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AM Carrfier Shift




 
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R. Fry SWL
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« Reply #25 on: June 04, 2019, 06:47:44 AM »

... I remember the popular Gates mod monitor used at most BC stations in the past having a "carrier" meter ...

Here is the FCC definition for the carrier shift of an AM broadcast station (from 47 CFR §73.14):

        Carrier-amplitude regulation (Carrier shift). The change in amplitude of the carrier wave
        in an amplitude-modulated transmitter when modulation is applied under conditions of
        symmetrical modulation.

In the past the FCC had a specified limit for AM broadcast station carrier shift, and all broadcast-approved AM modulation monitors metered/displayed it.  Now, due to "de-regulation" at the FCC, AM broadcast stations are not even required to have a modulation monitor.  However they still need to meet the technical standards required of AM stations by the FCC.

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« Reply #26 on: June 04, 2019, 07:26:27 AM »

Yes, that's right - the FCC doesn't require monitoring, but they have to meet the regs.

About half of the (now around 600) modulation monitors that Radio Engineering Associates has sold have gone to broadcast stations.

AM radio is getting harder and harder to listen to due to all of the RFI that the power grid and communications infrastructure (DSL and the like) is spitting out these days, but that's another discussion.  Usually I just stream the same stations, even in the car, rather than contend with all of the noise and static.
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« Reply #27 on: June 04, 2019, 09:00:49 AM »

Given that there are systems which intentionally produce carrier shift to save energy (perhaps by 3dB or more without severe effects) I don't know why there's a concern about, what I would assume to be, a relatively small amount that we may encounter in our transmitters (which aren't using extreme positive peak modulation). Sure, the old controlled carrier ham rigs sounded awful, but probably for more than that one reason.

So, aside from any difficulty in measuring carrier shift, why is it a problem? Will the guy on the receiving end notice that it's there, or not there? If this is an exercise in splitting hairs, I'm okay with that. But should I really be worried about how much carrier shift my transmitter has? I've never had a "you've got too much carrier shift" report, or heard anyone else ever get one, the exception being with a controlled carrier rig like the DX-60.

All that aside, I can see how an SDR transmitter could be a different animal. Having fooled around with a Flex-1500, there seem to more ways to make it play poorly than there are ways to make it work FB (which it can). Overdrive a single stage and it goes south quickly. It took some help from Rob, W1AEX for me to get my Flex settled in. Maybe SDR's are more susceptible to carrier shift if not correctly set-up? I dunno.


Don
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R. Fry SWL
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« Reply #28 on: June 04, 2019, 09:12:40 AM »


An interesting sidebar to this discussion is that, by itself, amplitude modulation of an r-f carrier to ±99.99... % does not change the carrier amplitude at all, even for asymmetric modulating waveforms.

All of the results of that modulation appear in the upper and lower sidebands, which occupy r-f spectra separated from the carrier by the sum and difference of the modulating frequency/frequencies.

A time-domain view of that modulated waveform has an r-f envelope amplitude that varies along the time base — which is produced by the vector sum of the (constant amplitude) carrier and the two, varying sidebands, at each instant of time.

The average value of the carrier may appear to vary in a time-domain display if the modulating waveform is not symmetric, but this effect is still produced by the vector sum at each instant of time of the (constant amplitude) carrier and the two sidebands.
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W4DNR
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« Reply #29 on: June 04, 2019, 01:01:28 PM »


So, aside from any difficulty in measuring carrier shift, why is it a problem? Will the guy on the receiving end notice that it's there, or not there? If this is an exercise in splitting hairs, I'm okay with that. But should I really be worried about how much carrier shift my transmitter has? I've never had a "you've got too much carrier shift" report, or heard anyone else ever get one, the exception being with a controlled carrier rig like the DX-60.
Don


Don,

I don't think it is as much of a problem as it is a curiosity that very few people understand what it is or how it could be created or how it could be eliminated.  I am one that "should" know, but just never stopped to fully examine the cause and effect.

I've worked with 50KW AMs ( and lower ) since 1976 and I must have been lucky to have had transmitters that had ample power supplies and lots of RF headroom.     

I think this is a good discussion.

