Duty cycle and pulse width for high power power amplifier

[enemra]

Hello,

I was looking at a datasheet of a general purpose high watt (100W) Broadband High Power Amplifier Module (SSPA), supporting 2GHz - 6GHz. The specifications list says Duty cycle is 10% typical and pulse width of min 2us, and max 200uS.

My question is, why do you specify a restriction on duty cycle and pulse width for RF in power amplifiers? How does switching RF ON/OFF affects the PA?

Spec in the attachment.

Regards
Enemra

[CaptDon]

Because the amplifier can only sustain an AVERAGE POWER OUTPUT of 10% of the rated maximum power output. So it comes down to duty cycle, however they have also given you a minimum and maximum PULSE WIDTH which apparently you must obey. Normally you could run that amplifier in continuous wave fashion at 10% of the rated maximum output power but the amplifier you specified may only be rated for pulse duty such as radar. A best example is a small boat radar. 4 Kilowatts pulse power but with a duty cycle of .001 percent the average output power is 4 watts. If you need an amplifier of that power level in your daily work you should have a fundamental understanding of how duty cycle directly effects power dissipation within typical amplifiers with efficiencies of 60 to 75%??? Longer pulse width = more heat. Duty cycle and pulse width also effect repetition rate. Small radars typically have repetition rates from 400 to 1000 p.p.s.

[enemra]

Understood. Thanks for the response. So, if the issue bottom down to "heating", then I can duty cycle a continuous rated power amplifier to any extent isn't it? For instance, if the spec says 100W continuous, I can duty cycle it to 0.01%?

[CaptDon]

Yes indeed!! If the amplifiers rise and fall times are fast enough (which they probably are) you can use less than 100% duty cycle with no harm. In fact the average heat and average power dissipation will fall directly as a function of duty cycle. With extremely narrow pulse widths down in the tens of nanoseconds or less you may need an amplifier with a bandwidth of double or more of your maximum R.F. operating frequency to preserve the shape of your R.F. pulse if that is an important factor such as in the old LORAN-C systems or perhaps the new E-LORAN if they ever get around to building it. Preserving pulse envelope shape and phase is also important in pulsed doppler systems. Cheers!!