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Rod Resistors

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Res-net Microwave Rod Resistors

Rod resistors are a cylindrical type resistor that are used in both RF and microwave applications. They are thin film in nature and can be made of alumina, beryllium oxide, or aluminum nitride.

Because of their thin film construction they can achieve very high frequencies. These frequencies can be as high as 26.5 GHz or even higher in certain applications. Res-net Microwave Inc. is one of the few companies that offer rod resistors in alumina, beryllium oxide, and aluminum nitride materials.

The resistance range for rod resistors is generally from 10 ohms up to 500 ohms, and the resistive tolerances are generally 1%, 2%, or 5%. The temperature range is -65 Degrees Celsius to +175 Degrees Celsius. The resistors are covered with a protective coating of high temperature epoxy in order to insulate them from these temperature extremes.

In addition to these classic thin film rod resistors, they can also be supplied using pyrolytic carbon. These carbon rod resistors also perform very well at microwave frequencies and have the added advantage of being able to work to the temperature of +200 Degrees Celsius. The sizes of the carbon resistors mimic those for the other thin film types. These carbon rod resistors will have a negative temperature coefficient as compared to the other resistors which will have either a plus or minus 100 ppm TC.

The power ratings can go from 0.050 watts all the way up to 75 watts, depending upon the material used. They are used extensively in the RF and microwave industries especially in the manufacture of other components such as, SMA terminations.

Carbon Film Resistors

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Vamistor Carbon Film Resistors
Source: Tepro of Florida, Inc.

Carbon film resistors are designed for both high temperature and high voltage applications. They are actually made in a vacuum by breaking down hydrocarbon gases at high temperatures. This in turn forms a carbon deposit on the substrate. The process is accomplished by heating methane or propane gas in a process called high temperature pyrolysis. When the heat is applied, molecular condensation results and hydrogen is released. The end result is carbon.

The main advantage and reason for using the carbon film resistor is that these components are designed to withstand high voltages as well as high temperatures. The highest voltage the devices can operate up to is 15kV with a nominal temperature of 350°C. They are available with tolerances of 2 percent, 5 percent, 10 percent, and 20 percent. Cutting a helical grove in the carbon film creates the resistive tolerances. The desired resistance value is achieved by regulating the pitch of the helix. The thinner the carbon layer, the finer the pitch and the higher the resistance value. After the helixing or spiraling is completed special alloy contact caps and tinned electrolytic copper connecting wires are pressed onto the ends of the resistor body. The final step involves coating several layers of tan lacquer or using a glass film as a shield. This is done primarily for electrical protection, but also for shielding from the climate.

Carbon film resistors are a good choice because they have a small size for such high ratings and they have a wide resistance range as well. The resistance can be anywhere from 500 Ohms to 100M Ohms. Another plus is that they have no outgassing. This means they will not deposit any vapor into the environment during operation. Outgassing is a condition that causes high material vapor pressure in the device to emit contamination deposits into the air. It corresponds directly to the temperature.

These deposits can cause problems with the surrounding equipment. Some of the other advantages are that they have less stray capacitance and inductance, so they are better at high frequencies. Also, they have a high stability of performance when compared to carbon composition resistors and, of course, they are flame proof.

Electrical noise is another factor when choosing a resistor. There are two main types of electrical noise, thermal and shot. Thermal noise is the product of the Brownian motion of ionized molecules. This noise cannot be eliminated because it is fundamental to resistance.

Carbon film resistors have less of this thermal noise than the carbon composition resistor. The lower values tend to be noise free while increasing with the higher values. Shot noise usually results from the flow of electrons through a highly charged field. It is more prevalent in solid state devices. It is not significant in carbon film resistors and until recently was not even known to exist, with respect to resistors.

There are also some disadvantages when using carbon film resistors. In the first place, they are limited to about 1 percent accuracy. Secondly, they exhibit drift with temperature and vibration. The resistors can have a TCR (temperature coefficient of resistance) range of -250 to -1000 ppm/°C, depending on the resistance value. The advantages, however, seem to outweigh most of the disadvantages, especially in specific applications.

Surface Mount Components

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Chip Resistors
Source: Res-net Microwave – Res-net Microwave, Inc.

