IN MY OPINION

Time to Reevaluate RF and Microwave Power Application Development

By Dan Viza, RF and Emerging Markets Manager Freescale Semiconductor

The methods for evaluating RF power components for use in RF and microwave applications have seen relatively little change over the years in spite of the expanding universe of RF applications.
Read More...

FROM WHERE WE SIT

Uncertain Times for DefenseIn Defense of DARPA; Lamenting Bell Labs
By Barry Manz

A federal agency like DARPA is a sitting duck for politicians and assorted other critics. It has come up with some truly bizarre programs over years that ultimately either delivered no tangible results, were canceled before they could cause any damage, or attempted to answer questions that nobody was asking or needed answers to. Read More...


CURRENT ISSUE PRODUCTS


Band Reject Filter Series
Higher frequency band reject (notch) filters are designed to operate over the frequency range of .01 to 28 GHz. These filters are characterized by having the reverse properties of band pass filters and are offered in multiple topologies. Available in compact sizes.
RLC Electronics


SP6T RF Switch
JSW6-33DR+ is a medium power reflective SP6T RF switch, with reflective short on output ports in the off condition. Made using Silicon-on-Insulator process, it has very high IP3, a built-in CMOS driver and negative voltage generator.
Mini-Circuits


Group Delay Equalized Bandpass Filter
Part number 2903 is a group delayed equalized elliptic type bandpass filter that has a typical 1 dB bandwidth of 94 MHz and a typical 60 dB bandwidth of 171 MHz. Insertion loss is <2 dB and group delay variation from 110 to 170 MHz is <3nsec.
KR Electronics


Absorptive Low Pass Filter
Model AF9350 is a UHF, low pass filter that covers the 10 to 500 MHz band and has an average power rating of 400W CW. It incurs a rejection of 45 dB minimum at the 750 to 3000 MHz band, and power rating of 25W CW from 501 to 5000 MHz.
Werlatone


LTE Band 14 Ceramic Duplexer
This high performance LTE ceramic duplexer was designed and built for use in public safety communication and commercial cellular applications. It operates in Band 14 and offers low insertion loss and high isolation to enable clear communications in the LTE network.
Networks International

See all products in this issue


May 2008

Isolator and Circulator Basics
By MECA Elecronics

An RF isolator is a two-port ferromagnetic passive device which is used to protect other RF components from excessive signal reflection. Isolators are common place in laboratory applications to separate a device under test (DUT) from sensitive signal sources. An RF circulator is a three-port ferromagnetic passive device used to control the direction of signal flow in a circuit and is a very effective, low-cost alternative to expensive cavity duplexers in base station and in-building mesh networks. Examples of both applications will be covered later in this article.

To understand how these components control the signal flow, think of a cup of water into which you place a spoon and stir in a clockwise motion. If you sprinkle some pepper into the cup and continue to stir, you will notice that the pepper easily follows the circular motion of the water. You can also see that it would be impossible for the pepper to move in a counterclockwise direction because the water motion is just too strong. The interaction of the magnetic field to the ferrite material inside isolators and circulators creates magnetic fields similar to the water flow in the cup. The rotary field is very strong and will cause any RF/microwave signals in the frequency band of interest at one port to follow the magnetic flow to the adjacent port and not in the opposite direction.

Figure 1 shows the schematics for a circulator and an isolator. Notice how an isolator is a circulator with the third port terminated. The arrows represent the direction of the magnetic fields and the signal when applied to any port of these devices. Example: if a signal is placed at port A, and port B is well matched, the signal will exit at port B with very little loss (typically 0.4dB). If there is a mismatch at port B, the reflected signal from port B will be directed to port C. As you will note, it makes no difference which port is the input of the circulator because the relationship at the outputs remains the same as these devices are electrically and mechanically symmetrical.

Isolation
An important consideration when specifying an isolator or circulator is to ensure the device has adequate isolation for your given application. Isolation is a unit of measure (in dB) that states the separation of signal levels on adjacent ports of a device. The greater the isolation value, the less interference from a signal on one port is present at the other. The amount of isolation is directly affected by the VSWR presented at port 3 of the isolator. If the match on port 3 is poor, you can expected isolation below 10 dB, but if the match is improved to 1.10:1 by using a good termination device in the circuit, then the isolation would improve to over 20 dB.

