Characterization of Reed Relays Capable of Handling Frequencies up to 10 Ghz

7 GHz RF Reed Relay MEDER electronic Characterization of Reed Relays Capable of Handling Frequencies up to 10 GHz

Introduction the signal to shield capacitance has dropped to 0.5 pf when the reed is in the open state. For years engineers had thought the best way to switch high frequencies and very short fast digital pulses was When designing in the frequency domain with semicon- to use special semiconductors designed to handle the ductors, one has to deal with special added circuitry to high frequencies – namely gallium arsenide mosfets. reduce or eliminate inter-modulation distortion (also a Today, however, gallium arsenide is not the only option; potential problem with fast digital circuits). By its nature, new semiconductor materials are being developed that no inter-modulation distortion exists in a Reed Relay are less expensive and good for RF. Also, in fact, the and therefore, no special circuitry is required. This is Reed Relay is making a very big impact in the world of particularly useful in attenuator networks constructed high frequency and fast digital pulses. from Reed Relays.

The Reed Relay by its basic geometry resembles a co- Form C Reed Relays (single pole double throw) have axial cable (see Figure 1). The magnetic reeds make the potential added advantage when in its normally up the center conductor with a glass envelope setting closed state, they require no external power. In T/R the spacing from the center conductor to the coaxial (Transmit/Receive) requirements this can be of real shield, and therefore, its characteristic impedance (typi- value, particularly if the receive mode represents 99 % cally 50 ohms). Generally the RF characteristics were of the duty cycle and the device is battery operated. No not considered significant in the early years of Reed battery power is drawn 99 % of the time. Here extended Relays because the Reed Switches were too big and battery life is a clear benefit over semiconductor switch- the corresponding Reed Relays were too large having ing devices where power is required all the time. a long signal path length. However, in the 1980’s the Reed Switches began to shrink in size offering shorter In test and measurement, particularly IC (Integrated and shorter signal path lengths. It was here that the all Circuit) testers, with parallel high switch point counts, important signal to shield capacitance began to drop leakage current becomes a real problem. Reed Relays below 1.0 picofarad and hence the improved RF perfor- specially designed to handle fast digital pulses will also mance. Today, with 5 mm or less Reed Switch lengths offer leakage currents on the order of 0.1 pico-amps or less, a clear requirement and benefit with this technology. No other technology currently offers anything close to this combination.

Frequency Domain vs Time Domain

Today, the use of RF components has dramatically increased, where only a few years ago, they were pri- marily used in military requirements and specialized Figure #1. Shows the similarities of a Reed Relay with a coax- test equipment. With the cell phone revolution coupled ial shield to that of a RF transmission line. Since the coil is ef- with dramatic increases in computer processor speeds, fectively screened by the coaxial shield, it has no effect on the transmission of RF signals along the center lead conductor. requirements were created to run high frequencies and

www.meder.com 72 MEDER electronic 7 GHz RF Reed Relay high speed digital pulses through a host of different components. The need to run bigger and bigger software programs necessitated the need for faster processor speeds, as well as increased efficiency in signal processing, when converting analog signals to digital. The clear need to process large amounts of information required increased and faster memory. A few years ago processor speed exceeded the 1 GHz level and has not slowed, as each year processor speeds continue to increase. With this increase all electronic components need to increase their ability to switch or pass these fast signals. Figure 2. A simplified transmission line

To adequately compare the time domain to the frequen- In Figure 3 below represents an instantaneous point cy domain one has to recognize the fact that it takes at along a transmission line. R is the DC resistance; L is least 5 harmonics of the base frequency to construct a the inductance; C the capacitance and G the admit- square wave (digital pulse). Equation # 1. represents tance per unit length. In a loss-less line the character- this mathematically. Therefore, circuitry processing or istic impedance (Zo ) would be defined as Zo = distributing a digital clock running at 1 GHz will require (L/C)1/2. For microwave circuits, 50 ohms has become components in the signal path to have a bandwidth up the standard in most cases. to 5 GHz on a CW (continuous wave) basis.

