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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