In theory low level modulation

[NightOwl3261]

In theory, say you have a radio that is only low level modulation such as many current day HF radios that can only provide lets say 70% modulation, and you run an amp behind and the combination is set up to run 1:4 ( or 4:1 I can't remember how that is stated) does that in any way make up for the low level modulation and provide more usable AM operation. The whole low level modulation is confusing so I am not sure what the end result is when using an amp...

 

[543_Dallas]

A good amplifier that is linear does not change the modulation percentage. If you have 70% modulation going in that's exactly what should come out. In reality there are slight variations but not much.

Just because you have a (low level) ssb rig with a balanced modulator doesn't mean it can't achieve 100% mod or more. The manufacturers sometimes do a half fast job in the AM dept.

The difference between high level and low level is how the modulation is mixed with the carrier. High level modulates the carrier at the finals. Can be more efficient because the final runs in class C but requires more modulator power. Low level modulation happens at an earlier stage. Requires less modulator power but the finals are less efficient. The finals in the rig are just a linear amplifier.

 

[Justme]

For most HF Ham rigs low level AM modulation is not really a main question and half heartedly done.
My new FT991A had a terrible Am modulation, too much carrier ( 40 watts ) for a 100 watt rig and too much audio drive in low level modulation.
So, went into the engineers menu, set the carrier back to 25 watts and the audio drive to a point that on another rig it sounded good without distortion.

My old FT2000-D 200 watt radio had from the factory an excellent am modulation 50 watt carrier and over 200 watts with audio, it also had the benefit to set the P.A. in class A.
Low level AM can be just as good as high level AM, but needs carefull attention how the rig is set up.
Ham rigs always run in class A/B in their pa's whatever the modulation is you are running if that is AM/FM/SSB/CW.
Or even in digital modes the FT991A runs in class A/B.
As i always repair my own stuff, or modify it and check them up bi annually you gett to know your transceivers well including schematics and set up.
The new 991A now is opened up, has a new much higher cfm fan with MagLev bearing so it keeps really cool even running high power modes.
I'm a sucker for cool electronics, expands their lifetime and prevents faults...

 

[543_Dallas]

I would consider 25 watts an absolute maximum for a 100 watt pep rig. Having plenty of headroom is important for AM. Most riceboxes will run better at 10 to 15 watts.

 

[Shadetree Mechanic]

So if the rig had 70% modulation, wouldn't reducing the carrier bring it up to 100%?

 

[Captain Kilowatt]

Low level modulation does NOT mean less than 100% modulation. The term low level modulation simply means that the modulation audio is applied at a low level RF stage and everything after that has to operate in a linear class of operation. High level modulation means the modulating audio is applied to the final RF stage like plate modulation or collector modulation like most CB's use. The terms "high" and "low" when it comes to modulation levels simply indicates what stage of RF is modulated and has nothing to do with the percentage of modulation. There is no reason whatsoever that a radio with low level modulation cannot meet or even exceed 100% modulation. I see this term being confused and misused all the time.

 

[Justme]

Standard is 25% carrier for a 100 watt rig and with modulation 100 Watt output.
Depending on the radio and cooling system that should work well, certainly if you like me replace the bog standard cooling fan with one of higher quality and more cf/m output.
Running 100 watt FT-8 for one hour made the transceiver luke warm, the fan didn't even enter the highest rpm.
that is 50% on 50% off in FT-8.
As Am has only 25% carrier and modulated decently gets about the same 50% stress time as FT-8.
As usual in FM it is advised to keep the output to 50% of the maximum power of the transceiver or 50 watt continuous.
The PA and cooling system should be able to deal with that easy.

 

[unit_399]

An example of an export rig with low level AM modulation is the original President Jackson.

- 399

 

[NZ8N]

The original Jackson and later the HR2510 / Lincoln line of radios modulate the TX mixer (an unbalanced balanced modulator) when transmitting AM, rather than the SSB balanced modulator.

Most recent ham radios like my FT857D put the modulation for AM at the balanced modulator used for SSB, which can introduce aliasing on negative peaks (watch the scope pattern to see what I mean), which is another form of distortion.

The FT101 series put the AM modulation on the AM buffer circuit after the AM carrier crystal, rather than the balanced modulator like in later models.

 

[543_Dallas]

So if the rig had 70% modulation, wouldn't reducing the carrier bring it up to 100%?

Sometimes. It depends on the rig.

 

[Handy Andy]

I'll let you guys run with this for an example to help understand the theory

Thanks to @unit_399 for the suggestion...

 

[Shockwave]

Most recent ham radios like my FT857D put the modulation for AM at the balanced modulator used for SSB, which can introduce aliasing on negative peaks (watch the scope pattern to see what I mean), which is another form of distortion.

This is very common and is done intentionally to exaggerate any distortion over 100% modulation to the point where it's not usable at that level. Running that IF signal through the crystal filter prevents splash but makes heavy audio sound like SSB as a result of the circuit used. You may be interested in knowing I've had success when using the standard Cobra 148GTL series pass modulator on the FET driver stages of the FT-857 in AM. Full broadcast fidelity is achieved with positive peaks, as strong at the largest SSB peaks. Unfortunately Yaesu was incapable of manufacturing a quality LCD display for this rig that could handle temperature changes in the car and like many other 857's the typical defective display, has made the radio almost unusable!

