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

[Manfred Mornhinweg]

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.

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.

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.

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.

>> 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". High 
enough to be plainly audible on TX, from a considerable distance, which 
means that the same radio on RX must have been awful. I have repaired 
radios with faulty PLLs that generated excessive phase noise, so I do 
know that radios can go bad in that regard, and that they can be 
repaired. Typical causes are degraded electrolytic capacitors, 
semiconductors going noisy, detuned bandpass filters somewhere in the 
multiple PLLs... In one Yaesu it was a totally dead 2SC535 transistor, 
which caused the input signal to one programmable divider to be some 
30dB lower than normal, and that PLL still worked but with abnormally 
high phase noise.

Manfred

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