8 RL ν(t) = V/2 Σ 2V/(πn) sin (2πnt) n = odd C G Equation 1. This equation is the generalized form representing a square wave or pulse depending on the boundary conditions. It takes at least five terms (or five odd harmonics) to start ap- proximating a square wave. Uniform Transmission Line

Transmission Line Review Figure #3. An instantaneous look at what a signal sees at a given point on a transmission line in terms of passive physical High frequency systems have a source of power measurements. whether digital or analog that is delivered to a load, With this basic view of transmission lines, there are usually through a series of components by means of several ways to express and develop relationships that transmission lines. Figure 2. represents this simplest define what happens to signals as they travel through form where Z is the impedance of the line; and Z is s L them. Probably the most popular is through the use of the impedance of the load. The power, whether a digital scattering parameters and a two-port network. pulse or a wave, travels both ways as shown. A portion of the incident wave will be reflected back to the source, where it may be reflected again. If ZL = Zs, there will be standing waves.

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S – Parameters Measurements V Z Z i 1 = 11 12 1 Scattering Parameters or S-Parameters are a param- V2 Z21 Z22 i2 eter set related to four variables associated with the model of a linear two-port network. They define the small signal gain and the input/output properties of a Equation. 2. Matrix representation of a two port network in a linear two-port network. S-Parameters are forward and Z-Parameter representation. reverse insertion gains, and input and output reflection coefficients taken with driven and non-driven ports both Here the matrix is presented in Z-parameter represen- terminated in equal impedance, usually 50 Ω real. They tation to show the parametric concept using the two differ from other parameter sets because of this termi- port network approach. As can be seen, knowing some nation (e.g., Y-Parameters or Short Circuit Admittance of these parameters through measurement the others Parameters are found by exciting one port and short can be calculated. However, when dealing with higher circuiting the other). frequencies it is much harder to measure voltages and currents, but much easier to measure power. To develop the S-parameters we start with a two-port approach. The beauty of a two-port analysis is we need In a similar manner the S-parameters are developed. only consider what is between the two ports as a black The equations become a little more complicated than box, where knowledge of the black box is immaterial simple algebraic equations generated by the above ma- to the development of the S-parameter equations. You trix. Manipulating Eqn 2, we can write: need not know anything about the internal circuitry to make use of the two-port concept. I = z-1v Eqn 3

Using this approach, we will first develop a simple ma- where z-1 is the inverse of the matrix Z. From this, we trix representation of the internal circuitry. In Figure #4., can further represent the equation in terms of power. a two port network is shown, which can easily be repre- However, obtaining the inverse of a matrix is very te- sented by a two by two matrix as given in Eqn. 2 using dious without the use of computers or calculators which

Z-parameter representation. Here V1 and V2 represent can easily calculate the matrix cofactors and determi- the input/output voltages of the two port network; Z11, nants as well as transposition. However, now canned

Z12, Z21, and Z22 represent the impedances entering the computer programs are very easy to use with simple nodes, and i1 and i2 represent two node currents. inputs and quick internal calculations yielding fast results. For this reason and the bountiful amount of information generated, we now look into how S-parameters are developed.

b S S a 1 = 11 12 1 b2 S21 S22 a2

Eqn 4

Figure 4. presents a two port network that can completely describe external functionality. One can treat any circuit as a two port network if you can select two pairs of nodes.

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In Figure #5 below, we now look at a two-port network S11 = S22 = 0 Eqn 9 where we can construct the following matrix: The input reflection coefficient (Γ) can be expressed in

terms of the S - parameters and the load Z L as

Γi = E1r / E1i or

Γi = b1 / a1

= S11 + (S12 S21 ΓL) / (1 - S22 ΓL) Eqn 10

Where

Γ0 = ( Z L - Z 0 ) / ( Z L + Z 0 ) Eqn 11

Also, the output reflection coefficient, with E1 = 0, can Figure #5. Two port transmission matrix be expressed in terms of the generator impedance Z1 and the S - parameters as From the matrix we can write the following set of equations ΓL = b2 / a2 ( for E1 = 0)