Electronic surface mount components are attached directly to the printed circuit board (PCB). They are also known as SMT components (surface mount technology) or SMD (surface mount devices). These components can be either active or passive electronic components. Some of the types of components made in these surface mount configurations include: capacitors, inductors, resistors, semiconductors, and thermistors,

The actual technology was developed in the 1960’s, but was not fully implemented or widely used until the 1980’s. This technology helped to allow for the progression of smaller and smaller electronic devices such as calculators and computers. It also helped to open the door to an entirely new industry, electronic contract manufacturing.

In this process, the components are first put on reels using a tape and reel machine. Then the boards are populated with the various SMD components usually using a pick and place machine. Next, the printed circuit boards are run through a re-flow soldering oven. This solders the components to the board itself with a minimal amount of hand labor.

Their smaller size is not without its drawbacks. Most of the surface mount components cannot carry as much power as the older thru-hole technology because of their smaller size. Sometimes the transformer or inductor must be mounted in a separate (usually by hand) process because they are too large to be tape and reeled. Hence these boards cannot be entirely populated by a pick and place machine.

All in all, however, the surface mount technology is how most components will be manufactured in the future. The call for power is decreasing, with the exception of specific applications, so the smaller size is not an issues for most computers, tablets, and cell phones. Going forward most components will be surface mount devices, if they are not already.

Disk Resistors

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Res-net Microwave Disk Resistors

A resistor is a commonly used two terminal passive electronic component. It produces a voltage across it’s terminals that is proportional to the current flowing through it. Resistors are used to limit or regulate the flow of electrical current in an electronic circuit. They are often used within a circuit for current control or to turn a current signal into a voltage signal. Resistors can be manufactured in two different ways, one by winding a resistive wire on a core (wirewound) or a second by using a film via a sputtering process or some other method (metal film).

Disk resistors, however, are not a very common component. They are a specific type of resistor that is physically small and circular. They can be either thick film or thin film and can therefore operate at higher frequencies. They can be used in lower frequency SMA terminations in lieu of a rod resistor. Since they have a symmetrical geometry there is essentially no magnetic field, which would normally create more inductance. Therefore, with the absence of this field, they are ideal for applications that require lower inductance’s. They are also often used for pulsed power systems.

A typical disk resistor, such as those produced by Res-net Microwave, are the thick film type. They have a resistance range of 5 ohms to 500 ohms. They also have a nominal power of 10 watts.

Wound Film Capacitors

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Wound Film Capacitors
Source: Arizona Capacitors, Inc.

Wound film capacitors are classified as passive electronic components that can store an electrical charge. They consist of a pair of conductors separated by a dielectric. The dielectric stores the charge when an electric static field is created between the two conductors. This capacitance that is stored is measured in a unit called farads.

Wound film capacitors are wound with a machine much like a wirewound resistor or a transformer. They can have many different types of dielectrics. These capacitors can be wound on paper, plastic, polystyrene, Teflon, polypropylene, polycarbonate, or polyester. The configurations can vary just as much as the dielectrics and they can be purchased as a standard product or custom designed to a customer’s specifications or requirements. They can be manufactured in the form of a wrap and fill, preformed case, tubular hermetic, molded, a welded case, and many more.

The capacitance can range from 50pf to 500uf and the voltages can range from 30 to 250 KVDC. So, the wound film capacitors can fill the needs of many different specific applications and for many different industries.

Some of the industries Arizona Capacitors, Inc. serves, for example, includes: railroad, audio, communications, power generation, medical equipment, aerospace, electronic controls, automotive, just to name a few. The wound film capacitor has been around for decades and will be a staple for energy storage for many decades to come.

Microwave SMA Terminations

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Res-net Microwave SMA Terminations

RF and Microwave SMA terminations are a type of high frequency passive electronic component. With most RF and microwave systems operating in 50 Ohm characteristic impedance, the need to insure maximum power transfer and minimize signal reflections is evident. This can be accomplished with an SMA termination. Just like an SMA connector, they screw into place and are thus also referred to as a connectorized or coaxial termination.

They are similar to all terminations, in that their purpose is to terminate a signal in any given circuit. In essence, they are absorbing the incident power. If unused ports, such as a test instrument, are not “terminated” they can cause interference by having the signal reflected back from the end.

The physical part consists of the connector type, SMA body, as well as a rod resistor, but disk resistors are also sometimes used as a substitute. The rod resistor is a cylindrical thin film type resistor that usually operates up to 26.5 GHz in frequency range, depending on the size of the rod.

The metal work or connector body can come in a variety of metals, but brass and copper are very popular. The SMA termination can also be plated in silver or gold and sometimes tin, depending upon the application and the customers’ preferences. Since it is a connector with threads, it is easy to install. It screws on just as a nut would on a bolt.

