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
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