SWITCH CONTACT VARIATIONS
General Microwave switches cover the frequency range from 100 MHz to 40 GHz, and come in a variety of configurations. The terms pole and throw are used to describe switch contact variations. The number of “poles” is the number of separate circuits controlled by a switch, and the number of “throws” is the number of separate positions that the switch can adopt. Here are some examples:
A single-pole, single-throw switch (abbreviated SPST) is a simple on-off switch, which can be used to control the switch the power supply to a circuit. In a SPST switch, there is a pair of contacts that can be either closed or open.
A single-pole, double-throw (abbreviated SPDT) switch can be on in both positions, switching on a separate device in each case. For example, a SPDT switch can be used to switch on a red lamp in one position and a green lamp in the other position.
General Microwave switches are available in various topologies, ranging from single-pole single-throw, to single-pole eight-throw (SP8T, in which a single switch can alternatively turn on eight separate devices).
Microwave switches can be either reflective or absorptive. In a reflective switch, the impedance of the port that is OFF will have a very high VSWR [= Voltage Standing Wave Ratio, the ratio between the maximum and minimum voltages in a standing wave]. The high VSWR indicates a loss of signal power resulting from the reflection caused at a discontinuity in a transmission line. A high reflection loss, however, will indicate that there is a low loss of power upon insertion of a device (insertion loss). On the other hand, an absorptive switch will have a low VSWR on each port, regardless of the switch mode. This indicates a minimum of power loss in the transmission line.
General Microwave switches are available in both reflective and non-reflective configurations, and a non-reflective SP16T unit.
SWITCH TOPOLOGY
PIN diodes are named for a region of undoped intrinsic semiconductor lying between their p- and n-type materials. A simple PIN diode switch is turned on by applying a DC bias that makes the diode conduct. This lets an RF source pass a signal through the diode to a load. Applying a negative DC bias turns off the diode by widening the diode depletion layer, and thus blocks the RF signal. There are two fundamental methods of connecting PIN diodes to a transmission line to provide a switching function:
1) In series with the transmission line, so that RF power is conducted when the PIN diode is forward-biased and reflected when reverse-biased; or
2) In shunt with the transmission line, so that the RF power is conducted when the diode is reverse-biased and reflected when forward-biased.
A simple reflective SPST switch can be designed utilizing one or more PIN diodes in either configuration, as shown in Fig. 1.
A multi-throw switch essentially consists of a combination of SPST switches connected to a common junction and biased so that each switch port can be enabled individually. Multi-throw switches can be constructed by connecting multiple diodes to a common junction and forward-biasing only those in the conducting paths. The common junction of the switch must be designed to minimize the resistive and reactive loading presented by the OFF ports, in order to obtain low insertion loss and VSWR for the ON port.
There are two basic methods of realizing a multi-throw switch common junction for optimum performance over a broad frequency range:
1) The first method employs series-mounted PIN diodes connected to the common junction. A path is selected by forward-biasing its series diode and simultaneously reverse-biasing all the other diodes. This provides the desired low-loss path for the ON port, with a minimum of loading from the OFF ports.
2) The second method utilizes shunt-mounted PIN diodes located a quarter-wavelength from the junction. The diode(s) of the selected ON port is reverse-biased, while the OFF ports are forward-biased, creating a short circuit across the transmission line. As a result of the quarter wavelength spacing, the short circuits are transformed to open circuits at the junction. By proper choice of transmission line impedances and minimization of stray reactance, it is possible to construct a switch of this type with low insertion loss and VSWR over a three-to-one bandwidth. The schematic diagrams for both switches are shown in Fig. 2.
ABSORPTIVE SWITCHES
It is often desirable to have a PIN diode switch present a low VSWR in its OFF position as well as in its ON state, in order to maintain desired system performance. Absorptive switches (also known as non-reflective or terminated switches) are considered more suitable for high-power applications, such as satellite communications, radars, and wide range Internet, since they absorb the incoming waves and do not generate potentially damaging reflections.
General Microwave offers a complete line of single- and multi-throw absorptive switches which incorporate 50W terminations in each of the output ports. Fig. 3 shows the schematic diagrams of the two versions of absorptive switches employed by GMC.
The shunt termination is used in GMC’s “all-series” configured absorptive switches which have a suffix ending in “T” or “W”. This style of absorptive switch offers the minimum penalty in insertion loss due to the addition of the terminating elements.
The series termination is used in GMC’s high speed “series-shunt” configured absorptive switches, since it provides the optimum in switching performance.
