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Author Topic: How fast do ALC loops react?  (Read 6605 times)

Posts: 5908

« Reply #15 on: March 25, 2012, 03:37:48 PM »

Yes, overshoot should be added as a possible cause of unexplained ALS amp failures.

This is alluded to by ZS6BIM in reply #5. He makes the good point that when the amp saturates, the feedback loops (local and also global via the ALC) are "broken" and the amp will be driven even harder. While the drive cannot be high enough to damage the FET gate oxide, what about the drain current when driving the LP filter with the flat topped waveform? I'll model this in SPICE and see what happens.

Keep the ideas coming. So far I don't see that the ALS amps operate the FETs close to the MRF-150 specs. But other things might kill the FETs such as:

Exciter overshoot
   the ALS has no protection for overdrive
Load arcing
   the ALS has no protection for this other than the slow reverse power protection

Improper application of heatsink compound
Debris between the FET and the heatsink
Heatsink flatness

Bias pots possibly changing during shipping (my observation)
Ameritron possibly not using matched FETs

« Last Edit: March 25, 2012, 03:40:29 PM by WX7G » Logged

Posts: 67

« Reply #16 on: March 26, 2012, 01:02:27 AM »

In addition to the points described by WX7G regarding improper mounting of RF flanged devices there can also be a problem with flange flatness.

To illustrate the point I lightly sanded a number of transistor flanges from different manufactures using a fine emery paper on a flat surface.

Have a look at the image below of different RF FETs and Bipolars (MRF150, MRF422, HF28-220 and others.

In the image the bottom row have flat flanges. These devices are from M/A Com and Spectrum - the top row showing a significant deviation from flatness is from other manufactures.

AN555 is a useful application note from Motorola on the “Mounting Stripline-Opposed- Emitter (SOE) Transistors”


Posts: 473

« Reply #17 on: March 26, 2012, 11:26:13 AM »

Scary, but unfortunately not surprising.
A modest surface grinder is your friend, particularly if buying parts from ah second tier suppliers.

The heatsinks can often be worse for both flatness and finish then the power devices, and between the two of them, budgeting for a little time on the machining centre for each amp is probably a good idea.

Push comes to shove, valve grinding paste has its place.
73, Dan.

Posts: 67

« Reply #18 on: March 31, 2012, 07:27:26 AM »

More thoughts on the importance of a good ALC.

For those interested here is a storage oscilloscope image of the internal ALC reaction time of an HF transceiver showing the first 20ms of the word “two” made at the beginning of a SSB transmission.

The ALC corrects the amplifier’s gain in 440 microseconds for the required output power, in this instance 125W.

To prevent continuous gain corrections at the syllabic rate that would result in splatter the gain is then held constant during normal speech.

The purpose of ALC is not only to set the transmit path gain for the rated output power but also to reduce the gain by the correct amount when the load deviates from 50, j0 Ohms so as to prevent nonlinear operation of the final and possibly excessive dissipation.

Again for interest here is the RF output spectrum of the same transceiver made by storing the accumulative frequency spectrum of 60s of SSB speech.

The above spectrum was made with the transceivers speech processor on (about 12dB's of amplitude compression) giving the following look to the classical Christmas tree "threee"

Due to the speech processing the envelope of a two tone signal through this transceiver, shown below,  looks rather bad like a seriously over-driven amplifier however if correctly done you can see it makes no significant contribution to band splatter.

As mentioned elsewhere on this forum a correctly implemented ALC and TX path gain distribution can make it impossible for a user to generate excessive distortion through overdrive, excessive speech processing or operating into a mismatched load.

Unfortunately this is not always the case with external power amplifiers that don’t contain their own ALC loop.


Posts: 494

« Reply #19 on: March 31, 2012, 10:12:51 PM »

Mikes reply is very telling and informative.
It agrees with much of what I have observed unscientifically.
My big advancement came from finding the best deep null antenna tuner setting using a noise bridge instead of just using any settings that looked good on the SWR meter.
The AL80B instantly seemed to like these findings by showing a much broader amplifier load setting sensitivity into the same antenna and being able to move frequency farther up ,or down band without as much retuning if any.
It also corrected a specific condition ( that bugged me) with the radio through with or without the ALC feedback.
Every different radio and amplifier combination can act differently so it's not always a plug and play deal without observing what's taking place.
Very specifically my radio is internally set to only allow 80% PEP modulation on AM mode.
This had always resulted in a backwards power movement on  the AL80B and any external power meter reading when using the AM mode.
I had attributed it to an internal radio control of the ALC and audio circuits by design.
Now with the new tuner settings the power will actually stay steady or move forward a noticeable amount instead of backward.
I can only guess at this time that there must be a change of impedance taking place over and above just the ALC and internal radio actions.
Bottom line is there has to be more going on than most of us know about.
Ken KM3F

