[Amps] FCC Denies Expert Linears' Request for Waiver of 15 dB Rule

[Jim Brown]

On Tue,1/3/2017 1:36 PM, Manfred Mornhinweg wrote:
> Jim,
>
>> USING ALC to control drive level for a power amplifier is a CAUSE of 
>> splatter and clicks. ALC should NEVER be used to set drive level.
>
> How do you do it, then? I would say that when transmitting SSB, using 
> ALC is pretty much a fact of life, like it or not. No ham in his sane 
> mind would always speak at a slavishy constant level, and hold the 
> distance to the microphone exactly constant to get just the right 
> meter deflection, so he can achieve a reasonably constant output power 
> without the use of ALC.

You are confusing speech processing, which is generally done at base 
band (that is, on the audio signal before it is applied as modulation), 
with ALC, which is control feedback between an RF amplifier and its 
driver.  Most modern transceivers include speech processing. In the pro 
world, we use both peak limiting and compression. Peak limiting being a 
short time constant that simply reduces gain on speech peaks, and 
compression being more of a dynamic gain-riding. Good signal processing 
can sound very good with up to 10 dB of gain reduction, and some systems 
are good for more than that.

Audio processing done entirely at baseband creates artifacts at 
baseband, but those baseband components won't get past the TX passband 
filter. W4TV has noted, however, that some rigs, notably Yaesu and ICOM, 
do part of their processing at RF, and can splatter pretty badly.

>
> When using an amp, certainly there is a choice between using ALC from 
> the amp, or just use the internal ALC of the transceiver and set the 
> output power just right to fully drive the amp but not overdrive it. 
> Either method should provide good results, if properly done, with the 
> method using ALC from the amp being more practical. What I meant in my 
> paragraph about hams causing splatter that way is that they don't use 
> ALC from the amp, and run the radio at full output, severely 
> overdriving the amp.
>
> ALC doesn't have to cause splatter and key clicks. It's _bad_ ALC, or 
> improperly used ALC, that does. Typically setting the mic gain far too 
> high and having the ALC throttle back the gain by 30dB or so, and 
> doing this on a poorly designed radio that has an ALC with a very slow 
> attack time.

Most amplifier manufacturers disagree with you. As long ago as 1980, Ten 
Tec's manual for their Titan amp advised against using ALC, and the 
manual for most power amps includes that advice. Perhaps it's the need 
for that careful matching of time constants that forms part of the basis 
for their advice. Most amp manufacturers also provide ALC output ONLY so 
that its omission won't be a buying obstacle for those customers who 
think they need it.

> In SDRs one can use the ever present delays in the signal processing 
> to implement "look ahead" ALC, that completely eliminates ALC-induced 
> splatter and key clicks. The same is true for AGC on RX. It makes such 
> receivers very pleasant to listen to, without any AGC "pumping" and no 
> distortion on the attacks.
>
>>> Radios with slow-acting ALC are also famous for causing IMD blasts 
>>> and key clicks without even needing an amp, and there are many. But 
>>> the most usual way of producing lousy signals is by intentionally 
>>> defeating the ALC of the transceivers. In my environment they call 
>>> it "liberating" the radio, because the poor radio was tied down to 
>>> just 100W by the evil manufacturer, and by defeating that "brake" it 
>>> can produce 150W or so, when turning the mic gain to full and then 
>>> screaming into the mike, right?
>>
>> I can't imagine what you are talking about. Since the days of 
>> separate TX and RX, I don't remember ever seeing a rig that didn't 
>> allow adjustment of output power.
>
> Vic mentioned the TS850 and its internal adjustment. That's exactly 
> what I was talking about. Every single solid state transceiver I have 
> ever worked on has this adjustment, and sometimes several of them to 
> cater for lower power limits on the higher bands. It basically adjusts 
> the power you get when you turn the front panel power control to max. 
> This has to be set low enough that the final stage doesn't severely 
> saturate. For push-pull output stages with 1:16 transformers and 
> powered from 13.8V, which is the industry standard, that's pretty 
> close to 100W. Without ALC such stages are easily driven to 150 or 
> 160W, but that's in deep saturation, causing horrible splatter, as Vic 
> correctly mentioned.

The pot that Vic was talking about sets the MAX output level. There's a 
front panel control to set the operating power.  I set it for about 50W 
of drive for my Titan.

>
> And this adjustment simply sets the trigger level for ALC. Nothing 
> else.  Let's face it: SSB transmitters control the output power by 
> means of ALC. It's the best method found to date, as far as I know, at 
> least for voice transmission. Instead in digimodes, including CW, it's 
> better to adjust the drive to stay just below the ALC activation 
> level. With voice you can't really do this, because the voice level 
> changes too much.

I completely disagree with you, Manfred.  Audio processing has been 
widely used in broadcasting for at least 60 years. The AM station where 
I worked as a student was using it in the early '60s. That is done 
entirely at AF, NOT at RF. VE7RF does extensive audio processing in his 
station. I've adjusted my K3 to roll off everything below 500 Hz and 
provide about 10 dB of compression on audio peaks.

>
>>> Sometimes radios develop faults that make the transmission dirty. I 
>>> remember a case of one station running a factory-made radio with a 
>>> bad PLL. It had an extremely high phase noise, and would transmit 
>>> modulated noise over a wide part of the band. That guy did reply to 
>>> my report, and told me that he had the same very high noise on RX, 
>>> so he thought that I was hearing what he thought was his local noise 
>>> floor! I tried to explain to him that probably his radio was faulty, 
>>> and I went on to explain about phase noise in frequency synthesizers 
>>> and how that can affect both TX and RX, but he totally rejected my 
>>> suggestion that his radio was faulty. He replied that his radio was 
>>> putting out "the full 100 watts and some more", and thus couldn't 
>>> possibly be faulty...
>>
>> The error in this paragraph is that the radio "developed" a fault. 
>> Many rigs are DESIGNED with massive phase noise and nasty clicks.
>
> It's not an error, Jim! While I agree with you on the fact that many 
> radios have rather high phase noise and poor ALC by design, in the 
> case I described the phase noise was far higher than normal even for a 
> rather bad radio, so it must have have been a fault, not a "feature".

Yes, it IS a design error. Any rig that is broader than necessary for 
the mode being transmitted is badly designed or badly operated. Quite 
common with the el-cheapo "do everything from 160M to 440 MHz for $1K US 
boxes, but also with the high priced rigs from some mfrs.

73, Jim K9YC