= S22 + ( S12 S21Γ1) / (1 – S11 Γ1) 11 b1 = S11 a1 + S12 a2 Eqn 5 Eqn 12

b2 = S21 a1 + S22 a2 Eqn 6 where

Γ1 = ( Z 1 - Z 0 ) / ( Z 1 + Z 0 ) Eqn 13

Here a1 and a2 represents the incident waves at ports

1 and 2 respectively; b1 and b2 represent the reflected Now for the case where Z 1 = Z 0 where Z0 = Charac- waves as shown in Figure #5. Just as the Z-parameter teristic Impedance = 50 Ω and E1, E2 = Electrical Stimuli set relates total voltages and total currents at the net- @ Port 1, Port 2 respectively, we can write the following work ports, S-parameters relate traveling waves. Here equations in the form of power: the incident waves a1 and a2 are the independent vari- 1/2 ables, and the reflected waves b1 and b2 are the depen- a1 = (Incoming power @ Port 1) Eqn 14 dent variables. 1/2 b1 = (Outgoing power @ Port 1) Eqn 15 For the S matrix, the off-diagonal terms represent volt- 1/2 age wave transmission coefficients, while the diagonal a2 = (Incoming power @ Port 2) Eqn 16 terms represent the reflection coefficients. If the net- 1/2 work is reciprocal, it will have the same transmission b2 = (Outgoing power @ Port2) Eqn 17 characteristics in either direction, i.e.

Now, for E2 = 0, then a2 = 0 and we have the following:

S12 = S21 Eqn 7

S11 = b1/a1 Eqn 18 If the network is symmetrical, then = [Outgoing Input Power / Incoming Input Power] 1/2

S11 = S22 Eqn 8 = Reflected Voltage / Incident Voltage For a matched two-port, the reflection coefficients are zero and = Input Reflection Coefficient

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S21 = b2/a1 Eqn 19 The Value of the S-parameters

= [Outgoing Output Power / Incoming S-parameters have a magnitude and an angle associ- Input Power] 1/2 ated with them, and are easily obtained from a suitable network analyzer when testing components or circuits. = [Forward Transducer Gain] 1/2 To take full advantage of the S-parameters taken from a network analyzer and the accompanying results above,

And for E1 = 0, then a1 = 0 we can reasonably accurately reproduce the Insertion Loss, the VSWR and Return Loss in conjunction with a

S12 = b1 / a2 Eqn 20 suitable MMICAD program. Here we can establish the effect on an RF circuit with the addition of a component = [Outgoing Input Power / Incoming without ever having to physically insert the component Output Power] 1/2 into the circuit. Development of an equivalent circuit for a Reed Relay for both its open and closed contact states = Reverse Transducer Gain will yield more accurate results when added to the MMI- CAD program. These equivalent circuits are shown in

S22 = b2 / a2 Eqn 21 Figures 6 and 7 below. By applying the S-parameters to the software program, an engineer can immediately = [Outgoing Output Power / Incoming find how the Reed Relay (or any other component) will Output Power] 1/2 interact in his circuit with other components.

= Output Reflection Coefficient

And finally, for the case of a linear two-port passive device these equations reduce to the following:

S11 = Input Reflection Coefficient

= VSWR Eqn 22

S21 = Forward Gain

= Insertion Loss Eqn 23

20 log 10 (S11)

= Return Loss in dB Eqn 24

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R1 R4 R7 R9

C1 C5 R2 L1 L2 R10

C2 C3 C4 C6 R3 R5 R6 R8 R11

Figure # 6. The equivalent circuit of the closed contacts of a coaxially shielded Reed Relay with two ground terminals on both the input and output of the Relay.

R1 R4 R7 R9

C1 C C5 R2 L1 L2 R10

C2 C3 C4 C6 R3 R5 R6 R8 R11

Figure # 7. The equivalent circuit of the open contacts of a coaxially shielded Reed Relay with two ground terminals on both the input and output of the Relay.

