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From: hanavin@udel.edu (Chuck Hanavin)
Newsgroups: sci.electronics
Subject: Re: AM Radio
Message-ID: <9602@nigel.udel.EDU>
Date: 30 Jan 90 16:09:50 GMT
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Reply-To: hanavin@udel.edu (Chuck Hanavin)
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>Slightly off the subject of the original question (re: improving the
>transmitted frequency response), What _is_ the current practice in AM
>transmitter design?  I am familiar with the classic plate-modulation
>methods, but many years ago studied other methods such as using two
>unmodulated carrier sources, phase-modulating them, and combining them to
>yield an AM signal.  This had the advantages of not needing all the iron
>of the plate-modulation schemes, possibly better efficiency, and better
>fidelity.

>Can anyone educate me as to the current state of the art?

-----------------------
The current state of the art being used is called Pulse Duration
Modulation. Here, the audio signal is digitized with a switching
rate of 40 to 60 khz. A power supply of twice the unmodulated carrier
value is needed. The power supply is turned on and off by the digital
audio. The amplitude of the original audio signal relating to the
duration the power supply is on, and the frequency of the audio signal
relating to the frequency the pulse is on. A filter is used between the
modulator and modulated amp to filter out the 40 to 60khz switching
frequncy. The modulator is directly coupled to the modualated amp, hence,
no mod transformer and excellent frequecy response. Since the moduater
is operated as switch (on or off), it has very high efficiency (80 to
95 percent). 

Chuck (WB3FJJ)