LDMOS FET Drives UHF TV Transmitters

May 13, 2008
With 450-W output power, this robust device helps simplify the design and reduce the cost of power amplifiers for UHF broadcast television transmitters.

Television broadcast systems in the ultrahigh-frequency (UHF) region from 470 to 860 MHz will make the transition from analog to digital technology next year, and broadcasters are taking advantage of digital performance while attempting to keep equipment costs low. Since one of the highest cost components is the transmitter, it is no surprise that the MRF6VP3450H 50 V LDMOS FET from Freescale Semiconductor (www.freescale.com/rfbroadcast) would appeal to these broadcasters. It delivers 50-percent more RF output power than any single UHF device, with efficiency as high as 45 percent (Fig. 1).

That high efficiency is available with 450 W CW output power from 470 to 860 MHz while operating at +50 VDC. Such high solid-state power from a single device allows the use of fewer devices for a given power-amplifier (PA) output power level, with the capability of achieving broadcast solid-state power levels once provided by less-reliable vacuum tubes. Current solid-state UHF TV broadcast transmitters are available with RF output power as high as16 kW.

The 450-W output of the MRF- 6VP3450H LDMOS exceeds a previous high of 300 W from a single UHF broadcast device. The MRF6VP3450H is designed using Freescale's sixth-generation very high voltage (VHV6) LDMOS technology, the same process delivering the industry's first 1-kW LDMOS FET and the first RF power transistors for broadcast applications available in over-molded plastic packages. Free scale's broadcast product portfolio now includes 32- and 50-V devices, with a frequency range of 10 MHz to 1 GHz, and RF output power levels ranging from 10 W to 1 kW.

The MRF6VP3450H achieves 26 dB and drain efficiency of 28 percent with an average output power of 90 W, based on a DVB-T OFDM signal, 64 QAM, test-signal conditions. The adjacent-channel power ratio (ACPR) at a 4 MHz offset is -62 dBc over a 4 kHz bandwidth. When measured using the more traditional two-tone test method, the device has drain efficiency of 45 percent, gain of 23 dB, and a peakenvelope- power (PEP) RF output level of 450 W. Third-order intermodulation distortion is -26.5 dBc (see table).

These performance metrics translate into remarkable savings. For example, a typical DVB-T TV transmitter rated at 250 W average power with OFDM signal with a peak-to-average ratio (PAR) of 10 requires a driver and four final output transistors to achieve input-to-output gain of 29 dB. Using the MRF6VP3450H in the same configuration will deliver 320 W average power with 42 dB gain (Fig. 2).

Broadcast transmitters require RF power transistors that are extremely rugged and can handle very high impedance mismatches (10.0:1 and more) and the high PARs that are typical of OFDM signal formats. The MRF6VP3450H is designed to accommodate this environment, and when operating at 50 V can produce its rated 90 W average output power into a 10:1 VSWR when amplifying a signal with a 10-dB PAR. Under pulsed conditions (10 ms pulse, 2.5 percent duty cycle), it can deliver its full 450 W into a 10:1 VSWR. The device also incorporates protection against electrostatic discharge (ESD).

The MRF6VP3450H is internally matched to 50 Ohms. It is housed in Freescale's NI-1230 air-cavity ceramic package and is RoHS compliant. Freescale Semiconductor, RF, Analog, and Sensors Group, 2100 E. Elliot Rd., Tempe, AZ 85284; (800) 521-6274, Internet: www.freescale.com/rfpower.

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