CONSIDERATIONS IN
RF/MICROWAVE RELAY
PERFORMANCE
Karl Kitts, Director of Development Engineering for High Performance Relays
TE Connectivity – Aerospace, Defense & Marine
Republished by RF Globalnet
AEROSPACE, DEFENSE & MARINE /// BYLINE ARTICLE
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CONSIDERATIONS IN
RF/MICROWAVE RELAY
PERFORMANCE
Karl Kitts, Director of Development Engineering for High Performance Relays
The hardware designer has unique performance requirements to consider when specifying RF/microwave relays and
switches. Signal-level DC and low-frequency AC relays are typically characterized using different specifications than
those used for RF/microwave. But the attributes can be compared. One reason for the difference is that the
characteristics of microwave relays are frequency dependent
Figure 1. RF/microwave relays are characterized and specified differently from DC and low-frequency AC relays.
(Source: TE Connectivity)
Insertion Loss
For both electromechanical and solid state signal relays, the most common performance specification is ON resistance.
Electromechanical performance is normally an order of magnitude lower because of the hard contact interfaces. For
example, values of less than 50 milliohms are common for MIL-PRF-39016 relays, while solid-state relays have
resistances of hundreds of milliohms.
Signal-level relays are typically applied for low-voltage DC systems or AC systems at less than 2000 Hz frequency. As
frequency increases into RF and certainly microwave applications, greater care must be taken with internal currentcarrying conduction paths and contact structures of the relay. Discrete contacts that perform well at low frequencies tend
to become resistive or reflective as frequency increases. In addition, at higher frequencies, skin effect tends to push
current to the outer edges of internal bus bars; as a result, certain geometries yield improved RF performance.
For high-frequency relays, ON resistance is typically replaced by Insertion Loss. It is a measure of signal loss through
the device, expressed in decibels. Typically increasing with frequency, insertion loss is influenced not only by the
heating or skin effect losses but also by leakage in the insulation materials and even by magnetic coupling in the part.
To illustrate, insertion loss values are typically <0.5 dB for TE Connectivity’s MW3 (3 GHz) relay.
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Figure 2. Performance of a RF/microwave relay rated for operation up to 6 GHz shows the frequency dependency of
characteristics. (Source: TE Connectivity)
Repeatability
In signal-level, low-frequency relays, repeat or cycle-to-cycle performance is generally not of high concern as long as the
relay remains within maximum levels of specifications. For RF/Microwave relays, Repeatability of insertion loss from one
closure to the next over product life can be critical, especially when the relay is used in test equipment or measurement
applications. RF relays are frequently characterized during validation testing for variation over life using traditional
statistical tools such as maximum deviation between readings and standard deviation.
Isolatoin
OFF or open-state resistance for signal-level relays is extremely high due to the full galvanic isolation of an open contact
set. Resistance values into megohms and voltage standoff in thousand of volts are typical.
For RF/microwave relays, this attribute is termed Isolation. Isolation is expressed again in dB as reduction in signal
strength across the open RF relay contact system. Due to the proximity or spacing limitations in relays, some signal
leakage is always present as frequency increases. This leakage is most often attributed to capacitive coupling across
the open contact sets or internal relay bus bars. Again, relay designers can influence this performance to some degree
through spacing in the relay and current path designs. Using RF field analysis software early in the relay design cycle
allows higher levels of isolation to be achieved.
VSWR
The final electrical attribute for RF/microwave relay performance has no practical equivalent in signal-level relays.
Voltage Standing Wave Ratio (VSWR) measures the percentage of the relay’s transmitted signal that is reflected back
to the source. Internal to the relay, care must be taken so that frequency resonance points are avoided within the
applied frequency band of the relay. Internal near-contact shielding and even the relay dust cover itself can make a
significant contribution to the overall relay VSWR and isolation performance. To minimize VSWR, it is absolutely critical
to match the transmission line impedance throughout the relay terminal lead interfaces and onto attached printed circuit
cards.
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Solid State Versus Electromechanical Relays
The clear advantages of solid-state relays (SSRs) are high life endurance from no moving parts, small size, and high
speed of operation (microseconds versus a few milliseconds for electromechanical designs).
Electromechanical relays, however, generally provide better RF characteristics with higher isolation levels and lower
insertion loss within the whole frequency range. In addition, hard-contact electromechanical relays are able to carry RF
signals superimposed on DC levels and to transmit higher RF power. They yield true broadband RF characteristics from
DC up to the specified frequency. Electromechanical relays offer better isolation properties than SSRs. Metal contacts
provide the lowest possible insertion loss. Furthermore, electromechanical RF relays are able to transmit RF signals
with minimal signal distortion, while non-linear performance of SSRs can distort signals.
Electromechanical relays also offer a distinct advantage over solid-state for multichannel applications or for applications
with normally closed and changeover-style contact forms. Achieving these forms with solid-state electronics can be
overly complex and require much larger physical space than needed for electromechanical relays. Recently launched
TE AXICOM (Figure 3) relays offer a combination of small physical size and superb RF characteristics, with isolation
values greater than 80 dB @ 1 GHz, insertion loss of 0.3 dB, and VSWR values less than 1.3:1 ratio.
Figure. 3. AXICOM relays offer enhance RF performance characteristics. (Source: TE Connectivity)
Printed Circuit Card/Relay Interface
A much higher degree of consideration is required for interface to RF/microwave relays as compared to that of DC and
low frequency products. Traditional DC and low frequency AC relays often just require validation of adequate spacing
and proper copper weights for reliable current carry performance. While these parameters remain for RF/microwave
application, the designer must also maintain adequate shielding and channel to channel isolation and ground plane
locations. Even terminal through-hole pitch dimensions can be optimized for overall PCB RF performance. As a
recommendation, the PCB designer should consult with the relay applications engineering team for design guidance
whenever possible.
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Author’s Bio
Karl Kitts serves as Director of Development Engineering for High Performance Relays for the
Global Aerospace, Defense & Marine business unit of TE Connectivity. With more than 25 years’
experience, his expertise is in high current MIL/AERO power distribution, electrical relays and
power contactors, electronics circuit protection, HVDC relays, sensors and time delays, and solid
state devices.
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and are subject to change without notice. Specifications are subject to change without notice. Consult TE Connectivity for the latest dimensions and design specifications.
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