Identify Friend or Foe (IFF) systems are one of those rare electronic assemblies that serves both civilian and military purposes. Although operating modes differ, military and civilian IFF systems still require high-power amplifiers for both the interrogator and the transponder portions of the system. Power Module Technology (www.pmtrf.com) offers modular IFF amplifier solutions at various power levels, in the form of compact pallet amplifiers that can be literally plugged into an existing system or used as building blocks in a higher-power system.

One major reason why IFF systems are a part of both military and civilian aircraft is that they are essential. In US-controlled airspace above 10,000 ft., the US Federal Aviation Administration (FAA) requires that all aircraft, military or civilian, be equipped with an operating IFF transponder system capable of automatic altitude reporting.

Although a somewhat archaic system design, the IFF architecture has been an effective part of high-altitude aircraft for some time. Operating almost like a radar with active targets, an IFF system is often referred to as an aircraft’s secondary radar, with the primary system bouncing RF pulses off other aircraft to determine their position. An IFF system sends out an interrogation in the form of a sequence of pulses. The IFF transponder on an aircraft that receives the interrogation sends out a reply with information about that aircraft. A friendly reply can confirm the identity of the receiving aircraft. Conversely, in the case of no reply, the receiving aircraft is still unknown.
An IFF system operates at two different frequencies, with the interrogator sending pulses at 1030 MHz and the transponder operating at 1090 MHz. The system can determine an aircraft’s position by comparing the antenna dish angle and the delay from the interrogator pulse to the received IFF pulses. The pulsed signals are actually a series of bits, including encrypted, parity, and information bits for security. An IFF message is typically encrypted with a secret key, and can only be decoded by IFF transponders with the same key.

A number of different IFF modes are used in civilian and military aircraft, with several of the military modes featuring a Selective Identification Feature (SIF) for security. Civilian operating modes may also include barometric pressure altitude information as part of the transmitted information. In all cases, these pulsed systems require robust power amplifiers at 1030 and 1090 MHz to operate over reasonable distances.

Power Module Technology currently offers three different IFF pallet amplifiers, with pulsed output power levels of 600, 1100, and 2000 W peak. All three are solid-state designs, based on reliable silicon lateral diffused metal-oxide-semiconductor (LDMOS) transistors from a major supplier. The transistors are biased in Class AB mode to achieve a high level of pulse linearity without compromising on amplifier efficiency.

The highest-power member of the pallet amplifier trio is model PM1030-1090-2000P, rated for 2000 W minimum peak output power at 1030 and 1090 MHz. It offers 16.0 dB minimum gain and 16.8 dB typical gain at those frequencies. An input signal level of 42 W is typically needed to achieve the rated 2 kW output power. The amplifier, which can operate into an infinite load mismatch without damage, incorporates thermal-tracking bias circuitry to maintain constant gain and output power even with changes in operating temperature. It is designed for pulsed signals at widths from 500 ns to 32 ìs at a typical duty cycle of 0.1% (maximum duty cycle of 4%). The amplifier’s maximum rise time is 25 ns and maximum fall time is 20 ns, so that imparts minimal temporal degradation to a pulsed signal. It exhibits maximum input return loss of 14 dB.

The compact LDMOS amplifier features 44% efficiency, drawing 99 A current during a 2-KW burst from a typical +46 VDC supply. The quiescent current during burst is a low 0.4 A. The amplifier generates few signal artifacts, with -45 dBc typical second harmonics and -50 dBc typical third harmonics. It is “ready to install” in a system, with 50-Ù input and output impedances and requiring no assembly or tuning. It measures 10.6 x 10.5 x 2.25 in.

The lowest-power unit is model PM1030-1090-600P, with 600 W minimum peak output power at 1030 and 1090 MHz. It has 16.6 dB minimum gain and 17.4 dB typical gain, with an 11-W input signal yielding the full output power. It is designed for signal pulse widths of 500 ns to 30 ìs at a typical duty cycle of 0.1% (4% maximum) and, like the other IFF pallet amplifiers, has 50-Ù input and output ports for ease of installation. It features 52% typical efficiency, drawing 25 A from a typical +46-VDC supply during a 600-W burst, with low 0.1-A quiescent current. The amplifier also boasts fast rise and fall times of 25 and 20 ns, respectively. It measures 3.6 x 8.0 x 2.25 in. (91.4 x 203.2 x 57.2 mm) and maintains second and third harmonic levels of -25 and -30 dBc, respectively.

In between those two amplifiers in terms of power, model PM1030-1090-1100P is rated for 1100 W minimum peak output power at 1030 and 1090 MHz, with 16.3 dB minimum gain and 17.1 dB typical gain. It requires a 21-W input signal to achieve full rated output power. Model PM1030-1090-1100P includes the thermal-tracking bias circuitry of the other two amplifiers, to maintain consistent gain and output power even with changing operating temperatures. It is designed to handle pulse widths from 500 ns to 30 ìs at typical duty cycle of 0.1% (maximum of 4%) and is a Class AB design like the other two pallet amplifiers. It achieves 48% efficiency, drawing 50-A current at a typical supply of +46 VDC during an 1100-W burst, with low quiescent current of 0.2. Model PM1030-1090-1100P measures 5.7 x 9.5 x 2.25 in. (144.8 x 241.3 x 57.2 mm) and holds second and third harmonics to typically -35 and -40 dBc.

These 50-Ù amplifiers can be added directly to an IFF rack or used as building blocks to achieve higher power levels. They are rugged enough to withstand the most severe load mismatches, and provide the high efficiency and reliability that have made LDMOS transistors mainstays in commercial cellular base-station designs.

Power Module Technology, 3107 North Deer Run Road, Suite 20, Carson City, NV 89701; (775) 883-1122, e-mail: sales@pmtrf.com, www.pmtrf.com.