Discussion in 'Amplitude Modulation' started by KI5OMM, Jul 2, 2021.
If the radio has an external ALC jack, Tom's idea might be the easiest and probably best solution.
The original Yaesu FT101 series ONLY uses ALC on SSB . . . it is disabled on CW, Tune and AM.
The ALC is just there to maintain Linearity on SSB, since overdriving the PA would cause distortion or splatter. But you can't overdrive on carrier-based modes, so why would you want ALC?
I don't think there's an SSB rig that comes close to producing such brilliant-sounding AM as the FT101 produces (but not the Z series, which is a different rig altogether).
Unlike most SSB rigs on AM, it's proper double-sideband, where the buffer of the CW Carrier oscillator is modulated. So you set the mic gain to get full modulation, and the Carrier control to set the power (the modulation level will stay the same). In reality, you can't even over-modulate, as it's rather like high-level Choke Modulation.
When I use my FT101E on AM I often get told I sound like the BBC !
Back in the early 1980s when I ran the repair shop at a two-way radio (land mobile) service provider, I worked on many FT-101s that local hams (and CBers) brought in, since they also dealt in some amateur equipment on the side. That rig has problems of its own: poor dynamic range at the receiver and a spurious image on 160m transmit, but the AM TX function is indeed superior to about any modern day plastic radio. Depending on the modulator board, the rig may be capable of modulating up to 150% positive without flat-topping or distortion. Some modulator boards were better than others. Timtron WA1HLR published a set modifications that greatly improved both the TX and RX audio, to make it sound near broadcast quality with a good microphone.
As long as the AM is generated using low-level modulation and linear amplified, you can overdrive the
amplifier chain in AM as well as in SSB.
Seen from the PA point of view, the modulation crest values for AM are indistinguishable from those of SSB.
A major difference between AM and SSB is however that ALC acts very differently on the SSB and AM waveforms. In the SSB case, it will just compress the envelope slightly, but in the AM case it reduces the modulation percentage, and introduces considerable envelope distortion.
For this reason, ALC was often switched off or its control characteristics changed to keep the ratio between peak modulation envelope and carrier as constant as possible.
Making such an ALC is hard, and often it was a quite
difficult balancing act to adjust an ALC circuit so it worked properly in both AM and SSB modes in marine transmitters that were supposed to transmit both with good quality.
More advanced professional gear circumvented this problem using TGC for level setting, and an ALC with a limited control range for handling any overshoots in the SSB waveform.
This is in effect how the FT-101 handles the issue in AM mode, but the TGC level setting is manual, and it becomes the responsibility of the operator not to overmodulate.
Most noticeably, it kicks the carrier level back as the gain of the stage under ALC control is reduced, thus generating negative carrier shift AKA "downward modulation". The result is sort of a reverse controlled control modulation, where the carrier level decreases with modulation as opposed to properly working carrier control where the carrier level increases with modulation.
In any case, for best quality, the carrier level of an AM signal should not bounce up or down, but remain steady under modulation. Controlled carrier, even when functioning properly, is a compromise at best.
If the rig does not have a means for manually disabling the ALC, refer to W8JI's solution, as explained in posting # 27.
"Reverse controlled carrier" is a quite apt description of what ALC does to AM.
This was a major problem in AM transmission for ground-air SELCAL where there is a limit for
how much envelope distortion the amplifier chain was allowed to introduce.
After considerable experimentation in connection with a change of exciters in the mid-80s, it was decided against using ALC in the AM mode, and using a function in the remote control interface to disable ALC. The previous exciters used envelope feedback for distortion reduction, which worked both in AM and SSB modes.
The envelope distortion on AM introduced by ALC is made worse by the wide-range ALC common in amateur gear. When the ALC is allowed to vary the system gain by 10 dB or so, the effects on the carrier level easily become disastrous. In writing specifications during the 80s for AM-capable SSB transmitters we decided to allow an ALC range of less than 3 dB having switchable time-constants.
Controlled-carrier AM is certainly at best a compromise, but can be tolerated in transmitters where plate dissipation or power supplies are marginal.
The businesses that own AM broadcast licenses today take advantage of variable carrier techniques to save money on the station utility bill.
Modulation-Dependent Carrier Level (MDCL) technology supposedly can cut overall power consumption by 20 percent.
That uses algorithms a lot more complex than simple carrier control used on amateur gear. One system increases the carrier level during modulation peaks. Another increases carrier level at intermediate levels and decreases it at both low and peak levels. None of them supposedly vary the carrier enough to be audible on a receiver with normal AM detector.
Nautel AM TX.
Their 50kW is no larger than a regular refrigerator.
Not much more than a giant computer hooked to a lightning rod...but they sound devine.
Obviously a result of the natural progression of technical evolution.
IMO, this process also ultimately removes the beauty, romanticism, and sexiness making the final product stunningly sterile and devoid of intrigue.