Transcript
High-Power Multi-Function Radar Receiver Protection B. M. Coaker, D. M. Dowthwaite & N. E. Priestley e2v technologies (uk) Limited, Carholme Road, Lincoln, UK LN1 1SF
Abstract - Modern solid-state limiter receiver protection devices are described, addressing the broad range of operating frequencies and system power levels encountered in modern radar systems. In particular, the replacement of legacy transmit/receive (TR) cells with solid-state receiver protectors (SSRPs) enables receiver protection performance without the added lifecycle costs of radio-active gas fillings and high-voltage power supply infrastructure. Where high power levels persist, due to operational or asynchronous conditions, then gas tube (pre-TR tube or gas switch) solutions can be used, neither of which require a radioactive or high-voltage corona primer within the tube. Multifunction devices allow the integration of the receiver protector with further system functions, such as noise sources, sensitivity time control (STC) attenuation and filtering, optimising electrical performance, physical size and mass of the device. Key words – Limiter Receiver Protector
devices to handle the high power levels and pulse duties encountered in modern radar systems. Consequently there are now three main options for replacing the TR limiter as a receiver protection element: II.
SOLID-STATE DIODE LIMITERS
From its background in TR cell duplexer technology, e2v has developed and broadened its range of solid-state receiver protectors and limiters, based upon its own packaged PIN, NIP and detector diodes [1]. Fundamentally, the waveguide SSRP designs incorporate a PIN (or NIP) diode mounted at the end of a post-coupled co-axial line. Figure 1 illustrates a simple waveguide PIN limiter structure, with a packaged PIN diode mounted with a sliding short-circuit choke, secured between two (threaded) boss structures within the waveguide wall [2].
PIN diode Gas Switch
I. INTRODUCTION With the development of solid-state limiter capabilities to increasing peak- and mean-power handling levels, former transmit/receive (TR) cell requirements are increasingly being addressed using a solid-state receiver protector (SSRP); in many cases, the stand-alone SSRP can solve the protection requirement, or may be integrated with a gas switch or preTR tube in order to encompass high-power fault or highpower duplexing applications. The TR limiter (TRL) has traditionally being used for duplexer applications with a magnetron based RF source. Here the limiter typically sees a short-pulse, low-duty signal, but with high peak-power – often many hundreds of kilowatts at S-band or many tens of kilowatts at X-band. However, the continued use of the TR tube-based TRL in modern radar systems does present some issues, namely: (i) the use of radio-active primers to achieve a rapid activation response in the gas filling, and (ii) the need for a high-voltage power supply infrastructure to support the high-voltage primer (‘keep-alive’) on the TR tube. The development of non-radioactive gas-filled tubes offer broad-band high-power protection, whilst the careful selection of diode devices and structures provides low-loss / low leakage / fast response within the limiter structure. Development of multiple-chip limiter stages, along with detector-driven limiters, has further enabled solid-state
Fig. 1. Schematic showing single-stage waveguide PIN limiter construction (above), and equivalent circuit (below)
By cascading a number of these diode stages within the protector body, an increased level of RF attenuation and low magnitude of RF leakage can be achieved. For each application, the protector design and diode selection is optimized to provide low quiescent loss, with high attenuation in the limiting state, resulting in minimum power absorption by the limiter diode(s) enabling very high peak and mean power handling. A.
Receiver Protector Configurations Depending upon the system application, the protector diodes can be biased in a number of configurations [3]: Self-biased within the limiter structure (or self-biased from an integrated detector module) (Passive) Active switching, triggered by a system command (e.g. TTL logic level) input (Active) Active diode switching circuit, triggered by (i) a system command input, or (ii) a trigger signal derived from a detector module coupled into the waveguide (Quasi-Active) Quasi-active driver, driven from (i) a system command trigger or (ii) self-biased from a detector module, with (iii) power-down bias functionality from a detector module within the limiter structure (Quasi-Passive) Multi-function SSRP sub-systems Additional features and functionality can be integrated within the SSRP, and are illustrated in the product example shown in Figure 2, depicting an e2v 500MHz-bandwidth Xband receiver protector, which also incorporates Ka-band rejection filter, waveguide pressure window, digitally-addressed sensitivity time control (STC) attenuation, and integrated noise generator.
