Solution
These considerations should be used whenever RF signals are implemented in any switching system. This can prevent damage to the RF hardware as well as preserve the expected lifetime of the relay. Listed below are some important considerations when switching RF signals.
- Characteristic Impedance Matching.
Characteristic impedance is a transmission line parameter that determines how propagating signals are transmitted or reflected in the line. For RF signals, this is an important parameter used to prevent reflections that might damage sensitive RF equipment.
When selecting an RF switch, verify that the impedance of the switch will match the overall impedance of the system. This is because the RF switch will act as a transmission line, and we must ensure that the transmission line (switch) will match the signal source impedance and load impedance. This will insure that the maximum signal power is transferred through the switch, as well as prevent reflections from damaging the signal source. Generally, most RF signals are 50 Ω, however some test systems using video signals will have a system impedance of 75 Ω.
Similar to ensuring that the characteristic impedance is matched for the entire system, we need to verify that signals are properly terminated to ensure that the entire system has the same characteristic impedance.
For example, in a typical RF application involving switches, your source, load, transmission line, and switch impedance may be 50 Ω. When the RF relay opens, the source and transmission line impedance will remain at 50 Ω, while your load impedance has jumped to a very large value. This impedance mismatch can cause RF signals to be reflected back into the signal source and damage equipment.
When switching sensitive RF equipment, it may be necessary to ensure that the signal is terminated to fulfill the requirement of keeping identical characteristic impedance throughout the system. A terminated switch is essentially a broadband impedance (resistor) to ground that matches the characteristic impedance of the system when the switch is closed. Effectively, it protects the device under test (DUT) from self-inflicted damage when open, particularly in high power situations such as some transceivers.
NOTE: A switch that is closed with termination generally cannot handle the full power of the switch. See device documentation for additional information.
- Switching Active RF Signals.
Active RF signals should not be switched in order to prevent damage to the RF signal source. This is because, as a relay actuates, the channel is momentarily unterminated. As discussed in the Termination section of this article, some RF sources can be damaged by reflections if their outputs are not properly terminated.
It is recommended that when switching RF signals, any test sequences should stop RF signal generation until after the switching process is complete. The switch relay state can be verified through the use of the "Wait For Debounce" function of the NI-SWITCH/NI Switch Executive API or the DAQmx Switch "Wait For Settling" function. Refer to your RF source documentation for more specific information on signal reflection tolerances.
- Switching Low Signal Levels
To obtain the highest performance and power, most RF relays are composed of Electromechanical Armature relays. These relays are generally able to handle higher voltage and current levels than other relay types and are commonly used in these applications. Unfortunately, all Electromechanical Armature style relays have problems when switching low level signals.
This is because inside of sealed relay housings there exist gases that are emitted from the plastics, adhesives, and other materials that go into making the relay. This material is organic and is adsorbed onto the contacts where it can then polymerize under low voltage and current switching conditions. Higher voltages and currents ensure that switching arcs are of sufficient energy to clean deposits from the contact surfaces. Low level contact loads do not provide enough energy to form an arc sufficient to clean off the contacts. Polymerization occurs instead of arcing, leaving behind a powdery substance on the contacts causing the resistance to rise. This will prevent the relay from performing well in frequently repeated tests and will decrease the performance of your RF switching application.
The best way to avoid creating high resistance contacts is to read the relay module specifications carefully. Most armature relays should have minimum voltage and current specifications. Make sure that your application will be above these values. If switching very low currents with armature relays is unavoidable, momentarily increasing the switching current or "Buzzing" the relay may prove to be beneficial if performed periodically.