The common port of the standard absorptive multi-throw switches in the GMC catalog will be reflective in the special circumstance when all ports are turned OFF. If there is a need for this port to remain matched under these conditions, this can be realized either by employing an additional port to which an external termination is connected or, in a custom design, by providing automatic connection of an internal termination to the common port.
DEFINITION OF PARAMETERS
INSERTION LOSS is the loss of signal power resulting from the insertion of a device in a transmission line or optical fiber. It is usually expressed in units of dBs. Insertion loss is the maximum loss measured in a 50 ohm system when only a single port of the switch is in the ON state.
ISOLATION is the ratio of the power level when the switch port is ON to the power level measured when the switch port is OFF. Good isolation prevents stray signals from leaking into the desired signal path. If these stray signals are allowed to get through, measurement integrity is severely compromised. In a multi-throw switch the isolation is measured with one of the other ports turned ON and terminated in 50 ohms.
VSWR is the ratio of the amplitude of a partial standing wave at an antinode (maximum) to the amplitude at an adjacent node (minimum). VSWR is an indicator of reflected waves bouncing back and forth within the transmission line. An increase in VSWR corresponds to an increase in power in the line beyond the actual transmitted power. This increased power will increase RF losses, as increased voltage increases dielectric losses, and increased current increases resistive losses. It is defined for the input and output ports of the selected ON path. For those switches with a “T”, “W” or “HT” suffix, the VSWR is also defined for the OFF state.
VIDEO LEAKAGE refers to the spurious signals present at the RF ports of the switch when it is switched without an RF signal present. These signals arise from the waveforms generated by the switch driver and, in particular, from the leading edge voltage spike required for high- speed switching of PIN diodes. When measured in a 50-ohm system, the magnitude of the video leakage can be as much as several volts. The frequency content is concentrated in the band below 200 MHz, although measurable levels for high-speed switches are observed as high as 6.0 GHz. The magnitude of the out of band video leakage can be reduced significantly by the inclusion of high-pass or “video filters” (1) in the switch. The General Microwave E-series switches are specially designed for low in-band video leakage, without sacrificing switching speed.
HARMONIC AND INTERMODULATION
PRODUCTS
Harmonic products are spurious signals that are multiples of the operating frequency of the device. Intermodulation products (or intermods) are spurious signals occurring when more than one carrier is transmitted simultaneously through a single non-linear device. Two new frequencies are created from the sum and difference of the original frequencies, resulting in intermodulation distortion. IM distortion is measured as a percentage of the original frequencies, and a lower specification is better, since these undesired tones interfere with communications in nearby channels if too large.
All PIN diode switches generate harmonics and inter-modulation products, since the PIN diodes are fundamentally non-linear devices. The magnitude of these spurious signals is typically small in a switch, since the diodes are usually either in their saturated forward-biased state or in their reversed-biased state.
The physics of the PIN diode cause a cut-off frequency phenomenon such that the level of harmonics ( = spurious signals at integer multiple frequencies of the fundamental frequency) and intermods greatly increase at low frequencies. These levels will vary with the minority carrier lifetime ( = the average time interval between the generation and recombination of minority carriers) of the diode. Thus, a high speed switch operating below 500 MHz may have a second-order intercept point of 35 dBm, while a slow switch operating at 8 GHz will have a second-order intercept point of 70 dBm. Table 1 summarizes typical performance of high- and low-speed switches:
Table 1: Typical Switch Intercept Points
| SWITCH | FREQUENCY | 2nd ORDER INTERCEPT |
3rd ORDER INTERCEPT |
| HIGH SPEED | 2.0 GHz | +50 dBm | +40 dBm |
| LOW SPEED | 2.0 GHz | +65 dBm | +50 dBm |
Since these levels vary significantly with frequency, switching speed and RF topology, please consult the factory for specific needs in this area.
(1) For switches with internal video filters, specify Option 41, Option 42, or Option 43. These filters reduce the leakage as shown in the chart page 91.
SWITCHING SPEED (2)
Switching speed is a measure of the rate at which a given electronic logic device is capable of changing the logic state of its output in response to changes at its input.
Port-To-Port Switching is the interval from the time the RF power level at the off-going port drops to 90% of its original value to the time the RF power level in the on-going port rises to 90% of its final value. See Fig. 4.
Rise Time is measured between the 10% and 90% points of the square-law detected RF power when the unit is switched from full OFF to full ON. See Fig. 5.
Fall Time is the time between the 90% and 10% points of the square-law detected RF power when the unit is switched from full ON to full OFF.
On Time is measured from the 50% level of the input control signal to the 90% point of the square-law detected RF power when the unit is switched from full OFF to full ON.
