Linear Amplifier Module Powers 20 to 512 MHz

Linear Amplifier Module Powers 20 to 512 MHz

This solidly built Class A/AB power amp incorporates numerous monitoring and protection circuits for safe operation even with 350 W output power at 1-dB compression.

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Low-distortion modulated signals are required for communications systems, which is the forte of the RFM20-512-350-HSD power-amplifier (PA) module from RF and Microwave Power Technology LLC: It provides clean amplification of communications signals from 20 to 512 MHz.

The Class A/AB PA module delivers 350 W output power at 1-dB compression, with high gain and high efficiency. It also includes numerous monitoring and protection circuits to help a user achieve safe operation. The PA module is a good fit for commercial and military communications systems that require high gain to transform low-level input signals at typically −2.3 dBm into high-level output signals for transmit purposes.

The RFM20-512-350-HSD (see figure) achieves at least 54.7-dB gain and typically 57.7-dB gain across the full frequency range, with worst-case gain flatness of ±1.2 dB and typical gain flatness of ±0.6 dB. It operates on two power supplies, drawing maximum current of 3.5 A from a voltage supply of +24 to +28 V dc and a maximum of 15 A from a voltage supply of +46 to +50 V dc.

Not So Hot

The robust amplifier is built to take whatever excess heat its output stage generates, with a nickel-plated copper base for efficient thermal flow away from the active device. It is also generously equipped with control and monitoring functions for protection, such as output-stage current sensing, an alarm when the package base exceeds +60ºC, and an output disable function with a response time of better than 1 μs. When combined, for example, with a user’s own VSWR-monitoring circuitry, the amplifier can be quickly shut down in the event of impedance-mismatch conditions.

The RFM20-512-350-HSD Class A/AB PA module delivers 350 W output power at 1-dB compression from 20 to 512 MHz with flat gain and high efficiency.

Also, the amplifier includes an input-stage monolithic microwave integrated circuit (MMIC) with a TTL-compatible enable/disable control pin that allows users to manually enable or disable the MMIC, achieving response times of 50 and 30 μs, respectively. The input-stage MMIC provides additional quieting for applications that may benefit from it, such as over-the-horizon (OTH) radar systems.

In addition to temperature-compensated bias circuitry, the PA makes use of a temperature-monitoring IC that provides an analog output voltage proportional to temperature. At room temperature (+25ºC), the IC’s nominal level is at +0.75 V dc, and it exhibits a positive voltage slope with temperature of 10 mV/ºC for temperature above +25ºC. This IC and its associated connection pin are meant to guide a user’s choice of PA cooling approach in a system, rather than provide absolute PA temperature measurements.

To control output-power levels, the PA features an integral voltage-variable attenuator (VVA) with attenuation range of better than 30 dB. It operates with control voltages from 0 to +5 V dc, with an attenuation tuning slope of approximately 14 dB/V from +1.4 to +3.6 V dc and maximum attenuation occurring at +4.4 V dc.

The amplifier’s second-harmonic performance is typically −46 dBc, with worst-case levels of −34 dBc. Third harmonics are typically −21 dBc, with worst-case levels of −10 dBc. Input return loss is typically −23 dB and maximum of −14 dB. The output-stage efficiency is at least 48% and typically 53%. Measured at 350 W peak envelope power (PEP) and 100-kHz offset, the third-order intermodulation distortion (IMD) is typically −36 dBc.

The multistage PA module runs with quiescent current (IDQ) of 3.3 A at +28 V dc and 0.8 A at +50 V dc. The amplifier comes in a rugged metal housing measuring 4.50 × 8.40 × 1.35 in. (114.30 × 213.36 × 34.29 mm) and weighing 83.2 oz. (2360 g). It includes an SMA input connector and Type N output connector, as well as a DB-9 connector for monitoring and control.

RF and Microwave Power Technology LLC, 2280 Solitude Dr., Reno, NV 89511; (775) 842-3280.

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