loss, the better the signal path.
• VSWR (Voltage Standing Wave Ratio): At high frequencies resonance is generated
from the interference between the input signal and the reflected signal. VSWR is
specified as the ratio of the maximum value to the minimum value of the waveform.
The lower the VSWR, the less the reflected wave.
When selecting a switching system, it is strongly recommended that the user understand
the application and test philosophy before configuring the system in order to optimize
the solution. However, since test stations today are sometimes designed concurrently
with the product to be tested, it is not always easy to determine the exact configuration.
Selecting a switching system, therefore, that is modular and easily expandable should
be key, putting particular focus on the selection of the correct building blocks.