I would still like to know  ( besides improper ALC on transceivers that weren't really designed for AM ) how to create and / or  eliminate *Carrier Shift*.          Even the DX-60 that you mention operating *controlled carrier* ...... maybe more properly called screen-grid modulation ?   and sounding bad ..... Is *carrier shift* associated with screen grid modulation, or just IMPROPERLY designed  screen grid modulation ?      If properly designed, why wouldn't a DX-60 sound just as good as any other modulation scheme ?   Splitting hairs ? No, I hope I'm going to learn something.

Don W4DNR
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KK4YY
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« Reply #30 on: June 04, 2019, 04:46:15 PM »

Don,

The DX-60 was screen modulated, but intentionally designed to produce a controlled carrier response. This was an attempt to thwart the low plate efficiency of the screen modulated amplifier stage at low modulation levels.

Screen modulation, by itself, does not produce a controlled carrier. Heath, and others, created methods to accomplish it. It was a rudimentary implementation that many DX-60 owners have had to undo. Yes, it worked, but it was torture to listen to. Anyone running a stock DX-60 will usually be politely informed that eliminating the controlled carrier would be a welcome improvement.

Don
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« Reply #31 on: June 04, 2019, 07:37:36 PM »

isn’t this definition

“Carrier-amplitude regulation (Carrier shift). The change in amplitude of the carrier wave
 in an amplitude-modulated transmitter when modulation is applied under conditions of
 symmetrical modulation.”

just another way to define modulation percentage ?
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« Reply #32 on: June 04, 2019, 08:59:31 PM »

Hey folks, interesting topic!

"Carrier Shift" isn't an abstract, nor is it always properly demonstrated by scopes or "average" wattmeters.

The "shift" nomenclature was initially created by the lack of power supply regulation in legacy transmitters (tube-type), which would include overall capacitance and the ability to respond to transients (mostly high-energy, low-end audio). The plate current would dip (or increase) based on that regulation. "Positive Swing" isn't always a good thing!. Steve, QIX has properly defined it as an  "average" power deviation which is absolutely correct. At any given time (suspended in a time-based scope or panadapter) both sidebands should be mirror images, even if asymmetrical. That would include "carrier controlled" schemes if properly implemented, at a given time.

Since the majority of my operation is with plate-modulated old-school stuff, a starting point is the plate current indication under modulation.. Any meter deviation with modulation is a flag that "carrier shift" could be occurring.
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« Reply #33 on: June 04, 2019, 10:49:56 PM »

Methinks this is where using separate power supplies for the modulator and RFPA come in.
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« Reply #34 on: June 04, 2019, 10:55:07 PM »

...Is *carrier shift* associated with screen grid modulation, or just IMPROPERLY designed  screen grid modulation ?      If properly designed, why wouldn't a DX-60 sound just as good as any other modulation scheme ?   Splitting hairs ? No, I hope I'm going to learn something.

Don W4DNR

There are a number of enhancements one can make to the DX-35, DX-40, DX-60, HT-40, Knight T-60 and T-150 and similar transmitters to improve the power supply and audio.

Once the power supply is beefed up you will automatically have less carrier droop during modulation.

By modifying the speech amplifier for more linear audio and wider audio frequency response, the audio can be much improved.

And finally, by removing the controlled carrier time constant circuit and going to direct screen modulation, higher modulation percentages can be had.

Down here,

http://amfone.net/Amforum/index.php?board=46.0 ,

and here,

http://amfone.net/Amforum/index.php?board=52.0

one can find improvements for most of the SGM transmitters mentioned above.


Phil - AC0OB
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« Reply #35 on: June 04, 2019, 11:35:25 PM »

There were a few questions.... "How do I get rid of carrier shift"  and  "What causes carrier shift?"

These questions are like asking "How do I get rid of audio distortion?" and "What causes audio distortion?"

There are many causes of carrier shift, distortion and non-linearity, thus many solutions. Jeff/ NBC came up with the elegant suggestion of just looking at the plate meter on a plate modulated rig.

