Automatic Level Control, ALC.

What characteristics of the load impedance are used to automatically
control the drive-level of the power amplifier?

Whatever they are must apply to CW, SSB and FM modes of operation.
----
Reg.

On Sat, 8 Oct 2005 18:54:44 +0000 (UTC), "Reg Edwards"
wrote:


Automatic Level Control, ALC.

What characteristics of the load impedance are used to automatically
control the drive-level of the power amplifier?

Well, as you no doubt know Reg, in the early days of application of
ALC to amateur transmitters it was literally to control the drive
level of the PA, and often did that is a closed loop where excess
drive in a class AB1 PA was detected by occurrence of grid current,
which was used to reduce gain in prior stages to limit the drive level
to AB1 conditions. Such a control system is indirectly affected by the
PA load.

Today, modern transceivers have further developed that idea to
controlling the RF output voltage or (differently) PEP power out (ie
be detecting Vout or power out and adjusting gain in prior stages to
limit output. This might more loosely be called Automatic Power
Control. In this case, the measurement point of the control loop is
more directly affected by the load.

Many modern solid state transceivers also detect poor loading using a
reflectometer calibrated for the specified load for rated output
power, and reduce drive based on the measured "reflected power" in the
reflectometer to protect the PA (if that needs to be done anymore for
that purpose - transistors are more robust). This might be properly
described as PA Protection. The measurement is directly affected by
the load, and is highly responsive to load changes beyond a configured
threshold.

All of these things may be included in the readers understanding of
ALC. Certainly, many if not most, modern solid state transceivers
perform all three of these functions, and display the control voltage
as "ALC". And many current model external valve based high power PAs
(Linear Amplifiers) still use the grid current detection scheme (if it
is hooked back to the transceiver).


Whatever they are must apply to CW, SSB and FM modes of operation.

And AM?

Owen
----
Reg.

--

"Owen Duffy" wrote various ways and means of automatically
controlling the power output of transmitters. Thanks very much.

Presumably control is needed to limit the power dissipated in the
output transistors which involves a time delay. And, which is a
different thing, to place limits on the instantaneous
collector/emitter voltage.

In my Icom 735 transceiver the first control requirement appears to be
met by by placing a limit on the DC collector current or power input.
The second requirement appears to be met by feeding back a
gain-control voltage proportional to the magnitude of the reflection
coefficient of the load impedance. I may not be entirely correct.
----
Reg.

On Sun, 9 Oct 2005 20:11:09 +0000 (UTC), "Reg Edwards"
wrote:


"Owen Duffy" wrote various ways and means of automatically
controlling the power output of transmitters. Thanks very much.

Presumably control is needed to limit the power dissipated in the
output transistors which involves a time delay. And, which is a
different thing, to place limits on the instantaneous
collector/emitter voltage.

Another method of PA protection is thermal protection of the PA,
sometimes integrated with cooling fan control. In my experience, where
drive control is deployed, it reduces drive to prevent the heatsink
exceeding about 60 deg C. I have found such protection commonly on
commercial FM land mobile equipment (two way radios), some amateur FM
mobiles, and less frequently in HF equipment.


In my Icom 735 transceiver the first control requirement appears to be
met by by placing a limit on the DC collector current or power input.

You are right, some radios do incorporate over-current protection of
the PA. It is in some Icom radios (IC706IIG, IC7400 / IC746PRO), and I
suspect many other Icoms will share this feature.

Whilst probably intended for PA protection, it has the great benefit
that the radio is less likely to draw excessive current while
adjusting an ATU, and tripping over-current protection in a power
supply. A radio that needs 16A in operation, but 25 during tune is a
pain in the butt on a nominal 20A power supply with over current
protection that kicks in above 22A!

The second requirement appears to be met by feeding back a
gain-control voltage proportional to the magnitude of the reflection
coefficient of the load impedance. I may not be entirely correct.

As I described, and for example, from the IC706IIG service manual:
"The reflected wave signal appears and increases on the
antenna connector when the antenna is mismatched. The
HF/50 MHz reflected signal level is detected at D10 (FILTER
board), and is amplified at the APC amplifier (IC1091c) and
applied to the ALC circuit as the reference voltage."

You are probably (almost certainly) correct about the 735 having a
"reflected power" limiting circuit, but Icom's view seems to be that
if you want to read their description of how the radio works (if you
had an interest in that sort of thing), you need to purchase the
service manual.

Where is all this leading Reg?

Owen
--

"Owen Duffy" wrote
Where is all this leading Reg?
=====================================

Owen, you sound very slightly suspicious. smiley

But it improves my and other readers education on a previously
undiscussed topic of conversation.

Thanks, Reg, G4FGQ.

A radio that needs 16A in operation, but 25 during tune is a
pain in the butt on a nominal 20A power supply with over current
protection that kicks in above 22A!

Myself, I never tune using full power. With my 706g, I either
put it on "AM", or reduce the drive to a pretty low level. I did the
same on my 730. Only with a good match would I ever test full
power output. I do the same with my tube TS-830.. heck, actually
any transmitter... :/
MK

Owen,
You have obviously have extensive knowledge and familiarity with some
equipment on this subject. I haven't studied this aspect of the radios I
have yet, but offer the following from my design experience.

There are several relatively independent considerations which have been
mentioned in this thread and I hope this clears up any confusion about which
is doing what.

