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Author Topic: Controlled CXR AM  (Read 3118 times)
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VE7RF
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« on: August 20, 2020, 01:56:03 PM »

This may well have  been  covered before.  My old  drake T4XB/C  had  'controlled cxr AM'.   This scheme worked pretty good if  I remember.  I think the heath  DX-60/B  also  had  CCM.

I see that  Continental Electronics in Dallas,Texas  still sells  100  and  300 kw  SW  AM broadcast TX...with the  CCM  feature (0-1-2-3-4-5-6 db cxr redux..set  by dip switches).  It was a good  heat redux  scheme....which would be nice to have in these  SS  modern Xcvr's....driving an external linear amp.

Jim  VE7RF
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KD6VXI
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« Reply #1 on: August 20, 2020, 07:21:14 PM »

The big flamethrower did it not for heat reduction but for power input reduction.

Saved hundreds to thousands of dollars a month on multi TX sites.

--Shane
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K1JJ
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« Reply #2 on: August 20, 2020, 09:09:29 PM »

I remember in the early 60s when 75M was mostly AM.   There were carrier heterodynes everywhere and the band was packed.

You could always tell the guys running controlled carrier because the carrier heterodynes would get louder and weaker as they talked. It was quite unnerving.  Unlike the slow gradual fading sound, the fast heterodyne tones following the voice syllabic rate was akin to hearing a modern sync detector with everyone off freq... :-)

I suppose a big clear channel BC station would have no problem, but when the band is changing, I'll bet a 6DB change is quite noticeable when there is interference.

It's a nice power reduction savings and clever idea,  1/4X  for 6 DB, BUT I'd choose a constant carrier for AM any day.

These days carrier heterodynes are not much of a problem, so maybe it's worth another look at for a big rig.

T

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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

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« Reply #3 on: August 21, 2020, 11:00:07 AM »

Each method is different.

Some lower the carrier during no audio periods.  This causes lower efficiency on a plate mod rig but lower power input on a solid state rig.

Some actually raise the carrier during low modulation.  This causes higher efficiency on a class C plate mod rig.  And clears the band more.

I've seen a homebrew version of it at the station in Simi Valley.  A big Ampliphase rig capable of 250Kw.  Its run at 100kw.  I presented the circuit here before, but not in it's entirety.  The guy who designed it lost a page of his original design over the years.

I may ask the engineer of the station if he has a copy of the full schematic.

I can imagine the effect you're describing Tom.  That would be nasty to listen to I am sure.

--Shane
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K1JJ
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« Reply #4 on: August 21, 2020, 04:30:12 PM »

Each method is different.

Some actually raise the carrier during low modulation.  This causes higher efficiency on a class C plate mod rig.  And clears the band more.
Some lower the carrier during no audio periods.  This causes lower efficiency on a plate mod rig...
--Shane
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Hi Shane,

How is lowering the carrier on a class C  plate modulated rig accomplished?  Is it a reduction in screen or plate voltage or something else?    If it's not too complex, I might play around with the technique on one of my rigs to see how it works out...   I can imagine a detected audio waveform driving a MOSFET which then varies the carrier.

A similar technique: My big linear amp (for SSB) has a bias control that idles the tube low between words.  It goes from a class B almost cutoff to normal idle every few seconds or so as I speak and pause. It really does save power and I don't see any attack artifacts cuz the lower idle is still  enuff for a clean sig.

T
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Use an "AM Courtesy Filter" to limit transmit audio bandwidth  +-4.5 KHz, +-6.0 KHz or +-8.0 KHz when needed.  Easily done in DSP.

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« Reply #5 on: August 21, 2020, 05:37:01 PM »

Tom,

On the big PDM style rigs, it's an algorithm using circuitry designed into the PDM generator.

On the continental rigs that Jim mentioned they use an EER system.  Side benefit is that allows for modulation other than straight amplitude.

On the ampliphase transmitter I saw it work on, the modulator (phasing, for the peak tube) as well as one of the IPA tubes where controlled.  I'm not sure what it changed on the IPA to vary the carrier level, probably screen voltage.  That system increased the carrier during low modulation levels.  It was designed to copy the effect of BBC (Brown Bovery, not British Broadcasting) system.  All done with op amps, and worked pretty good.  Was odd to watch it negative swing though!

Their is another setup that allows for the lowering of the carrier in solid state series modulators...  A guy named Bill Eitner came up with it.  He basically took a sample of the audio modulating waveform, rectified it on the positive side via a cap and diode, then applied it to the carrier control.  He called it NPC for Negative Peak Compression.  It was pretty easily accomplished and worked.  IMD was slightly raised as I recall.  Was popular with the CBers 20 years ago whom owned series modulated chassis.  It didn't compress the negative peaks, but instead rode the carrier at a syllabic rate.  Shwing the monkey and all that crap ....  Smiley

--Shane
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« Reply #6 on: August 23, 2020, 12:07:21 AM »

This may well have  been  covered before.  My old  drake T4XB/C  had  'controlled cxr AM'.   This scheme worked pretty good if  I remember.  I think the heath  DX-60/B  also  had  CCM.

I see that  Continental Electronics in Dallas,Texas  still sells  100  and  300 kw  SW  AM broadcast TX...with the  CCM  feature (0-1-2-3-4-5-6 db cxr redux..set  by dip switches).  It was a good  heat redux  scheme....which would be nice to have in these  SS  modern Xcvr's....driving an external linear amp.

Jim  VE7RF

Two different methods. Controlled Carrier modulation in the older rigs was a form of Screen Grid Modulation and has no equivalent to CCM in modern broadcast transmitters.

See section 4 of this document:

https://www.nrscstandards.org/standards-and-guidelines/documents/archive/nrsc-g101.pdf

Quote
...DESCRIPTION OF ALGORITHMS
MDCL algorithms can be classified into two categories. The first category consists of a single system, Amplitude Modulation Companding (AMC), for which both the carrier level and the sideband level are dynamically reduced together. For the second category, Dynamic Carrier Systems (DCS), only the carrier is dynamically reduced and the sideband power remains constant at the normal AM level. A number of systems fall into this category...



Phil

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