This solid-state passive receiver protector has waveguide input, SMA co-axial output, and a digitally-controlled attenuator giving an STC attenuation function from 0 to 60dB. It includes three PIN diode stages, with the front stage comprising a PIN doublet across the waveguide, and operates under 55kŴ / 0.1% duty / 1µs pulse and 10kŴ / 2% duty / 25µs pulse conditions. An integrated noise generator is incorporated (14dB ENR), for system calibration. Frequency range is 8.7GHz to 9.2GHz, with an operating temperature range of -20°C to +85°C. C.
Multi-function Drop-in Limiters e2v technologies have taken the microstrip limiter design approach, and adapted it to surface-mount, true ‘drop-in’ packaged devices, particularly suited to phased array radar modules. e2v have developed a surface-mount S-band SPDT reflective switch, with passive limiter protection on its input (J2) – see Figure 3.
B.
Fig. 3. S-band 10W passive limiter with SPST switch (external and internal views)
The limiter section utilises proven e2v diode limiter technology, providing protection at power levels up to +36dBm CW / +40dBm peak, and recovery times better than 200ns. An integrated SPDT switch circuit within the module offers in excess of 40dB isolation, with a switching speed better than 2µs. The packaged device is hermetically sealed, and is designed for re-flow soldering onto a circuit substrate. The unit operates from a +5V DC supply, typically drawing 20mA max. The SPDT switch control is from a –8V / 0V complementary pair, drawing 1mA maximum bias current. D.
Fig. 2 . General arrangement of e2v X-band four-stage SSRP (waveguide input, co-axial output)
Solid-state limiter power handling capability The power handling characteristics of Pin and Limiter diode structures employed in the range of e2v receiver protectors [1] are illustrated in Figure 4 below. It is clear that the incident power handling capability falls sharply as the operating pulse width is increased, establishing a plateau range of power handling limits at pulse widths in excess of 50µs. Data for low-power Limiter diode (VRRV = 150V) and high-power PIN diode (VRRV = 300V) devices are shown, together with a multiple-chip high-power PIN limiter configuration. This analysis is based upon a consideration of junction area and thickness in the PIN (or limiter) diode types, and
calculation of the level of power dissipation required to raise the diode junction temperature to +150°C.
demanding. Shown here in Figure 5 and Figure 6 is an example of a recent X-band duplexer development, with the replacement of a twin 50kW TR limiter (TRL) unit with a twin pre-TR limiter. In this application the dual-channel TRL unit was mounted between 3dB hybrid couplers, and so the incident power (from a 100kW magnetron transmitter source) was split equally between the two channels. Therefore the maximum rated power handling requirement per channel, was approximately 60kW peak.
Fig. 4. Summary of solid-state limiter power handling capability for e2v technologies Passive designs
Clearly for systems operating with sub-microsecond pulse widths, full system power faults can be addressed with a totally solid-state limiter solution. However, where the system operates with pulse widths longer than a microsecond, and/or there is a likelihood of long-pulse / high-power asynchronous pulses, then the solidstate limiter should be combined with a gas switch. In this hybrid configuration, the gas switch offers protection against high-power / long duration synchronous pulses from the system (in the embodiment of a Pre-TR Tube), or protection from high-power / long duration incident Fault pulses (in the configuration of a Gas Switch).
Fig. 5.
Legacy dual-channel X-band 50kW TR limiter
The updated protector design is shown in Figure 6, and includes a gas tube pre-TR input section, followed by a three-stage solid-stage limiter in each channel. The gas switch is in an H-plane orientation, in order to minimise the arc-loss in the gas tube: since the device operates continuously at the high-power levels, low arc-loss is a requirement for RF and thermal efficiency.
III HIGH-POWER TUBE PROTECTORS In some radar systems, the presence of very high operating peak power levels and/or the threat of very high amplitude asynchronous signals can require many stages of multiplechip diode protection in a solid-state limiter, in order to offer sufficient limiting attenuation of the high-level input signals. However, it is possible to combine a gas tube input element with a reduced series of limiter diode stages, to offer an optimum size and mass of Receiver Protector, whilst providing protection capability against very high operational and/or Fault powers. A.
