[Boatanchors] Modern AM Modulation techniques

[Sheldon Daitch]

I pulled out the DX-200 water cooled power block manual and each power block has 224 amplifier modules, Harris says 220 are big step and four are binary RF amplifiers.
The 220 amplifiers are set up to run full power level, the four binary RF amplifiers operate at /16, 1/8, 1/4 and 1/2 the power level of the big step, thus smoothing the modulation waveform.

There are also 14 PA driver modules, all identical to the 224 discussed above.

Each PA module uses 8 IRFP350 MOSFETs, normally in a full quad bridged configuration.  If one of the sides fails, the PA module continues to operate in half quad single ended mode.

Unlike the modules in some of Harris' FM transmitters and maybe even in the smaller MW transmitters, these modules are not hot replaceable.  

73

Sheldon



-----Original Message-----
From: boatanchors-bounces at mailman.qth.net [mailto:boatanchors-bounces at mailman.qth.net] On Behalf Of Sheldon Daitch
Sent: Monday, October 05, 2015 10:09 PM
To: Rob Atkinson
Cc: boatanchors at mailman.qth.net; D C _Mac_ Macdonald
Subject: Re: [Boatanchors] Modern AM Modulation techniques


Yep,

I met Hilmer once when I was out at the Harris factory mid-1990s or so, when I was going through one of the DX series training courses.

A friend of mine who worked for Harris before coming to work with us told me that Hilmer was a gentleman farmer and he suspected that some of Swanson's ideas for Harris' designs might have come to mind while driving around the farm.

I am not familiar with the smaller Harris DX transmitters, only the DX-200 power blocks, and I can't remember exactly how many power amplifier modules there were in each DX-200 power block, but I am thinking it was about 175 or so.  (Reminder - look in the manual in the morning at work.)

The PA modules for the DX-100 and DX-50 were not the same as the DX-200, and even in the DX-200, there were water cooled modules and air cooled modules.  

I saw one of the 2 2 megawatt versions while in Quincy, it was on a heat run for the customer.  Ten 200kW power blocks.  

73
Sheldon
________________________________________
From: Rob Atkinson <ranchorobbo at gmail.com>
Sent: Monday, October 5, 2015 7:30 PM
To: Sheldon Daitch
Cc: D C _Mac_ Macdonald; boatanchors at mailman.qth.net
Subject: Re: [Boatanchors] Modern AM Modulation techniques

All of the AM medium wave broadcast rigs are solid state with class D RF modules producing various power levels that are stepped on and off by a streaming binary coded signal derived from an A to D circuit sampling the program audio.  This is fed to a logic board that steps on and off needed combinations of RF modules that are paralleled with each other to provide the needed RF level at any instant.  A 50 KW rig will have 50 to 100 modules, each using two FETs in power blocks of
500 to maybe 2 KW.  The modules are hot swapable.   Harris can make
rigs up to one or two megawatts by simply building bigger boxes with more FET modules.
The master mind for this design was now deceased Harris engineer Hilmer Swanson.  These rigs are about 90% efficient.

here's a brief description of how it works:

http://hawkins.pair.com/wabcnow.shtml#digmodsect

73

Rob
K5UJ

On Mon, Oct 5, 2015 at 10:31 AM, Sheldon Daitch <SDAITCH at bbg.gov> wrote:
> I don't think there is a modern AM MW transmitter in the broadcast world that uses the typical Class B audio modulator with a Class C RF amplifier anymore, well, not many produced in the last 10-15 years or so were built that way.
>
> Most of the modern MW broadcast band transmitters are solid state and even at the higher powers found in HF broadcasting, even with the tube RF amplifiers, many of those transmitters now have solid state modulation sections.
>
> The PEP to carrier power ratio still applies, just the way the transmitters work are different.
>
> 73
> Sheldon
>
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