Insertion Loss
Another important consideration when specifying circulators and isolators is to ensure the device has minimal insertion loss when inserted in a transmission path. Generally, the insertion loss of a circulator/isolator (or any microwave device for that matter) becomes more significant at higher frequency, namely because loss increases with frequency and higher frequency power sources are considerably more expensive. Accordingly, the criteria of low insertion loss will prevent precious power from being wasted.

Common Applications
As described earlier, a common application for a circulator is as an inexpensive duplexer (a transmitter and receiver sharing one antenna). Figure 2 shows that when the transmitter sends a signal, the output goes directly to the antenna port and is isolated from the receiver. Good isolation is key to ensure that a high-power transmitter output signal does not get back the receiver front end as is governed by the return loss of the antenna. In this configuration, all signals from the antenna go straight to the receiver and not the transmitter because of the circular signal flow (remember the cup of water).

Figure 3 illustrates the most common application for an isolator. The isolator is placed in the measurement path of a test bench between a signal source and the device under test (DUT) so that any reflections caused by any mismatches will end up at the termination of the isolator and not back into the signal source. This example also clearly illustrates the need to be certain that the termination at the isolated port is sufficient to handle 100% of the reflected power should the DUT be disconnected while the signal source is at full power. If the termination is damaged due to excessive power levels, the reflected signals will be directed back to the receiver because of the circular signal flow.

Power Ratings
MECA isolators are designed with an internal 10w load capability. However, the recommended maximum power that our devices can sustain is 2w to allow for de-rating and heat transfer. Higher isolator power levels can be achieved utilizing our circulators with an external load which would make the limiting factor the ferrite material and not an internal resistor. As previously outlined, if the match on the terminated port is poor, you can expect isolation below 10 dB, but if the match is improved to 1.10:1 by using a good termination device in the circuit, then the isolation would improve to over 20 dB. MECA manufactures an extensive selection of high power, low loss RF loads. Please consult with a MECA applications engineer to discuss your requirements and select the proper termination for your high power isolator.

Special Handling and Storage
Isolators and circulators have magnets that produce strong fields to control signal flow. As is the case with any magnet, when placed in close proximity to another, the magnetic fields oppose one another, and over time, will weaken the strength of the magnets. This is called degaussing. A similar effect can be seen when stored in close proximity to ferrous metals. Special care should be taken when storing any isolators/circulators and MECA recommends that the devices should be separated by 3 inches from each other and all ferrous surfaces to reduce degaussing effects.

MECA offers twenty-four models of isolators and circulators in both N and SMA-female connectors with average power ratings from 2 - 250 watts. The most “popular” frequency bands between 0.8 - 18.0 GHz are readily available and can ship from STOCK in 4 weeks after receipt of your order.

MECA Electronics, Inc.
Since 1961, MECA has designed and manufactured an extensive line of RF/Microwave components with industry leading performance, including fixed attenuators, directional and hybrid couplers, isolators/circulators, power divider/combiners, RF loads, DC blocks and bias tees. MECA serves all areas of the RF and Microwave industries, including world class network providers and supporting supply chain infrastructure, and has long been the “backbone” of high performance wired and air-interfaced networks, such as in-building applications, satellite communications, radar, radio communications, telemetry applications, mobile radio, aviation and air traffic communications.

MECA Electronics
www.e-meca.com
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WHITE PAPERS

The Design of Ultra Narrowband Amplifiers Using Small-Signal Varactor Upconverters
This paper presents a method of realizing tunable microwave amplifiers with ultra narrow bandwidths that can be less than 0.5% by the use of a varactor up-converter (UC).
Planar Monolithics

Directivity and VSWR Measurements
Return loss and VSWR measurements are complicated by the finite performance of the directional device used to measure the reflected power. The only accurate and convenient way to make return loss measurements is with a well matched high directivity directional coupler or bridge.
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Switch Solutions for Systems with Low PIM Requirements
Dow-Key Microwave has invested in R&D for new RF switch products designed specifically to reduce intermodulation (IM) in coaxial switches.
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How to Specify RF and Microwave Filters
Covers cavity, ceramic, LC, crystal and helical filters.
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Mounting Considerations for Medium Power Surface-Mount RF Devices
Covers all factors that must be considered when mounting SMT devices.
TriQuint Semiconductor

Biasing MMIC Amplifiers
How to bias MMICs along with theory and techniques.
Mini-Circuits


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