This is all about the stability of all supplies feeding the rig, using good tubes, good iron, regulated screens and grids when needed, operating parameters adjusted right, etc. And be sure the loading cap is adjusted on the heavier side (less C) when using a linear amp or when screen modulating. I've heard more than a few BIG screen modulated rigs that sounded horrible with negative carrier shift and crunchiness under modulation until the operator simply decreased the C2 loading cap value and it cleaned up.  The Slab Bacon with the big meat a-shaking was a perfect example with his 4X1 screen modulated rig. He found that rig sounded very FB once the loading was increased as the plate efficiency was deliberately reduced. (more heat = cleaner operation)

Carrier shift is just another symptom / problem of a rig that needs work - that can be cured with a shotgun approach - by running tones thru the rig, triangle, sine - watching the spectrum analyzer and oscilloscope, sweeping, etc., to trick out the rig.   Once a rig is optimized in all areas, fine tuned and is transparent with exquisite cleanliness, the carrier shift will be gone - just like the distortion, splatter and tuning instabilities.  IE, carrier shift is a matter of degree. It can be slight and hardly noticeable, just like a bit of audio non-linearity. Or it can be really bad as the carrier varies wildly under modulation like a controlled carrier system. If there is a bad carrier shift, then there is likely other audio problems related to non-linearity caused by the same source(s). I like to look at a rig as a complete system where everything depends on everything else, like a chain.  One bad link and the problems start.

T
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« Reply #36 on: June 05, 2019, 01:08:02 AM »

Please avoid having any sort of poor power supply voltage regulation 'modulate' the transmitter.

By using CCS-rated, high quality parts (Dahl, UTC, Thordarson, Amertran, et al.) having current ratings of 2-3x the maximum expected amount, voltage regulation of 2.5 to 3.1% can be got over a 5:1 to 6:1 current range. This would be a FWCT or bridge with a choke input type of filter.
Even the older, 'high-DCR' parts work fairly well in a choke input circuit when run at 1/3 to 1/2 their continuous rating.
We (hopefully) don't skimp on RF circuit parts, pay the same attention to the power!

Such a supply won't fit inside the transmitters mentioned, but could be put in a separate enclosure. Hey lots of gear has a separate power supply.

Power supplies with capacitor input filters have poor regulation and are avoided. leave them for desktop SSB amplifiers.



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« Reply #37 on: June 05, 2019, 01:50:31 PM »

A while back, I put together a small amplifier that allows me to use a QRP transmitter at 100W PEP or to drive a larger amplifier. It was an RF deck from a Transworld TW-100 and the filter ass'y from a TS-440.

Running it from battery power, as I typically do in the shack, I added a large hold-up cap inside the amp to counter the DC voltage drop of the cable from the battery. I used what I had in the junk-box, but one of those huge caps that are used on mobile stereo installations would have been a nice choice too.

I believe any 12V radio installation could benefit from doing similar, whether it be mobile, in-shack or battery, power supply configured. Having a stiff supply voltage should help with carrier shift and perhaps IMD on a linear amp.

I did similar with an FT-101, replacing several hold-up caps in various parts of the circuitry, typically doubling the values. Compared to regulating voltages, adding more C is easy. Modern caps are physically smaller/uf and larger values can easily be placed in the vacated space. When re-capping an old rig, why not make it better than new?

As to the OP's original concern with carrier shift on an Icom IC7300, maybe its external power supply wasn't up to the task. I'd hate to see that radio get a bad rep for something that's out of the manufacturers control. Just sayin'.


Don
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Jim, W5JO
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« Reply #38 on: June 05, 2019, 04:50:58 PM »

As to the OP's original concern with carrier shift on an Icom IC7300, maybe its external power supply wasn't up to the task. I'd hate to see that radio get a bad rep for something that's out of the manufacturers control. Just sayin'.
Don

You are right.   There is a lot we don't know about the installation.  There have been numerous complaints about voltage drop across the blade fuses and the power supply cable.  We also don't know what kind of power supply is used.  We don't know the settings he is using on the radio, all of which could cause problems.

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« Reply #39 on: June 06, 2019, 09:42:34 AM »

a better solution to voltage droop in dc supply situations is to use remote voltage sensing ... a well designed supply can dynamically regulate this to just a few millivolts
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« Reply #40 on: June 06, 2019, 10:34:47 AM »

If there is a bad carrier shift, then there is likely other audio problems related to non-linearity caused by the same source(s).
T

Steve points out that , "  I regularly modulate 150% in the positive direction, and 90% in the negative direction (at the same time) and there is no carrier shift."

Wouldn't  "asymmetrical modulation" ( +150  -90 )  be the definition of " non-linearity"  ( maybe "purposeful"  non-linearity ? )

At 150% positive modulation, I'm pretty sure that the average reading RF Ammeter  or the Bird 43 Voltage needle will increase in readings.   And I see a case  for zero 'carrier shift".       I think that would be a terrific accomplishment.   