My understanding of the true purpose of (what we call) ALC is to prevent
distortion in the final amplifier and the resulting "splatter" it causes,
while allowing the maximum power out. As you so correctly point out, this
originated in the linear amplifiers of old. Distortion at this point causes
increased intermodulation (mixing) of the desired signal components and the
resulting products appear out of the normal bandwidth = "splatter". There
is always SOME of this going on, but as long as it is below about -30 dBc
this is considered acceptable.
Because there is no Grid current to sense in solid state amplifiers, some
other method of detecting the onset of distorting is required. Not knowing
what _IS_ done, I can only speculate that knowing the highest level of
output power which causes only a small, allowable increase in distortion, an
output level detector can be designed which begins to provide feed back
which cuts drive when this point is reached; thus paralleling the function
of the grid current detection method of old. Because this is an
instantaneous feedback system with a relatively high gain, the feed back
voltage will exhibit a considerable rise as the device is driven closer and
closer to the undesirable point. This must be what we are looking at that
is called ALC on the front of the radio. As long as the level of this
signal is within the manufacturer's limits, the distortion is limited to the
specification and you are assured of the highest power with in-spec
distortion.

With solid state comes other considerations which are actually separate
from the distortion issue although from a circuitry standpoint they may seem
identical. Because these devices have been and are still somewhat sensitive
to mis-treatment and can be destroyed instantaneously, various methods are
available to help save the devices. However, had they been easily available
in tube days, they would have been equally applicable.

Sensing and placing a limit on collector/drain current is one method.
Some load conditions can cause excessive current resulting in device
destruction, so sensing over-current electronically and reducing drive can
prevent this.

With the advent of simple reverse power sensing circuitry, another fault
detection scheme can be used to reduce drive and protect the final power
device. Sensing SWR and reducing drive can prevent high device current and
destruction, as well.

Heat rise can also destroy the device, so sensing device temp and
reducing drive controls this also.

Because a design has one or more of these, the identity of each could be
confused by combining the feedback signal and calling it only "ALC" when in
actuality, there are two or more protection schemes present along with the
(distortion limiting) ALC.

73, Steve, K'9;D.C,I



snipping quite a bit:

"Owen Duffy" wrote in message
news
On Sun, 9 Oct 2005 20:11:09 +0000 (UTC), "Reg Edwards"
wrote:


"Owen Duffy" wrote various ways and means of automatically
controlling the power output of transmitters. Thanks very much.

Presumably control is needed to limit the power dissipated ...

Another method of PA protection is thermal protection of the PA...


In my Icom 735 transceiver the first control requirement appears to be
met by by placing a limit on the DC collector current or power input.

You are right, some radios do incorporate over-current protection ...

Whilst probably intended for PA protection, it has the great benefit
that the radio is less likely to draw excessive current while
adjusting an ATU, ...

As I described, and for example, from the IC706IIG service manual:
"The reflected wave signal appears ...the antenna is mismatched. The
...signal level is detected ...and
applied to the ALC circuit as the reference voltage." ....

Owen
--

On 11 Oct 2005 14:03:55 -0700, wrote:

A radio that needs 16A in operation, but 25 during tune is a
pain in the butt on a nominal 20A power supply with over current
protection that kicks in above 22A!

Myself, I never tune using full power. With my 706g, I either
put it on "AM", or reduce the drive to a pretty low level. I did the
same on my 730. Only with a good match would I ever test full
power output. I do the same with my tube TS-830.. heck, actually
any transmitter... :/

Quite wise.

Nevertheless, large currents are drawn by some radios in low power.

For example, I owned a Alinco DX70-TH and their EDX-2 autotuner. When
it autotuned, the tx power was reduced to about 10W out, but it still
drew in excess of 20A tuning into some loads on some frequencies...
which would trip the overcurrent protection in one of my power
supplies, so it was unusable on that power supply.

People do rough tune radios on full power, and I, like yourself,
question the wisdom of that practice.

I designed a device to make manual low power tuning with an IC706x
more convenient ( http://www.vk1od.net/IC706TuneMate/IC706TuneMate.htm
). It has been interesting that almost all of the feedback that I have
received on the article is from screwdriver antenna users who insist
that it is absolutely unecessary, they tune their screwdriver for
maximum RF Po indication on the IC706x meter using full power in RTTY
mode. (Achieving high RF Po indication depends indirectly on low VSWR
because of the PA load protection scheme.)

Owen
--

wrote:
Myself, I never tune using full power.

Don't trust the foldback circuitry or just being polite?
--
73, Cecil http://www.qsl.net/w5dxp

On Tue, 11 Oct 2005 16:32:59 -0500, "Steve Nosko"
wrote:


Because a design has one or more of these, the identity of each could be
confused by combining the feedback signal and calling it only "ALC" when in
actuality, there are two or more protection schemes present along with the
(distortion limiting) ALC.

Yes Steve, once you have the gain control mechanism, it becomes the
obvious control point for various over-x closed loop control systems.
But you are right that the highest priority function is for limitation
of distortion due to exessive signal (for the current conditions), and
it is that loop that has the most onerous dynamic performance
requirements (eg fast attack) for it to perform the intended fuction
well.

Unfortunately, there is a growing common belief that unless the ALC
meter is high upscale, then the rig isn't being talked up enough (the
dumbing down of ham radio). There seems a common ignorance that most
ALC detectors have a threshold (usually the rated PEP for solid state
PAs), and that the slightest deflection of the indicator means that
some peak signal has reached that threshold, further deflection causes
greater gain reduction in the IF stages, causing the ALC to act more
and more as a compressor where overdrive of the PA is likely to occur
on transients during ALC attack time. Exceeding the red line, even for
occasional transients, is to exceed the capability of the ALC to
contain PA distortion due to overload. My thought is that if you want
audio compression, use a speech processor, not the ALC.

Mentioning speech processors. A correctly adjusted speech processor is
proably better protection against overdrive than depending on ALC
alone. The peaks are contained (clipped) and the distortion products
filtered off, before getting near the PA which does not have effective
post filtering for clipping distorion.

Owen
--