Smith Charting

In a likewise manner, the S-parameters can be plotted This information can be most helpful when trying to tune on a Smith chart revealing further information concern- a given RF circuit, particularly, if there is a net inductive ing the characteristic impedance over a wide frequency or capacitive reactance. The engineer will know what to range. Here the exact impedance for a given frequency add or subtract and where. is plotted, and whether that impedance has a net inductive or capacitive reactance.

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RF Parameters defined for usage in Frequency Domain RF Parameters the Time Domain and Frequency Do- Isolation (open circuit transmission loss). Isolation rep- main resents the energy loss stated in dB when energy is transmitted through the open contacts. The following define the parameters we want to measure on a Reed Relay, whether the relay is used in the Isolation Loss = E Transmission Eqn 25 time domain or the frequency domain. Insertion Loss (closed circuit transmission): Indicates Time Domain RF Parameters or TDR device losses or reflections occurring when energy is incident on the relay and is reflected back rather than To measure time domain parameters, Time Domain transmitted through the relay. Reflectometry (TDR) is employed. Time domain reflectometry allows one to characterize a transmission line Insertion Loss = E Incident - E Transmission Eqn 26 or series of components by the reflections or disconti- nuities occurring when sending a pulse of known amplitude and rise time into the line or circuitry. A trans- Voltage Standing Wave Ratio (VSWR) mission line terminated by its characteristic impedance appears as an infinitely long line (no reflections). No termination (open circuit) gives reflections due to mis- Incident Transmitted matches. Detection of relative position of discontinui- Wave Wave ties, whether inductive or capacitive, depend upon the polarity of the reflected signal. However, knowing the Standing polarity of the reflection, redesign of the component to Device Wave eliminate that capacitive or inductive point in the signal path of the component can yield a smoother signal with less reflections and better transmission characteristics. Reflected Also, characterizing the component on an ‘as is’ basis, Wave allows the user the ability to add compensation circuitry quickly and easily by knowing ahead of time, the type of Figure #8. VSWR compensation needed. VSWR = (EI + ER) / (EI - ER) Rise Time is the time between 10% and 90% of the full amplitude of the leading edge of a pulse. A pulse = (1 + ρ) / (1 - ρ) Eqn 27 incident upon a relay with a perfect rise time (0) will be altered once it exits the relay with a rise time stated where the Reflection Coefficient Γ = ρ∠φ as the Relay Rise Time. Any system dealing with fast Ideal conditions: ρ = 0, VSWR = 1 digital pulses must consider the rise time through the components where rounding off and/or distortion of the Return Loss: square wave can occur. 20 log 10 (VSWR) = Return Loss in dB Eqn 28 Characteristic Impedance (Z) (50 ohms). Represents the distributed impedance at any instantaneous point at the entry, through and exiting the relay. A pulse or signal traveling through the path of the relay seeing any impedance changes will reflect some of its signal strength. Standing waves can occur at these reflection points.

www.meder.com 78 MEDER electronic 7 GHz RF Reed Relay Introduction of the First Patent Pending Ceramic Reed Relay