I wasn't here to bash the Icom 7300 ....   My friend has the matching Icom power supply, but I can't speak for it's ability to supply sufficient current through it's power leads.   I just thought that "carrier shift"  was an interesting subject. 

Don W4DNR





   
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Jim, W5JO
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« Reply #41 on: June 06, 2019, 11:43:17 AM »


I just thought that "carrier shift"  was an interesting subject. 

Don W4DNR   

As you can see Don, it is like discussing SWR.
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« Reply #42 on: June 06, 2019, 12:44:17 PM »


I just thought that "carrier shift"  was an interesting subject. 

Don W4DNR   

As you can see Don, it is like discussing SWR.


... or PEP regarding the "legal limit".
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« Reply #43 on: June 06, 2019, 05:47:28 PM »


I wasn't here to bash the Icom 7300 ....   My friend has the matching Icom power supply, but I can't speak for it's ability to supply sufficient current through it's power leads.   I just thought that "carrier shift"  was an interesting subject. 

Don W4DNR

I have only one plastic transceiver which is the Icom-7200 and use a 35 Amp Astron linear supply with the voltage set at 14.5V. No real voltage drop at max power with the supplied cables.

Again, it is the modulation method combined with the internal ALC that results in carrier shift.


Phil - AC0OB
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« Reply #44 on: June 06, 2019, 07:47:30 PM »

If there is a bad carrier shift, then there is likely other audio problems related to non-linearity caused by the same source(s).
T

Steve points out that , "  I regularly modulate 150% in the positive direction, and 90% in the negative direction (at the same time) and there is no carrier shift."

Wouldn't  "asymmetrical modulation" ( +150  -90 )  be the definition of " non-linearity"  ( maybe "purposeful"  non-linearity ? )

At 150% positive modulation, I'm pretty sure that the average reading RF Ammeter  or the Bird 43 Voltage needle will increase in readings.   And I see a case  for zero 'carrier shift".       I think that would be a terrific accomplishment.    

I wasn't here to bash the Icom 7300 ....   My friend has the matching Icom power supply, but I can't speak for it's ability to supply sufficient current through it's power leads.   I just thought that "carrier shift"  was an interesting subject.  

Don W4DNR
 

The male voice (at most males) reproduced by a very good audio system (including the microphone) has a LOT of natural asymmetry.  

Asymmetry is quite natural and normal.  In my own transmitter, the output meter does not move under modulation, unless I modulated with such a low frequency that the meter actually responds to the modulation.  In most cases, the positive modulation is up around 150% and the negative modulation around 90%.

Non-linearity is another story, and usually DOES cause carrier shift  (often negative).
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« Reply #45 on: June 07, 2019, 12:14:39 PM »

I should post some pictures.  It would help illustrate the behavior.
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« Reply #46 on: June 07, 2019, 01:01:42 PM »

The male voice (at most males) reproduced by a very good audio system (including the microphone) has a LOT of natural asymmetry.  

Asymmetry is quite natural and normal.


Yes, With commercial AM Broadcasting, we would start at the transmitter and swap output leads on every device between the transmitter and the microphone , looking for the combination ( phasing ) that produced the most asymmetry.

I worked on a Bauer 10J once that would do 110% positive 95% negative feeding it with an HP Audio Generator ... either polarity.  A 4CX15,000 modulated with a pair of 4CX5,000s.  All but one DJ could get that transmitter to modulate past 125% positive.       Later, some of the better AM processors like the Orban Optimod could even fix that.

Don W4DNR

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« Reply #47 on: June 08, 2019, 11:52:49 AM »

That unit is well-worth having its schematic as one page for study.

* Gates-M-5693-Modulation-Monitor-1959_full-sch.pdf (991.88 KB - downloaded 52 times.)
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« Reply #48 on: June 09, 2019, 06:03:07 AM »

That unit is well-worth having its schematic as one page for study.


I have an old General Radio mod monitor that is similar, as well as one of the Gates units shown in the schematic.

The design of these old units is fairly impressive, considering the limited technology available at the time.

Of course, these days we can do all of this using software and a small hardware interface to the transmitter, with much greater accuracy (particularly the meters) and at significantly lower cost.

However, it is always interesting to see what people were able to accomplish way back when, considering the limitations of what was available at the time for components.
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