Synchronous High-power Protection: pre-TR Tube plus Diode Limiter Where the gas tube is operating under normal operating conditions, the gas tube in this scenario is usually referred to as a pre-TR tube. A pre-TR tube is used in systems where the pulse conditions (duration, duty) may be more severe at lower power levels, and the recovery specification is not as
Fig. 6. General arrangement of updated X-band dual 50kW pre-TR limiter, showing internal section of twin channel pre-TR limiter structure (left) and actual device embodiment (right), viewed from device input
The power handling capability of each three-stage limiter section is approaching 30kW, owing to the careful selection and use of multiple-chip limiter structures, although limited by electrical breakdown across the waveguide. The limiter performance provides a large power threshold overlap between (i) the turn-on level of the pre-TR tube (at
~10kW) and (ii) the high maximum rating of the solid-state limiter (30kW). This limiter performance presents greater margin over the turn-on power threshold of the pre-TR tube, which in turn allows for the use of a primer-less, nonradioactive gas fill in the pre-TR tube: consequently neither radiation nor high-voltage hazards are presented to the user. B.
Asynchronous High-power Protection: Gas Switch plus Diode Limiter A gas switch is usually fitted when the normal operating conditions require (i) the fast response performance of a diode limiter, plus (ii) the high-level protection of a gas tube under a high-power fault condition: this fault condition is usually present for a short duration. Classically, a gas tube element is embodied at the protector input, to handle high power levels during normal operation. Incident RF energy first activates the PIN diode stages, which provide optimum protection during the RF pulse leading edge, and optimum spike leakage performance. The increasing E-field level across the waveguide then causes ionization of the gas fill in the tube element, leading to a plasma discharge in the gas tube at a high-power threshold level. This discharge presents an effective short-circuit to the incident RF pulse, protecting the receiver across a wide frequency bandwidth from incident high-power threats. This arrangement is illustrated in Figure 7, showing the gas switch (located in H-plane configuration in this case) within an iris tuning structure across the waveguide input, in front of the three-stage solid-state limiter section.
(recovery time, leakage level, insertion loss) without the high fault-power levels being a concern. Indeed, the power-handling capability of the solid-state limiter architecture means that the gas switch should never need to operate during normal operation, and can be designed only to activate during a system fault condition: therefore, assuming compliant fault shut-down performance within the radar system, the gas switch will only operate momentarily during a system fault, and should not need to be treated as an operationally-lifed item. IV. CONCLUSIONS Solid-state receiver protection has been embodied in conventional and phased-array radar system architectures, providing low-loss protection elements that are capable of withstanding high incident powers from local and remote radar transmitters, along with other EMC threats. The growth of power-handling capability in the solid-state limiter, through diode design, packaging and detector-drive schemes, has enabled SSRPs to replace gas-filled TR cells in the majority of radar systems. Where a high-power fault condition persists, and whilst the solid-state limiter can handle all normal operating power levels, then a nonradioactive gas switch may be used to operate under fault power levels. In the extreme case of very high peak operational power levels, the high power handling of the solid-state limiter provides a large power margin overlap with the turn-on threshold of the pre-TR tube, and permits a non-radioactive, primer-less pre-TR tube to be used for the normal power switching (duplexing) function. The integration of further system functionality within the receiver protector assembly enables an optimisation of the protector performance and loss, whilst providing reduced system mass, size and cost. ACKNOWLEDGEMENT The authors are pleased to acknowledge the support of colleagues at e2v, for technical inputs from John Pollard and Gary Fletcher, manuscript review by Dr Paul John, and detail drafting and photography by Mark Jacklin and Mike Lincoln respectively. REFERENCES
Fig. 7 General arrangement (above) and internal detail (below) of X-band passive limiter with integral pressure window and gas switch
With the protection that this limiter / gas switch configuration offers in the fault condition, the diodes for the limiter can be optimised to meet other operating parameters
[1]
e2v technologies Limited, e2v Microwave Product Guide, http://www.e2v.com/datasheets/publications/brochures/microwave_p roduct_guide.pdf
[2]
N Roberts, EEV limited, “A Review of Solid-State Radar Receiver Protection Devices”, Microwave Journal, February 1991, pp. 121125.
[3]
B M Coaker, e2v technologies Limited, Receiver Protection Technology, September 2005, http://e2v.com/datasheets/publications/papers/receiverpaper.pdf