Introduction of the first new Reed Relay actually de- relay design increasing its reliability and reducing its signed from its inception for high frequency and fast susceptibility to environmental conditions. pulse requirements will now be presented. 3. RF Capability For many component designs, the application either Early samples tested to 6 GHz had insertion loss under backs into the design, or the application is made to 1 dB. With a few minor improvements, we expect us- ‘fit’ an existing design. Here, our design incorporates age to 10 GHz a real achievable goal. The low switch from the beginning, key design features improving path to shield capacitance is a key ingredient to achieve this length, capacitance, distributed characteristic imped- exceptional RF performance. ance, high conductivity, and extremely low leakage currents to all points. 4. Very Small Size Customary relays in the past have been much larger. CRF Ceramic Relay Characteristics This design was predicated on small size to take full advantage of the clear benefits associated with size – less 1. Ceramic Base PCB space, shorter signal paths and better performance We start with a ceramic substrate with very short, high in high frequency and fast pulse applications. Also, its conductivity, gold signal paths. The ceramic was cho- low profile allows for tighter spacing when stacking sen for several reasons: inexpensive, great material for PCBs very close together, or in tight areas where a low plating patterns onto the substrate, hard robust packag- profile is critical such as in PDAs and Cell Phones. ing, thermal coefficient of expansion matching the thermoset overmolding, and extremely good thermal con- 5. Thermoset Epoxy Over-mold ductivity. The thermal conductivity serves to dissipate Thermoset epoxy over-mold added to the ceramic base any heat very efficiently reducing any potential thermal gives a very rugged final package capable of withstand- offset voltages from being generated when thermal gra- ing almost all environmental conditions. This relay se- dients are present. It also can aid in the removal of ries has the ability to withstand temperatures as low as the component from a PCB by heating only one area, –65 oC to as high as 155 oC under steady state non- the substrate conducts the heat to all areas of the sub- operating conditions. This package, having no internal strate allowing for ease of removal. The ceramic also solder connections is capable of withstanding vapor eliminates the need of a costly, capacitive lead frame, phase and IR reflow with temperatures up to 260 oC and the circuit connections are made on the bottom of without any degradation in performance. The relays the ceramic with a ball grid solder array eliminating the passed qualification testing of 5 repeated IR reflow im- quality sensitive coplanarity and lead skewing issues. mersions with temperatures reaching up to 270 oC with no performance reduction. 2. No Internal Solder Joints No internal solder joints are used, eliminating the tem- 6. TCE Matching perature restrictions when trying to solder reflow com- Probably the most important property of this design over ponents to a PCB. All internal connections are welded other relay designs is the attention paid to the matching and have been qualification tested for several IR reflow of the thermal coefficients of expansion (TCE). Here cycles. This approach adds to the robustness of the the ceramic and thermoset epoxy’s TCEs are closely

www.meder.com 79 7 GHz RF Reed Relay MEDER electronic matched to prevent any stress buildup on the fragile and turning off the equipment, or if the equipment is Reed Switch and fine copper wire making up the ener- used in an outside environment where natural temper- gizing coil. Most of the failure modes associated with ature excursions occur, the mismatch will eventually Reed Relays, particularly in the field, are associated with fatigue the solder connection. Here the use of solder stress induced on the Reed Switch and stretching of the balls is the great equalizer. The solder being very mal- fine copper wire to its elastic limit. This usually occurs leable absorbs the mismatch eliminating this potential during board mounting of the Relay or under changing problem. This has been qualification tested over sev- environmental conditions. Stress may induce a fine eral temperature cycles and many products to find it crack on the Reed Switch that may not manifest itself works very well. for an extended period of time. Air will slowly leak into the capsule, oxidizing the metal reeds producing faulty Gold has been used to improve the RF signal path and closure of the contacts. offers high conductivity. However, care has been placed in its thickness. Gold, like copper, tends to migrate in This TCE matching is probably the single most impor- tin/lead and can form an intermetallic. This intermetallic tant part of the design that dramatically increases its is essentially un-solderable. The thickness we use is quality and reliability. Particularly, when using large well below the thickness where this potential problem populations of relays in one system, this relay would could occur. In fact, in and around the solder joint the be ideal for achieving fault free operation over a long amount of gold is safely absorbed into the solder joint. period of time. Switching signal level loads, this Reed Pull testing on the solder joints under qualification test- Relay is designed to achieve over a billion opera- ing has verified this. No degradation of the solder joint tions. was observed with each solder joint able to withstand greater than 5 lbs. of pull force. 7. Ball Grid Array No lead frame is used in this design eliminating skew- 8. Internal Magnetic Shielding ing and coplanarity issues. The solder ball approach, All Reed Relays in this series are internally magneti- now used extensively in more and more surface mount cally shielded eliminating the need for costly external applications, clearly offers exceptional coplanarity with magnetic shields. These relays, being as small as they fewer soldering issues during the soldering process. are, users will want to take advantage of its size by mounting them as close as possible to other compo- Matching the TCE internal to the Reed Relay produces nents; they can/will mount them on both sides of the the negative result of not matching to a PCB, where board; and in large systems, adjacent PCBs may be TCE’s for many types of PCB’s range in the 50 to 100 mounted as close together as possible. When these parts per million per unit length. The TCE for ceramic adjacent components are other Reed Relays or mag- is approximately 10 parts per million per unit length. netic components, the likelihood of interaction occurring This mismatch in TCEs is not a major problem if, once becomes very probable. This is particularly true if the the Reed Relay is mounted to the PCB, there is rela- relays are operated together or operated when adjacent tively small temperature variation over the life of the relays are already closed. With our internal magnetic product. If, however, there are daily temperature ex- shield this problem is eliminated. Our qualification test- cursions in the equipment, associated with turning on ing has confirmed this.

