[Amps] Modern AM Broadcast Transmitters

[donroden at hiwaay.net]

I'd have to pull my manuals, but I believe you are correct on the  
75KHZ on the MW-50 .

On the SX series, Harris did something "cute" by running the switch rate
( 60KHZ ) through a chip that produced a 0 degree phase. a 90 degree  
phase, a 180 degree phase, and a 270 degree phase.  Each feeding 1/4  
of the total RF switching quads of IRF-350s ...... effectively making  
the switching 240KHZ, which simplified filtering out the artifacts of  
the switching.

Don W4DNR


Quoting Bryan Swadener via Amps <amps at contesting.com>:

> Thanks Don,
>
> PWM has been around since at least the early 1970s.It's NOT new.  
> Originally, it used hollow state devices.
>
> I'll add that the switch rate needs to be at least twice
> the highest modulation frequency (Nyquist-Shannon
> Sampling Theorem). IIRC, the Gates MW50 50 KW
> AM BC xmtr (used at two Seattle stations) samples
> at 75 KHz.
>
> Bryan WA7PRC
>
> ----------------------------------------------------------------------
>
> Date: Sun, 18 Dec 2016 05:31:06 -0600
> From: Don W4DNR
> To: amps at contesting.com
> Subject: [Amps] Modern AM Broadcast Transmitters
>
> Modern AM Broadcast transmitters are pulse width modulated with the  
> rated carrier power at 40% width of the pulses.
>
> Negative modulation moves the pulse width towards zero pulse width
> ( carrier cut-off )
>
> Positive modulation moves the pulse width towards full pulse width.
> 80% pulse width = 100% modulation or 4 x carrier power.
>
> 100% pulse width = 125% Positive modulation or 5 x carrier power.
>
> Conventional Plate Modulation using audio transformers and Pulse Width
> Modulation ( sometimes referred to as Pulse Duration Modulation or PDM )
> looks identical on an oscilloscope and is detected identically with  
> receivers.
>
> Don W4DNR
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DonR