Air Coils

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Air coils are actually inductors that do not contain a core, but instead use the surrounding air. Being classified as an air coil or an inductor means they are considered a passive electronic component.

Air coils are sometimes referred to as “self supporting” coils because there is no core to hold their shape. They are wound on a fixture or mandrel. Usually a special bondable wire must be used with either an electric current being passed through the wire to heat and cure the coating, or a solvent is used to activate it. These methods enable the wire to adhere to itself and thus hold its shape. In order for this to work, the heat or applied solvent must be applied while the winding is still on the metal mandrel. They are then removed once the wire “cures”.

Some air coils are wound by hand, but most are done on an automatic winding machine. This minimizes the about of direct labor and thus the component’s cost. This is one of the major benefits to using an air coil.

There are several other benefits to using an air coil in a design, if the electrical parameters allow for it. One of these benefits, as mentioned above, is the price. Not just because of the reduced labor, however, but a minimum amount of raw material as well helps to keep this product at a lower price than many of its counterparts.

A second advantage is that the inductance’s are unaffected by the current that is being carried. This benefit is due to the lack of an actual core.

A third benefit is the lack of iron or core losses. This, as one might imagine, is also due to the fact that there is not an actual core present to create these losses in the winding.

Finally, the fourth benefit is that they can operate at higher frequencies than those with a physical core. This ties back to the core losses, as they go higher in frequency the losses become greater. As electronics go higher and higher in the frequency spectrum, this becomes a tremendous advantage for using air coils in future circuit board designs.

1553 Data Bus Transformers

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Raycom 1553 Data Bus Transformers

The first MIL-STD-1553 data bus specifications were published by the United States Air Force in 1973 for the F-16 Fighting Falcon. Now, however, it is used extensively by many branches of the United States military, as well as for NATO.

The “Bus” itself is a wire pair that operates with an impedance of between 60 and 85 ohms at a frequency of 1 MHz. The actual data bus transformers are used in this specification where the transmitters and receivers couple. The transformer is used in tandem with a resistor to ensure that the bus does not conduct current through the aircraft. Also, it is used to reduce any impact from a short circuit.

There are only a few transformer manufacturers that are certified to produce the MIL-STD-1553 data bus transformers. This is mainly because it is a difficult specification to meet and an even more difficult one to become certified to build. Since this is used in flight applications, quality must be the primary focus in the manufacture and testing of these parts.

Furthermore, there are only about three companies that produce a transformer that can actually interface in all the various 1553 data bus circuits. There have been some compatibility issues identified with transformers and transceivers and Raycom Electronics has redone their design to address these specific issues. It is one thing to meet a specification, but quite another to have a product that will work in all the various types of applications. Raycom Electronics, is one of these companies.

There has been discussions over the years that fiber-optics would eventually make the 1553 data bus obsolete. but time has proved this to be incorrect. One of the main reasons being that fiber-optics would be almost impossible to repair. It is more likely that RF would eventually supplant the 1553 data bus, but that appears to be very far off in the future.

EMI Filters

CS-EMI-FilterCustom Suppression EMI Filter

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EMI Filter by Custom Suppression

 

EMI filters (Electromagnetic Interference), also known as RFI filters (Radio Frequency Interference), are vital passive electronic components. Filters in general, as the name implies, are used to “filter” out the unwanted frequencies in an electronic circuit.

EMI filters are used more specifically to suppress the electromagnetic interference, which is essentially just a high frequency noise. This noise is an undesired emission that can cause the malfunctioning of a circuit or equipment. These types of EMI/RFI (interference) can be found in power supplies, microprocessors, and AC motors. This interference can be generated inside an electrical device by impedance and voltage variances in the conductors. It can also be generated outside the device via telephone lines and power cords, just to name just a few.

EMI filters can be further categorized as “Low Pass” filters. This means that they block out the high, and in this case, unwanted electromagnetic frequencies and allow only the lower frequencies to pass through.

Two of the most important components of an EMI filter are inductors and capacitors. They are critical in the reduction of the interferences that are being suppressed. The capacitors that are used are referred to as shunting capacitors because they feed the current in a specific range into the inductors that reduce the voltage as the pass through.

The EMI filter reduces the EMI or RFI by restricting the flow of high frequency signals thereby attenuating the unwanted signal strength. Without these critical components certain circuits wound be unable to function properly, making EMI/RFI filters a vital part of many electronic circuits.