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Author Topic: AM stereo Generator designs plus 300 watt Digital RF  (Read 2772 times)

Offline digitalmod

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AM stereo Generator designs plus 300 watt Digital RF
« on: December 08, 2017, 1906 UTC »
 8)

Seems like we need more Captain Jolly engineers from broadcasting. I am one, albeit a kid reborn at age 70! Question: Seems to me we need some kits for shortwave DDS and not less than 250 watt transmitters. Yes, we need engineers that design and build.
I was thinking I own several stereo analog generators. I have designed switching type generator with 38 kHz L-R and 19 kHz pilots, but that's FM! Yeah and we piroots want shortwave .. so I would ask for any chips out there for a roll it easy AM stereo generator.
The station I was engineer for went AM stereo, but gave it up because public rather did not want that with 5 kHz fidelity. However two sidebands allow some nice potentials for Stereo Shortwave. Beyond that the DRM mode is easy as its just software. But, DRM has NOT really caught on either.
 ;)
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Offline redhat

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Re: AM stereo Generator designs plus 300 watt Digital RF
« Reply #1 on: December 08, 2017, 2023 UTC »
There never was a am stereo modulator IC akin to the BA1404, all the generators that are out there that I have seen use discrete modulator stages, delay and equalization networks, then finally a clipper stage to recover the phase modulated carrier.

For those folks who are happy with low level modulation followed by some serious linear amplification, you can do what I did in the old transmitter.  Take the signal from a c-quam test signal generator and mix it with a tunable LO which you could get from a DDS.  Filter and amplify the output....simple.  I do have PCB artwork for the mixer and filter stage in gif if anyone is interested....schematics too.

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Offline digitalmod

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Re: AM stereo Generator designs plus 300 watt Digital RF
« Reply #2 on: December 08, 2017, 2327 UTC »
Thanks.. I was afraid the am stereo thing really was a lost cause. Although I was reading Dr. Kahn really was planning on becoming a multi millionaire with his side band approach, but FCC did not really give him much shift there.


 8)
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Offline digitalmod

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Re: AM stereo Generator designs plus 300 watt Digital RF
« Reply #3 on: December 09, 2017, 0119 UTC »
 Well, its not a lost cause. AM stereo has been on in pirate mod 43 meter band and Utube videos show same. This dude has built, I think a KIT with Cquam format. Looks like a Motorola chip set. Maybe extinct now?? But I believe you do not need a linear RF amp as the station I used to work at was CQuam and didn't change the X mitter 5kw from a standard Gates Class Final Amp!

Video Here:

This dude sounds Canadian.. too much worry about part 15 dude!!

https://www.youtube.com/watch?v=_Fp-vuO7s6A
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Offline redhat

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Re: AM stereo Generator designs plus 300 watt Digital RF
« Reply #4 on: December 09, 2017, 0205 UTC »
The large IC is a PLL synthesizer used to generate the carrier frequency, most likely times 4.  In a conventional design, this would be followed by a quadrature divider, usually realized as a two stage johnson divider feeding two balanced modulator stages, one for I and one for Q.  The I stage has a dc bias applied to the modulating input to created the 1+L+R term.  The other is left as a balanced modulator being fed L-R and pilot at its modulation input.  The two signals are then summed, low pass filtered to allow the clipper to recover the carrier correctly, and then limited.  This phase modulated term is used as the carrier drive for a stereo transmitter.

Any practical implementation also involves adjustable delay and equalization networks necessary to allow the exciter to precisely duplicate the amplitude and phase response of the transmitters modulator stage.  Because real world transmitters have both group delay and amplitude roll off in the modulator, the response must be duplicated for the signals feeding the quadrature modulator or the stereo separation loop will never be closed, and poor separation will result.

It should also be noted that a precise demodulator is required to properly setup such a system, such as a modulation monitor modified to receive your transmit frequency, or with less precision, a stereo tuner with wide IF filters installed.

For the average joe, the linear amplification approach is a lot less involved and requires far less precision and hard to build/buy gear.

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Offline digitalmod

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Re: AM stereo Generator designs plus 300 watt Digital RF
« Reply #5 on: December 09, 2017, 1902 UTC »
 Thanks for info and the quad digital approach. The old style was quarter wave feed lines?  Not really a realistic thing at MW and surely a pain most other places. I read a 25 hz pilot is also included that the Rx using to demod. I am convinced, the station I left in 1980, must have had a field engineer set the whole thing up. It doesn't sound plug and play and that's the level of just about all the techs today in AM. ::)
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Offline redhat

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Re: AM stereo Generator designs plus 300 watt Digital RF
« Reply #6 on: December 09, 2017, 1919 UTC »
With the exception of Nautel's NX series, all modern stereo capable transmitters require a fairly complicated alignment process using single channel modulated tones, a reference demodulator, and a scope.

The original Motorola equipment was usually installed and aligned by a field service engineer, not the station chief.  When 'Moto started allowing third parties to license and manufacture equipment, all that changed.  To my knowledge, the only gear you can still buy is made Broadcast Electronics, or Delta Electronics.  BE for a time made a modulation monitor for C-QUAM, I've only ever seen one in the field.  I have seen a few original Motorola exciters and monitors out there too.  Delta will still sell a stereo modulation monitor and exciter, but they are the last to do so.  BE offers a stereo generator board for their A and E series transmitters.  Motorola got out of the broadcast end of the business in the late 80's or early 90's.

The options are not cheap.  The last time I priced a mod monitor from Delta, it was around $4K.

+-RH
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Offline digitalmod

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Re: AM stereo Generator designs plus 300 watt Digital RF
« Reply #7 on: December 15, 2017, 0236 UTC »
 8)
Thanks for great Info. I used to be a transmitter engineer, but wanna be DJ at a 5 kw in Vt. I left to go back to college at 27 year old. Never worked as a bc engineer again, but continued interest in stereo FM and designed such a multiplex switch generator with a divided by two xtal. Gave the 19khz pilot. My design , all with craze of day gates on chips, got me a staff job at University.
Oh well, that was.. then..
But, my desire now is to get some basic lab gear for RF design and especially scope and watt meter with dummy load.
The basic idea of relatively easy Class E design fascinates. Now, of course no naughty things here FCC is such a piece..

But, thanks for insights and I did get into the equations. Wow have I slipped in mathematics, and I used to teach it.
Found two YES two big variable air rotor capacitors in a unused (MFJ) mighty fine junk antenna tuner. That was one thing I thought I lacked, high voltage variable capacitors.
Radioboy
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