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A Miniature SPDT Switch for Hi-rel Space Applications

A single-pole, double-throw (SPDT) elctromechanical switch that provides a long awaitedsolution for miniaturizing space equipment

January 1, 1999
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A Miniature SPDT Switch for Hi-rel Space Applications

Dow-Key Microwave Corp.
Ventura, CA

The trend toward component miniaturization is not new; it has already become prevalent in the commercial industry, especially in the wireless sector. However, the space industry is more conservative and, although there is good reason to rely on mature technology, the pressure to reduce payload weight, size and cost is forcing space equipment manufacturers to look for smaller components.

Most of the electromechanical switches used currently for space applications were developed more than 20 years ago and, although many improvements have been incorporated over the last few years to improve the product's reliability, no major step has been taken toward miniaturization. This need is now being addressed with a new switch design.

A new 509 series single-pole double-throw (SPDT) electromechanical switch has been introduced that provides the long-awaited solution for miniaturizing space equipment. The new switch is suitable for use with low noise amplifier circuitry where low insertion loss is essential. The connectorless switch also can replace many of the connectorized switches in typical redundancy configurations.

The need for miniaturization of electromechanical switches has already been noted by the DC relay industry. Recently, a trend has emerged to extend the frequency range of standard DC relays, especially for those in TO-5 packages. Although these new relays feature good RF performance, the 509 series switch exceeds their performance in terms of insertion loss, isolation and SWR. The reason for this improvement is that the 509 series switch is a miniature version of a standard RF coaxial switch that utilizes a rectangular 50 W enclosed RF cavity. This design configuration provides almost perfect impedance matching, which cannot be achieved with DC relays that utilize an open contact structure. In addition, the power handling capability of the new switch is much better than TO-5-type relays. Table 1 lists the differences in RF performance between advanced DC relays and the 509 series switches.

Table I
Typical RF Characteristics

 

Advanced DC Relay

509 Series Switch

Insertion Loss (dB) (max)

DC to 1 GHz

0.20

0.15

1 to 2 GHz

0.40

0.20

2 to 3 GHz

0.50

0.25

SWR

DC to 1 GHz

1.12

1.10

1 to 2 GHz

1.30

1.15

2 to 3 GHz

1.40

1.20

Isolation (dB) (min)

DC to 1 GHz

25

65

1 to 2 GHz

20

60

2 to 3 GHz

20

60

Product Description

The 509 series coaxial switch is a subminiature, vented, magnetic latching device featuring excellent RF performance. Its small size and pin layout make it suitable for stripline and microstrip applications. The drive mechanism is a miniaturized version of the standard latching drive used for many years in space applications. It is highly integrated with the housing and provides magnetic efficiency and mechanical rigidity. The drive mechanism also has been optimized to improve the force-to-mass ratio, providing an extremely high resistance to shock and vibration.

Figure 1 shows the switch's drive circuit, which is composed of two coils, two soft iron cores, a rocker, a soft iron plate and a permanent magnet. The components are configured such that two magnetic paths are established - one closed (through coil 2) and the other open (through coil 1). Since the closed path is significantly stronger than the open path, the rocker remains in position until acted upon by an opposing field. When a voltage pulse is applied to coil 2 (the closed path), the rocker holding force is reduced drastically and the rocker is attracted by the core of the opposite position. In turn, the rocker acts against the dielectrics that control the RF contacts. Drives of this type are inherently reliable and have been cycled for millions of operations without failure.

Performance Specifications

The new SPDT switch is designed to operate from DC to 3 GHz, but will perform within somewhat degraded specifications well above 3 GHz. Switching time is 5 ms (max) and contact resistance is 100 mW (max). Table 2 lists additional specifications. Figure 2 shows the unit's outline drawing and configuration.

Table II
General Specifications

Frequency Range (GHz)

DC to 3

Insertion Loss Repeatability (dB)

< ± 0.03

Insertion Phase Repeatability (° )

< ± 0.05

Contact Arrangement

SPDT

Contact Resistance (mW ) (max)

100

Switching Time (ms) (max)

5

Operating Voltage (V DC)

24 to 35

Operating Current (mA) (max)

250

Coil Resistance at 20° C (W )

230 ± 10%

Operating Temperature (° C)

-40 to 85

Sine Vibration (g)

20

Random Vibration

20 to 100 Hz: +6dB/octave

100 to 400 Hz: 1g2 /Hz

400 to 2000 Hz: -9dB/octave

Weight (g) (max)

7

Manufacturability

A careful study was conducted to produce a design that is easy to manufacture. The main goal was to minimize the number of required parts. In addition, all parts were analyzed to select the best processes and fabrication techniques. All processes were carefully selected, designed and documented to ensure a high level of consistency. Process improvements also include the utilization of proprietary advanced cleaning techniques to provide maximum internal cleanliness and proprietary plating processes for the contacts.

Conclusion

The 509 series SPDT electromechanical RF switch offers a new level of miniaturization while providing reliability consistent with space application requirements. The current S-band connectorless design is the first step in addressing the industry's needs for high reliability miniature surface-mount devices.

Dow-Key Microwave Corp.,
Ventura, CA
(805) 650-0260.

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