Electromechanical relays can be manufactured with high-frequency RF characteristics that allow them to pass and switch signals in the 7-GHz range and beyond with the proper packaging. Two key RF parameters are the insertion loss and return loss. These parameters measure how much of the RF power passes through the relay when the contacts are closed and how much is reflected back to the source. RF relays are built with a specific characteristic impedance to match the application, and an impedance of 50 Ω is typical.
Insertion loss is a measure of the power being transmitted through the relay, typically calculated as
A point of particular interest is the -3-dB point, the frequency where only 50% of the power is being transmitted through the relay. This point usually defines the maximum usable application frequency range for the relay.
Return loss is the amount of power being reflected back to the source. For an ideal relay, if the characteristic impedance were matched to the transmission line and the output terminated to this impedance, the reflected power would be very small across the frequency range.
The return loss can be specified by the VSWR, the measurement of the portion of the incoming signal that is reflected back to the source. The relationship between the return loss expressed as VSWR or decibels is expressed by the equation
For an ideal network, the VSWR would be close to 1, and the return loss in decibels would be >20 dB. Figure 1 shows insertion and return losses for a microminiature RF reed relay mounted in a gullwing SMD package. The insertion loss is shown on the lower plot line and represented in decibels. For this particular example, the -3-dB roll-off frequency is approximately 5.7 GHz. The return loss is represented by the top line and shown as VSWR.
Another measure of the RF performance of a relay is its isolation. This is measured with the relay contacts open. At low frequencies, none of the incoming signal would pass through the relay, and the isolation would be >20 dB.
However, at higher frequencies, a portion of the incoming signal would pass through due to the capacitance across the open contacts. Typically, armature relays can be built with better isolation at high frequencies since the contacts can have wider spacing than reed relays, resulting in lower capacitance.
The insertion and return losses are shown in Figure 2 for a microminiature RF reed relay with open contacts. The plot shows the insertion loss is high, approximately -22 dB at 2 GHz, but drops off to approximately -3 dB for frequencies over 4 GHz. This relay is passing half of the input power to the output for frequencies above 4 GHz.
The return-loss VSWR also is high at approximately 18 at 2 GHz but drops to approximately 5 for frequencies over 4 GHz. This translates to about 3.5 dB, or again half of the power is being reflected back to the source and half is passing through the relay. The usable upper frequency for this relay would be approximately 3 GHz due to this isolation performance.
Another measurement of the relay's RF performance is the time for the output signal to rise from 10% to 90% of its final value. This time can be approximated by the equation
The 3-dB or 50% power roll-off frequency is defined by the equation
Now an equation that ties the rise time to the 3-dB frequency can be derived from the two preceding equations
With this equation, the rise time can be approximated from the insertion loss plot by dividing the -3 dB roll-off frequency into 0.35.
Importance of Setup and Packaging
For RF frequency ranges above 2 GHz to 3 GHz, the package selected and the PCB environment are of significant importance. The key is to match the transmission line impedance from the application PCB through the relay package leads and the relay contacts.
Any discontinuities throughout this path would result in reflection of the incoming signal and losses to the output. The higher the frequency of operation, the more important the impedance match for the signal path. This path should be in a straight line without sharp turns. Turns provide discontinuities that result in either peaks or valleys in the characteristic impedance throughout the signal path.
An important tool for analyzing the signal path is the TDR. This instrument sends a high-frequency, fast rise-time pulse into the RF system and measures the time and amplitude of the return signal. The TDR plot provides a representation of the impedance through the signal path.
Figure 3 shows a TDR plot of one of the RF reed relays from this study. On this plot, the impedance of the PCB trace is shown along with the discontinuities through the relay lead and the relay contacts. These discontinuities result in higher insertion losses and higher return losses. The ideal TDR plot would be a straight line at the desired characteristic impedance.
The relay package has a large impact upon the RF characteristics of the device. SMD packages offer better performance than through-hole packages. The leadless and axial packages provide the overall best performance and highest bandwidths. Figure 4 shows the performance of a device similar to a gullwing SMD relay mounted in a leadless ceramic package.
The -3-dB roll-off points for the gullwing device shown in Figure 1 and the SMD leadless device shown in Figure 4 are approximately 5.7 GHz and 7.5 GHz, respectively. The rise times of the leadless SMD and the gullwing device are calculated at 61 ps and 47 ps, respectively. This improved RF performance for the leadless device generally is due to the improved signal path through the relay.
Aging Effects on RF Performance
Normally, as relays are switched for many millions of cycles, the contact resistance will start to degrade to a point of failure. For low-frequency applications, failures typically are defined as a point where the contact resistance has increased to some high resistance. For most small reed relays, the contact resistance typically is in the range of 50 mΩ for a new relay. A life-test failure generally is defined as the point when the contact resistance exceeds 1 Ω.
An experiment was set up with 10 microminiature SM gullwing reed relays mounted on a PCB (Figure 5). The PCB had a ground plane with the trace width designed for 50-Ω impedance. Side-mount SMA connectors were used to keep the signal path a straight line from input to output.
The relays and PCB were characterized using a TDR, and the insertion loss and return loss were measured before the relays were aged. The relays analyzed in this study all had characteristic impedances at the contacts of approximately 70 Ω, not the ideal 50 Ω as advertised.
To accelerate the aging, the relays switched a 10-V 10-mA resistive load for up to 300 million cycles. The insertion and return losses were measured at incremental points as the relays aged.
A total of four relays with contact resistances over 1 Ω failed during this life test. The first failure occurred by 250 Mcycles with a contact resistance of 1.6 Ω. The remaining three failed by 300 Mcycles with contact resistance readings of 1.6 Ω, 1.1 Ω, and 1.9 Ω. There were no appreciable changes in the TDR, the insertion loss, or the return loss measurements for any of the devices that failed the standard life test. All of the relays had almost identical RF readings before and after the life test.
Conclusion and Recommendations
Depending upon the application, a range of RF relays is available. An important aspect of the high-frequency operation is the relay package and pin configuration. The highest speed operations can be achieved with SM packages that minimize the lead direction discontinuities. The J-lead and gullwing configurations are superior to the standard through-hole. However, for the highest frequency operation, either leadless SM packages or axial lead packages offer the best performance since signal-path discontinuities are minimized.
The capability of the relays to effectively transmit the RF signals based on the insertion and return loss measurements over the frequency range was not altered as the relays were aged. The contact resistance could increase by more than an order of magnitude without altering the characteristic impedance of the relay or its insertion and return loss characteristics over the frequency spectrum. Accordingly, an RF relay should be able to deliver a total number of life cycles equivalent to its mechanical performance for most low-level RF switching applications.
About the Author
Phil Roettjer is president of Relay Testing Services. Before joining RTS six years ago, he worked for more than 20 years on the development of hard disk drives, his last positions as the director of quality for the High-End Disk-Drive Division of Maxtor and as vice president of customer engineering with Quantum. Relay Testing Services, 89 Hartford Ave. East, Mendon, MA 01756, 508-473-5005, e-mail: [email protected]