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SW transmitter and modulation

Posted: Tue Sep 16, 2008 11:26 am
by K9EZ
Well this could have gone under AM as well....

I am working on a 1kW SW transmitter (AM). When its modulated, the power goes DOWN. Not UP. :?: :?: :?:

I am not sure why it would do that. It even says in a manual it will go down. HUH???? What am I missing??

Re: SW transmitter and modulation

Posted: Tue Sep 16, 2008 12:20 pm
by Deep Thought
100% negative carrier shift? :mrgreen:

How much does it go down? What kind of modulation does it use? For an example, the modulator in a Potomac Instruments synthesizer does it's thing by reducing the power output at the audio rate...if you want symmetrical modulation you would have to cut the 'carrier' to 50% and modify the bias on the modulated section. Not worth it considering how it works.

Sounds like your transmitter is doing the same thing. PEP=carrier!

Re: SW transmitter and modulation

Posted: Tue Sep 16, 2008 1:20 pm
by K9EZ
Deep Thought wrote:100% negative carrier shift? :mrgreen:

How much does it go down? What kind of modulation does it use? For an example, the modulator in a Potomac Instruments synthesizer does it's thing by reducing the power output at the audio rate...if you want symmetrical modulation you would have to cut the 'carrier' to 50% and modify the bias on the modulated section. Not worth it considering how it works.

Sounds like your transmitter is doing the same thing. PEP=carrier!

It is a Collins pt2200 exciter. With 1kW carrier, we can see 400watts at full modulation. Scwewy or what?

Re: SW transmitter and modulation

Posted: Tue Sep 16, 2008 1:25 pm
by Deep Thought
Can you sample the output of the exciter and see what it looks like? I know nothing about this unit but something about it sounds really odd. It must sound pretty bad...

Re: SW transmitter and modulation

Posted: Tue Sep 16, 2008 3:36 pm
by davedybas
At first thought....it sounds like a "weak" power supply.

Re: SW transmitter and modulation

Posted: Tue Sep 16, 2008 4:16 pm
by BroadcastDoc
That's what I was thinking too... I've seen that with both weak transformers and bad utility power, though never to that extreme.

Re: SW transmitter and modulation

Posted: Tue Sep 16, 2008 6:45 pm
by Deep Thought
I was going to head in that direction myself, but...

"It even says in a manual it will go down"

...sent me in search of a design reason for that. Haven't found one yet.

Re: SW transmitter and modulation

Posted: Tue Sep 16, 2008 10:30 pm
by BroadcastDoc
Good eye. I didn't catch that on the first read. I can't imagine why they'd design it like that...I guess you don't have to worry about excessive modulation!

Re: SW transmitter and modulation

Posted: Wed Sep 17, 2008 6:01 am
by K9EZ
I am trying to dig up the original transmitter manual. When I ask some of the engineers, they say to me "it has always been that way". Uh noooooooooooo not the AM that I know and love.

Stand by as I dig.

Re: SW transmitter and modulation

Posted: Thu Sep 18, 2008 6:11 am
by w4cl
This probably isn't it but I throw it out anyway (I like the weak or underrated power supply answer myself) but have you checked the tuning to see what the coupling is? Early in my career (back when I THOUGHT I knew everything, now I have been shown through time to know absolutely nothing,is THAT what they call wisdom?) I had an old 1 kw Gates that no one had been able to get a positive swing on it. I spend a month or more looking and then out of naive desperation re tuned the box without using the manual and overcoupled the crap out of it and the positive swing magically appeared! It ran that way for years until a leaky roof took it out.

Like they say, I would rather be lucky then good.

Re: SW transmitter and modulation

Posted: Fri Sep 19, 2008 8:38 pm
by K4WRF
Is the transmitter primarily intended as an ssb device that will produce AM? You may find this is common for older multipurpose com transmitters of that ilk and purpose. Is modulation envelope generated in exciter then amplified? So many questions!

K4WRF

Re: SW transmitter and modulation

Posted: Wed Oct 01, 2008 6:21 am
by K9EZ
K4WRF wrote:Is the transmitter primarily intended as an ssb device that will produce AM? You may find this is common for older multipurpose com transmitters of that ilk and purpose. Is modulation envelope generated in exciter then amplified? So many questions!

K4WRF

This is a weird device.... Here is the unit off of the Rockwell site: http://www.rockwellcollins.com/ecat/gs/RT-2200.html. We then put it into a 1kW amp. So yes it is as you described.

Why they chose this thing is beyond me. But that is the government for ya. if it says military on it, it means that it will do things you dont need it to do and be built to a tougher standard (like it could take a direct atomic blast), and the like for 3000X the price.

But I digress.....

Oh, and you gotta believe that a bunch of us are railing on buyers and people up above to save taxpayers money. I tell every soldier how much an item is and what that means to us (yes they are taxpayers too). Gotta start somewhere.

Re: SW transmitter and modulation

Posted: Thu Oct 02, 2008 3:55 pm
by K4WRF
Sounds like its single sideband with full carrier injected, quick and dirty AM. Not pretty but none-the-less AM.
“It is what it is”.
73’
K4WRF