Off Time is measured from the 50% level of the input control signal to the 10% point of the square-law detected RF power when the unit is switched from full ON to full OFF.
In addition to the above definitions, the following information about switching performance may be useful to the system designer:
Switching To Isolation – Although catalog switching speed specifications are usually defined to the 10% level of detected RF (equivalent to 10 dB isolation), the user of a switch may be more interested in the time that the switch requires to reach rated isolation—Switching To Isolation time. The Switching to Isolation time is strongly dependent on the topology of the switch. For all-shunt mounted or combination series and shunt mounted topologies, the time to reach final isolation is usually less than twice the fall time. For an all-series topology, the time to reach final isolation may be as much as ten times the fall time.
Switching To Insertion Loss – For multi-throw switches, the ON time depends on whether the switch is being operated in a commutating or single port mode. In commutating mode, where there is reversal of voltage polarity or current direction, switching speed is slower than in single port mode, due to the loading effect at the junction of the port turning OFF. All switching speed measurements at GMC are performed in the commutating mode.
(2) For a unit without an integrated driver, the specifications apply to conditions when it is driven by an appropriately shaped switching waveform.
PHASE AND AMPLITUDE MATCHING
Switches are available on a custom basis with phase and/or amplitude matching. Matching can be either between ports of a switch, between like ports on different switches, or a combination of the two. The uniformity of broadband catalog switches is quite good and is usually better than ±0.75 dB and ±15 degrees over the entire operating frequency of the switch. Please consult the factory for special requirements.
POWER HANDLING
The power handling of PIN diode switches is dependent on the RF topology, forward and reverse biasing levels, and speed of the switch. This catalog addresses both the maximum operating power levels and the survival limits of the components.
Maximum operating limits are usually set at the power level which will cause the reversed biased diodes to begin conduction and thereby degrade the insertion loss, VSWR, or isolation of the switch.
The survival power limits are based on the maximum ratings of the semiconductors in the switch. For special applications, significantly higher operational power levels can be provided, particularly for narrow band requirements. Please consult the factory for specific applications.
| VIDEO FILTER OPTIONS | |||
| Applicability: F91 and G91 Switch Series | |||
| Peak to Peak (mV) | Bandwidth (MHz) | ||
| Video Leakage with Video Filter Options: | 50 max | 100 | |
| INSERTION LOSS DEGRADATION | |||
| Option | Affected Ports | Frequency | Additional IL |
| 41 | Common Port Only | 1-12.4 GHz 12.4-18 GHz |
0.1 dB 0.2 dB |
| 42 | Output Ports Only | 1-12.4 GHz 12.4-18 GHz |
0.1 dB 0.2 dB |
| 43 | All Ports | 1-12.4 GHz 12.4-18 GHz |
0.2 dB 0.4 dB |
| VSWR DEGRADATION | |||
| Option | Affected Ports | Frequency | VSWR |
| 41, 42, 43 | All Ports | 1-4 GHz 4-18 GHz |
1.7 :1* No Change |
* As shown for switches whose VSWR specification from 1-4 GHz is less than 1.7. No change for switches whose VSWR specification from 1-4 GHz is 1.7 or greater.
OPTION 55 – EXTENDED FREQUENCIES
When Option 55 is applicable, a switch in our catalog that covers 1-18 GHz can be modified to cover 0.5 to 18 GHz with following specification changes:
1. Specification for insertion loss and isolation from 0.5 to 1.0 GHz is the same as the 1-2 GHz specification. VSWR degrades to 2.0 :1.
2. Insertion loss in the 12.4-18 GHz band increases by 0.3 dB, Consult factory for cost.
Herley – General Microwave specializes in developing and producing customized microwave components and millimeter wave products for the defense and aerospace industries as well as for non-defense applications such as communication systems. Herley General Microwave produces the industry standard General Microwave line of off-the-shelf catalog RF componentד. If you are looking for a Solid State Power Amplifier, Microwave Synthesizers or other microwave oscillators, Microwave Receiver, RF Switches, Microwave attenuator, Microwave Limiter, Microwave Phase Shifter, or Microwave IQ Vector Modulator we can produce components meeting your requirements at a very competitive price. We also produce high quality customized Integrated Microwave Assemblies such as Up and Down Converters, DLVAs, Beam Forming Networks, Front Ends, or Switched Bank Filters, that can be used in a wide variety of demanding applications. Herley General Microwave (HGMI), a subsidiary of Herley Industries provides solutions for electronic warfare systems, Phased Array Radar systems, electronic warfare simulators, test Equipment and test Systems and other defense and non-defense systems. We look forward to working with you, so please contact us today.