<Li> a turntable or metal wave guide stirring fan . </Li> <Li> a digital / manual control panel </Li> <P> Modern microwave ovens use either an analog dial - type timer or a digital control panel for operation . Control panels feature an LED, liquid crystal or vacuum fluorescent display, in the 90s brands such as Panasonic and GE began offering models with a scrolling - text display showing cooking instructions, numeric buttons for entering the cook time, a power level selection feature and other possible functions such as a defrost setting and pre-programmed settings for different food types, such as meat, fish, poultry, vegetables, frozen vegetables, frozen dinners, and popcorn . In most ovens, the magnetron is driven by a linear transformer which can only feasibly be switched completely on or off . As such, the choice of power level does not affect the intensity of the microwave radiation; instead, the magnetron is cycled on and off every few seconds, thus altering the large scale duty cycle . Newer models have inverter power supplies that use pulse - width modulation to provide effectively continuous heating at reduced power, so that foods are heated more evenly at a given power level and can be heated more quickly without being damaged by uneven heating . </P> <P> The microwave frequencies used in microwave ovens are chosen based on regulatory and cost constraints . The first is that they should be in one of the industrial, scientific, and medical (ISM) frequency bands set aside for non-communication purposes . For household purposes, 2.45 GHz has the advantage over 915 MHz in that 915 MHz is only an ISM band in the ITU Region 2 while 2.45 GHz is available worldwide . Three additional ISM bands exist in the microwave frequencies, but are not used for microwave cooking . Two of them are centered on 5.8 GHz and 24.125 GHz, but are not used for microwave cooking because of the very high cost of power generation at these frequencies . The third, centered on 433.92 MHz, is a narrow band that would require expensive equipment to generate sufficient power without creating interference outside the band, and is only available in some countries . </P>

What does an inverter do in a microwave