On Wed, 11 Jul 2001 11:31:20 -0500 David VanHorn writes: > > > > This was introduced something like 50 years after class C > amplifiers > >were first used in AM transmitters. You'd have thought it was a > mature > >technology by then! > > How is it that AM transmitters use class C? > I thought that would destroy the AM, or do they do that in front of > a > modulator?? > All current AM transmitters use high level modulation. The modulator, by various means, just varies the power supply voltage to the final amplifier. The old way of doing this was to just put the audio in series with the plate voltage (generally using a modulating reactor or choke to carry the DC with a modulation transformer capacitor coupled across the modulation reactor). The modulator was just a big class B audio amplifier. Newer transmitters use PWM to generate the varying power supply voltage to the RF amplifier. They are now using multiphase PWM so the ripple frequency is higher making it easier to filter out. The low pass filter after the PWM modulator needs suppress the switching frequency enough to meet the FCC bandwidth requirments (the switching frequency causes sidebands at the switching frequency and its harmonics away from the carrier). Attenuating the switching frequency while not messing up the audio is a trick, especially with 50 to 100,000 watts of DC plus audio. Phase linearity of the filter is also important since standard audio processing clips the audio peaks to avoid overmodulation. Nonlinear phase response turns these clipped peaks on their side, causing more overmodulation. Several years ago (maybe 20), Harris Corporation patented a new AM modulator called the Progressive Series Modulator. It was a very simple circuit (two transistors and a diode), but no one had thought of it before. In a standard series modulator, you have a pass element (transistor or tube) through which the DC goes from the supply to the RF amplifier. Varying this pass element varies the "DC" to the RF amplifier, modulating it. However, a series modulator is very inefficient. With no modulation, at least 50% of the power is dissipated by the modulator (the other 50% going to the RF amplifier). With the progressive series modulator, the two transistors are connected in series. A diode is inserted between a 1/2 voltage supply and the junction of the two transistors. With no modulation, the top transistor is off and the bottom transistor is saturated, giving close to 100% efficiency. The bottom transistor varies towards off to modulate down. The top transistor varies towards on to modulate up. Very clever! Still not as efficient as PWM, but it made possible high level transmitters "with no iron" giving very low distortion and wide bandwidth. I think Harris manufactured one model with this scheme, then went to PWM for everything. Harold FCC Rules Online at http://hallikainen.com/FccRules Lighting control for theatre and television at http://www.dovesystems.com ________________________________________________________________ GET INTERNET ACCESS FROM JUNO! Juno offers FREE or PREMIUM Internet access for less! Join Juno today! For your FREE software, visit: http://dl.www.juno.com/get/tagj. -- http://www.piclist.com hint: PICList Posts must start with ONE topic: [PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads