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Re: [Amps] Modern AM Broadcast Transmitters

To: amps@contesting.com
Subject: Re: [Amps] Modern AM Broadcast Transmitters
From: donroden@hiwaay.net
Date: Sun, 18 Dec 2016 14:34:12 -0600
List-post: <amps@contesting.com">mailto:amps@contesting.com>
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@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@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
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