What Do the Specifications for NI Switch Products Mean?

Updated Nov 15, 2018

Reported In

Hardware

  • PXI Switch Controller Module

Driver

  • NI-SWITCH

Issue Details

I am looking into purchasing an NI switch product and am having trouble understanding what each of the specifications mean in regards to my application. Where is this information available?

Solution

Listed below are the types of specifications found in most NI switch specifications documents. Directly following each specification is a brief description of the specification and why a user might be concerned with the specification. These specifications are grouped by their relative location in the specifications document.

General

Topology: organizational representation of the channels and relays in a switch module. A switch module is physically composed of relays, usually of one or more specific types and form. Relays are combined in software (and sometimes additional accessories) to create various topologies. For information about relay types and forms, as well as graphical representations of topologies for a specific switch module, refer to NI Switches Help.
 

Input Characteristics

Maximum switching voltage: the maximum voltage that can be present across relays in the switch module while they are closing. (generally specified by the relay manufacturer). For information on measurement categories, refer to Read Me First: Safety and Electromagnetic Compatibility.

Minimum switching capacity: the minimum signal that needs to be on the relay when hot switching devices to prevent contact resistance fluctuations. This only affects electromechanical style relays and is caused by materials within the relay housing settling on the contacts of the relay. When signals above the Minimum Switching Capacity are used, the material will burn off. Signals below this level will not burn off these materials and can result in contact resistance changes. This is also referred to as minimum switch load. 

Maximum switching current: the maximum current, measured in amps, that can be flowing through a relay as it is closing. If you close relays while there is voltage present it is known as 'hot switching' and will cause the relays to degrade more quickly than 'cold switching' or 'dry switching'. Cold switching is when relays switch with the power supply disabled. This eliminates all current from flowing through the relay as it closes and increases relay life. Current is the flow of positive electrical charge through a cable or path. Voltage is the electrical potential difference that causes a flow of electricity (current). With a higher voltage more current will flow through the relay.

Maximum carry current: the maximum current, measured in amps, that can flow through a relay once the relay has fully closed. When the software command is sent to close a relay, it takes some time to finish closing (known as the operate time). During this time only 'Maximum switching current' should be present. After this operate time has passed the relay can withstand up to the Maximum carry current.

Maximum RF power: the level of power that a system can handle at various given frequencies.

DC path resistance: the resistance of a complete signal path from source to destination terminals on a switch module. The total resistance includes the resistance of PCB traces, relays, and connectors. Trace and connector resistance is generally stable, but relay contact resistance increases with use.

DC isolation resistance: the impedance between two channels when a relay is open.

Thermal EMF: the voltage that is caused by the thermocouple created at the juncture of two dissimilar metals in the relay. This is only a problem with electromechanical armature and reed relays. FET and SSR relays have a similar 'offset voltage' that has the same impact to the customer but is caused by a different effect.

Bandwidth: the frequency range of signals that a module can carry. If the frequency of a signal is too high, the signal's amplitude will be attenuated (reduced).

Channel-to-channel isolation: the electrical separation between two inputs (channels).

Open channel isolation: the electrical separation between input and output (channel and COM).
 

RF Performance Characteristics

Minimum Input Frequency: Some RF Switches (i.e. FET) have a minimum input frequency due to the capacitive components in the circuit which are used to eliminate DC offsets in the signal.
Characteristic impedance: all RF signal paths act as a transmission line containing inductance, capacitance, and resistance. These attributes are used to determine a Characteristic Impedance of the path. It is important to have the same Characteristic Impedance through the entire RF signal path to minimize reflections. Most RF systems have a characteristic impedance of either 50 or 75 W.
Insertion Loss: measures the attenuation and power loss induced by the switch. This loss increases at higher frequencies.

Typical isolation: the ability to keep a signal on an unused channel from appearing on an active, terminated channel. The higher the isolation, the less of the unused channel's signal is seen on the active channel. This is similar to "crosstalk".

Typical channel-to-channel skew: the time difference in signal transmission that occurs when an identical signal is sent through two different channels.

Typical propagation delay: the length of time it takes for a signal to pass through the entire circuit.

Typical rise time: the time required for an output signal voltage to rise from 10% to 90%.

Voltage standing wave ratio (VSWR): the ratio of the maximum to minimum voltage in the standing wave pattern on the transmission line. This value measures the power of a reflected signal when it propagates through electrical system components with varying characteristic impedences. The VSWR should be minimized.

Open channel isolation: the electrical separation between input and output (channel and COM).

RF carry power: the maximum RF power that can be transmitted through a relay after the relay has fully closed.
 

Dynamic Characteristics

Relay operate time/Relay release time: the maximum amount of time it takes the relay to either close or open. This includes the time electromechanical relays may bounce as they are closing.

Maximum scan rate/Recommended cycle speed: a measure of how fast a switch can cycle, or open and close, while taking a measurement at each closed position. 

Expected relay life: expected number of cycles that a relay can complete over a lifetime under various electrical loads. 

Mechanical relay life: is the number of times a relay can be switched before it is expected to fail mechanically. 

Electrical relay life: documents how various different signal levels will affect the life of the relay. The actual relay life is greatly dependent on the voltage, current, inductance, and capacitance of the signal being switched. Most modules have on-board relay count tracking and instructions for field-replacing relays that have failed.
 

Trigger Characteristics

Input trigger: specifies the possible locations for an input or "Scan Advance" trigger. These are often the PXI trigger lines and some modules support external trigger inputs on the front panel or terminal block.
Pulse width: the period of time between the rising and falling edges of a the output trigger’s pulse (or between the falling and rising edges of a negative output trigger pulse).

Maximum pulse width: longest period of allowable time between the rising and falling edges of a positive input trigger’s pulse (or between the falling and rising edges of a negative input trigger's pulse). If longer, the switch cannot process the trigger event.

Minimum pulse width: shortest period of allowable time between the rising and falling edges of a positive input trigger’s pulse (or between the falling and rising edges of a negative input trigger's pulse). If shorter, the switch cannot process the trigger event.

Output trigger: specifies the possible locations for an output or "Advance complete" trigger. These are often the PXI trigger lines and some modules support external trigger outputs on the front panel or terminal block.
 

Physical Characteristics

Relay type: there are four different relay types. Armature and reed relays are electromechanical (they have moving parts); FETs and SSRs do not have moving parts.

Relay contact material: the material of which the contacting parts of the relay are made.

I/O connector: located on the front of the module. There are numerous types of connectors.

SMA torque: the torque that should be applied using a regulated torque wrench when connecting SMA connectors to the module. Using too little torque could result in poor signal quality, using excessive torque could damage the device.

PXI power requirement: how much power the module uses from each power supply rail of the PXI chassis.

Dimensions: physical dimensions of the module.

Weight: Total weight of the module.
 

Environment

Operating temperature: temperature range (in Celsius) within which the switch should be operated.

Storage temperature: temperature range (in Celsius) within which the switch should be stored.

Relative humidity: amount of water vapor that can exist in the air for the switch to function properly.

Pollution degree: degree of environmental pollutants with which the switch can operate.

Maximum altitude: maximum altitude at which the switch can operate.
 

Shock and Vibration

Operational shock: the level of shaking that the switch module can endure and remain operational.

Random vibration (operating): the level of vibration that the switch module can endure and remain operational.

Random vibration (nonoperating): the level of vibration that the switch module can endure while not operating.

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