The FT101 series put the AM modulation on the AM buffer circuit after the AM carrier crystal, rather than the balanced modulator like in later models.

These were some of the last HF rigs able to still provide maximum AM performance right out of the box. Depending on where the AM carrier oscillator is located in the IF stage, it can be an ideal place to inject modulation.

It's almost sad to see how many HF rigs were manufactured with horrible AM audio while still having just about all of the needed parts to produce awesome AM audio. Simply changing the point where the AF is mixed with RF, solves most of the AM problems in these rigs, with the only added parts being the ones used to switch the AF away from the balanced modulator in AM and to a later stage within the rig.

 

[CCC GGG]

First a little theory; you can represent an AM signal (which is composed of a carrier and upper and lower sideband) using phasors (not Star Trek type) or vectors as follows: Draw the carrier vector as a reference voltage arrow pointing straight up. Then add a single tone upper sideband vector connected at the tail of the carrier vector pointing to the right but make its length half as long as the carrier vector. Now add a single tone lower sideband vector pointing to the left. The tail end of these three vectors all start at the same point. Now imagine the upper sideband vector rotating around clockwise while the lower sideband vector rotates around counterclockwise, with both sideband vectors rotating at the same rate. That rate would be, for example, one kilohertz if the single tone modulation frequency was one kilohertz. Now you'll see the advantage of this vectoral representation. Can you see how, as they rotate in each direction, there will be a moment when they meet, lined up vertically with the carrier vector. That will make the carrier voltage value double at that moment. In fact that's exactly what occurs. The entire composite signal voltage doubles. And when you consider that power is 20log of voltage that means that the power will be 4 times the original carrier power at that moment since the composite voltage has doubled. That's a vectoral explanation of why the PEP power of an AM signal is 4 times the unmodulated carrier power. Now let the two sideband vectors keep rotating at 1 kHz until they are pointing directly downward. At that moment the carrier vector is totally nulled by the two sideband vectors. That corresponds to that pinch off point you see on an oscilloscope. The moment where the sideband vectors added with the carrier vector corresponds to the peak envelope width you see on the scope. Ain't phasors great?! Mr Spock didn't have a clue about this. OK enough for AM theory, except I wanted to emphasize that this vector representation exists for AM signals, whether they were generated using low level modulation or using plate modulation in the final amp stage.

Regarding 100% modulation, that's when the sideband vectors are indeed each 1/2 the voltage value of the carrier vector.

Practically speaking, when I use my HT-37 or Johnson Viking Ranger I can very readily achieve 100% modulation. No amount of post amplification will change the modulation percentage. Only the overall power will be increased.

With many of the newer solid state ham rigs including my TS-590SG, what I have observed is that the ALC is designed mainly with SSB operation in mind. When you try to run AM on these "100 Watt PEP" rigs, you run into issues with the AM peak being held back due to the presence of the carrier. Without going inside the radio and modifying the ALC loop components (although there's a trick using an externally supplied fixed ALC voltage) you can get very close to a nice 100% AM envelope by simply running the rig at perhaps a 15 Watt carrier and 60 Watts PEP. Don't worry about not quite getting 100 Watts PEP AM out of it since you can now run that signal through your favorite amp such as an SB-220 with about 10 dB of gain giving you a 150 Watt unmodulated carrier and a clean 600 Watts PEP AM signal. Or put a smaller amp in front of the SB-220 so you end up running the 220 with a 375 Watt unmodulated carrier and 1500 Watts PEP AM output.

 

[FredTx63]

For most HF Ham rigs low level AM modulation is not really a main question and half heartedly done.
My new FT991A had a terrible Am modulation, too much carrier ( 40 watts ) for a 100 watt rig and too much audio drive in low level modulation.
So, went into the engineers menu, set the carrier back to 25 watts and the audio drive to a point that on another rig it sounded good without distortion.

My old FT2000-D 200 watt radio had from the factory an excellent am modulation 50 watt carrier and over 200 watts with audio, it also had the benefit to set the P.A. in class A.
Low level AM can be just as good as high level AM, but needs carefull attention how the rig is set up.
Ham rigs always run in class A/B in their pa's whatever the modulation is you are running if that is AM/FM/SSB/CW.
Or even in digital modes the FT991A runs in class A/B.
As i always repair my own stuff, or modify it and check them up bi annually you gett to know your transceivers well including schematics and set up.
The new 991A now is opened up, has a new much higher cfm fan with MagLev bearing so it keeps really cool even running high power modes.
I'm a sucker for cool electronics, expands their lifetime and prevents faults...

Could you not drop the carrier below 40w without going into the service menu?

 

[Justme]

Did not help, tried that first even backed up to 5 watts.
Only when i went into the service menu and lowered power in AM to just under 25 watts carrier and listening on my FT 2000-D as receiver i could get it right without distortion.
Now having 23 watt carrier and almost 100 watts fully modulated without distortion.
Seems regulating the P.A. back did nothing i needed to get the drive back into the section where the AM modulation is done through the service menu.
That is now done and it sounds good, now getting good reports in AM about the modulation.
Not using that a lot, but if it is on the radio it must work properly.