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Applications

Automated Test Equipment CRF for wafer, memory, and integrated circuit test systems. Integrated circuit and wafer testers have contin- CRR for functional test systems. Functional test sys- ued to take on an ever more complex format with the tems continue to grow in size, pin count and complexity. need for faster and faster clock rates. With clock rates Each pin usually requires 3 to 5 test connections. Each in the 2 GHz range, components must be able to pass test connection needs to be isolated from all the oth- continuous wave signals with frequency responses in ers. Introducing any leakage paths thwarts the signals the 8 to 10 GHz range. These fast switching high speed under test potentially shunting them to the point where digital signals require these new frequency responses they lose their functionality. so that signals are not slewed or reflected going through the switching components in these systems. With the Because of the high pin counts, the number of test pin counts still going up on integrated circuits, the need connections grows dramatically. Here the need to sat- for a high number of switching points continues to grow. isfy these test connections with an ultra small surface The CRF Reed Relay represents an ideal switch in mounted relay (CRR series) becomes ideal for the fol- these component testers for the following reasons: lowing reasons: 1. The frequency response of 7 GHz or greater is a 1. Extremely small size current critical need. 2. Ability to mount the Reed Relays on both sides of 2. Rise time change through the relay of 40 picosec- the board onds typical. 3. Standard internal magnetic shielding eliminating 3. 50 ohms characteristic impedance. any magnetic interaction even in the tightest ma- 4. Insertion loss less than 1 dB at 6 GHz. trices 5. Extremely small size. 4. Insulation resistance to all points typically 1014 6. Ability to mount the Reed Relays on both sides of ohms. the board (with internal magnetic shielding elimi- 5. Over 200 volts isolation across the contacts. nating any magnetic interaction). 6. A minimum of 1500 volts isolation between switch 7. Insulation resistance to all points typically 1014 and coil. ohms. 7. Thermal offset voltage across the contacts in the 8. Over 200 volts isolation across the contacts. one microvolt or less range 9. A minimum of 1500 volts between switch and coil. 8. Contact capacitance less than 0.2 pf 10. Thermal offset voltage across the contacts in the one microvolt or less range. 11. Contact capacitance less than 0.2 pf. 12. Open contacts to shield capacitance 0.6 pf.

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Instrumentation (CRR and CRF) Multi-pole Configurations

1. On the front end of multimeters where voltage iso- When circuits require common points tied together, ca- lation is required, low voltage offsets (on the order pacitance becomes a real problem. Trying to reduce of 1 microvolt or less) and very low sub-picoamp this capacitance can be a real effort with no clear so- leakages are needed. lution. Using our new relay approach multi-pole relays 2. Feedback loops where high frequency, low leak- with common tie points are no problem configuring with age, and voltage isolation are required resulting reduced capacitance. Relay drivers, connec- 3. In Attenuators where a high frequency response is tors, etc. can be easily added forming RF switching required, low leakage paths are essential, long life modules, RF attenuators, T/R switches, ‘T’ switches, (in excess of 100 million operations), and elimina- etc. tion of any inter-modular distortion is a clear need.

The CRF can also be used in cell phone applications, TX/RX switching, two way pagers, and PDAs.

DIMENSIONS *All dimensions in mm (inches)

PIN OUT PAD LAYOUT POST REFLOW

Height: max.

Figure # 9 Mechanical outlays with Ball Grid Array (BGA).