Tech Articles & Whitepapers
- Basic Facts About Cirulators & Isolators
- Intermodulation Distortion (IMD) in Medium-Power Drop-In Ferrite Isolators & Circulators
- Ferrite Circulator Switches and Their Applications
- How to Specify Isolators & Circulators
- Microwave Circulators Using Ceramic & NdFeB Magnets
- Waveguide Designators (WR)
- Common Frequency Bands
Glossary of Terms
RF Microwave Components
Connector: Coax units are equipped with SMA or N-Type connectors (other types can be fitted by request).
Circuit: The stripline (or microstrip) element that with the groundplane forms the resonator in which circulator activity occurs. This element has 120° symmetry, may be circular or triangular, and may have cuts and shapes to adjust the circuit RF properties. Three transmission lines, which are part of the circuit, connect the circuit to the three input/output connectors.
Circulator: An RF device with three ports (typically) that allows RF power to be directed with low loss from one port to another , but none out of the third port. The device is symmetrical, and any port may be selected for the input port. Circulators are made in coaxial, waveguide, stripline and drop-in geometries.
dB: The ratio, on a logarithmic scale, between two values of a unit such as power. For power, the ratio is given by 10Log(P1/P2) where P1 and P2 are the two power levels. For voltage the ratio is given by 20 Log(V1/V2) where V1 and V2 are the two voltage levels. dB is short for deciBel, 1/10 of a Bel, which is the unit that was named after Alexander Graham Bell.
dBc: The ratio in dB of a power level that is being compared to the power level of the reference carrier signal. Normally used for comparing noise and distortion products present on a carrier signal.
dBm: The ratio in dB of a power level that is compared to 1 mW. For example, a level of 1W would be 30 dBm.
Drop in: A component that does not require connectors, but is typically soldered in place. The body of the component may be bolted to the system ground plane.
Ferrite: The ferrite material is the key component of a circulator. It is a ceramic material that interacts with the applied RF field depending on the level of stationary magnetic field across it. When placed in a resonator circuit, the RF fields form standing waves that are positioned to behave as a circulator.
IMD: Intermodulation Distortion is the result of two or more RF signals interacting in a non-linear medium. Power amplifiers produce IMD products at high power levels when amplification begins to saturate and the gain is no longer linear. Ferrite devices produce IMD since ferrite materials are inherently non-linear. Dissimilar metal junctions also cause IMD products, but at a very low level. Normally third order products are significant. These have frequencies given by 2F₁ - F₂ and 2F₂ - F₁, where F₁ and F₂ are the input frequencies.
Insertion Loss: The transmission loss from input to output, measured in dB. Typically quoted as the worst loss over the passband.
Isoadapter: An isolator that includes a coax to waveguide transition. The isolator is usually contained in the coax section. This device has the advantage of combining the matching elements of the transition and the isolator.
Isolation: The transmission loss from output to input, measured in dB. This parameter is not applicable to a circulator. Typically quoted as the worst isolation over the passband.
Isolator: An RF device that allows RF energy to flow with low loss in one direction, but presents high attenuation in the other direction. Typically an isolator is made by terminating one port (3) of a circulator. Power reflected back into the output port (2) is absorbed by the termination.
Magnet: Powerful ceramic magnets provide the stationary field that causes the ferrite material to exhibit circulator properties. Typically a pair of magnets is placed either side of the ferrite components.
Microstrip: A transmission line having a strip center conductor and a ground plane on just one side, separated by a dielectric material.
Pole Piece: Thin plates of steel that are used to help complete the magnetic circuit, maximizing the field in the ferrite material and reducing the stray field outside the device. Pole pieces are normally placed outside the magnets.
Power: Power is normally measured in units of Watts (named after the inventor).
Resonance: When a magnetic bias field is present with an RF field in a ferrimagnetic material, electrons precess about the axis of the bias field. With sufficient magnetic bias field, the electrons align with the bias field, and exhibit a resonant frequency in the microwave spectrum. This causes considerable loss to an incident RF signal having this resonant frequency. The RF frequency ω₀ (ω0 = 2ΠF₀) at which resonance occurs depends on the magnetic bias field H, according to the relationship F₀ = γH, where γ has the value 2.80 MHz/Oersted.
Resonance, Above/Below: Below resonance ferrite devices are designed so that for a given operating frequency, the magnetic bias field is below the level causing resonance. Similarly, if the bias field is high enough to exceed the resonant field, the device is operating in the “above resonance” mode.
RFI: Radio Frequency Interference. In order to minimize radiated emissions of RF energy, seams of coaxial devices can be sealed with conductive sealant.
Stripline: A transmission line having a strip center conductor and ground planes either side, separated by a dielectric material.
Surface Mount: Surface mount components are soldered directly to circuit traces. Solder pads on the component may be under the component.
Temperature Compensation: Ferrite and magnet materials are selected to minimize variations of circulator properties with temperature. Where excessive variation occurs, sheets of temperature compensation steel are used to reduce the variation. These steels show considerable reduction in permeability with temperature and compensate for excess magnetic field in the ferrite material.
Termination: A load on a transmission line that matches the transmission line impedance, normally 50 ohms. A termination must be able to handle all the power transmitted through the transmission line.
VSWR: (Voltage Standing Wave Ratio) is a ratio ranging from 1 to infinity, expressing the amount of reflected energy. Reflected energy sets up “standing waves” in the voltage pattern, and the height of maximum to minimum value determines the ratio. A value of one indicates that all of the energy will pass through, while any higher value indicates that a portion of the energy will be reflected.
Waveguide: Rectangular tube which passes RF energy with very low loss. The waveguide cross section determines the operating frequency range, which is about 20% of the nominal center frequency. Waveguide circulators are made with a Y geometry, in which the ferrite is placed close to the center of the Y. Waveguide flanges are used to connect the device to other components.
Wrap: Thin plates of steel that are wrapped around coax and waveguide circulators (typically 3 sides) to complete the magnetic circuit, maximizing the field in the ferrite material and reducing the stray magnetic field outside the device.