To maintain signal integrity in an automatic test system requires:
- An understanding of the device under test (DUT)
- Characteristics of the signals being switched
- Characteristics of the test instruments and switching system
There are many different aspects to understanding these things, but in this post we will concentrate on the advantages and disadvantages of the types of relays used in the switching system. There are four main types used in ATE switching systems:
- Dry reed relays
- Mercury-wetted reed relays
- Solid-state relays
- Electromechanical relays
Dry Reed Relays
Dry reed relays have contacts made from ferromagnetic material (reeds) that are encapsulated in glass. The energizing coil is wrapped around the glass, and when a current passes through the coil, a magnetic field generated that causes the reeds to come together.
- Contacts are hermetically sealed, reducing oxidation build-up
- Provides high isolation (about 1014Ω)
- The magnetic fields may affect adjacent relays, so shielding relays may be needed in high-density applications
- Inconsistent contact resistance
Mercury-Wetted Reed Relays
Mercury-wetted reed relays are similar in construction and operation to dry reed relays, but a small amount of mercury is added to the glass tube to provide more consistent contact resistance.
- Handles more power than dry reed relays
- Consistently low contact resistance
- More expensive than dry reed relays
- Position sensitive
- Mercury is a sensitive material
A solid-state relay typically consists of an opto-isolator that controls a FET, SCR, or triac to switch the load.
- Long mean time between failures (MTBF)
- Very fast switching times (typically in microseconds)
- Small size allows them to be used in high-density switching applications.
- Degrades signal integrity
- Typically not bidirectional
- Not useful for high voltage swings
- Typically application specific
An electromechanical rely consists of an electromagnet that operates an armature to bring two contacts, such as gold over silver, together. This method provides consistent contact resistances. There are many different types of armature-controlled, electromechanical relays, including bifurcated contacts for general-purpose switching, coaxial switches for RF and microwave applications, and contactors for high-power switching.
- Ideal for general-purpose switching
- Consistent contact resistance
- Many variations available
- Can switch high power loads
- Can switch RF/microwave signals
- Not ideal for low thermal, very low voltage switching
For more information on the types of relays used in VTI Instruments' switch cards, or on automatic test switching systems in general, contact one of our sales representative by visiting powerandtest.com/sales. You can also email us at firstname.lastname@example.org or call 800-733-5427 or 858-450-0085.