Posts: 5908

« Reply #20 on: April 01, 2012, 12:33:21 PM »

Ken, how much carrier power are you running your AL-80B at?

Posts: 494

« Reply #21 on: April 01, 2012, 02:07:42 PM »

I never go above 100 watts carrier using the ALC as the carrier level control.
The radio drive can be lowered to about 10 watts drive to attain the carier level but dynamic range is lost by restricting in that manner.
This allows the radio to run it's full 25 watt AM level into the amplifier allowing both to have their full dynamic ranges with lots of head room (dynamically) for the amplifier.
In this setup the automatic bias circuit in the AL80B has no action as it would on SSB.
I run a w2ihy EQ using only the 200 to 2400 EQ setting with both a Heil
Hn10 HC5/4 elements and Heil Pro Elite headset with boom mike HC6 element that has a lot more low end response.
With this set up the audio is excellent without going wide band and taking up any more bandwidth than stock.
As far as real ESSB, this is not at that level but is still very good clean audio.

Posts: 906

« Reply #22 on: April 06, 2012, 05:08:13 PM »

ALC is a dated concept that has no real place in modern amateur equipment designed.  Its  a flawed concept from the past, designers have  much better options today.

Unfortunately most ham equipment designs are copied or recycled, so we will never see  a simple concept such as TGC(transmitter gain control) circuits used on ham transceivers.

SDR radios can deliver  feedforward  power  control, however ham radio companies dont have engineers who are experienced designing these advanced  amplifier gain control systems. Its bread and butter
engineering in cell phone and mil communications systems.

Cobra 148 engineering is the standard!

Posts: 67

« Reply #23 on: April 07, 2012, 02:43:52 AM »

Zenki I believe your criticism of the conventional methods of ALC that rely on a feedback principle are unreasonably harsh.

(ALC = Automatic Level Control and Automatic Load Control – both equally important.)

ALC when properly implemented functions extremely well to rapidly establish the correct transmitter path gain and to protect the final stage against any unforeseen load changes.

I can’t see how a feed-forward system could do the same as it requires prior knowledge of the unknown! i.e. what impedance could the load change to at any given time.

Agreed with the advent of CPU based radios it is now possible to characterize the transmit path gain and together with DSP amplitude compression and filtering prevent overdrive however this is only half the story so as to say.

Perhaps however you would like to explain your interpretation in more detail?

From Wikipedia

“Feedforward control is a term that has specific meaning within the field of CPU based Automatic Controls. The discipline of “feedforward controls” as it relates to modern, CPU based automatic controls is widely discussed, but is seldom practiced due to the difficulty and expense of developing or providing for the mathematical model required to facilitate this type of control.

“In feedforward control there is a coupling from the set point and/or from the disturbance directly to the control variable, that is, a coupling from an input signal to the control variable. The control variable adjustment is not error-based. In stead it is based on knowledge about the process in the form of a mathematical model of the process and knowledge about or measurements of the process disturbances.” From: Basic Dynamics and Control, Haugen, F, 2009”


Posts: 473

« Reply #24 on: April 07, 2012, 04:56:57 AM »

The first part of the problem can be trivially addressed in a DSP based exciter by noting that the envelope of any modulation scheme is trivially bounded by sqrt (I^2 + Q^2) at baseband.

Here is a band limited square wave (The pathological case for SSB): square wave.png 
Here is the calculated envelope by this method
And here is the calculated RF signal plotted on the same plot as the above calculated envelope:

Thus a limiter operating in the DSP on |Z| where Z is the analytic pair will suffice to limit the eventual RF output at any desired level.

There does need to be a feedback path returning at least VSWR, or more usefully a sample of peak drain voltage and current to the modulation DSP so it can adjust the limiter threshold level to keep the PA in safe operation, but extending this to an external amp while not particularly tough is not something the market expects (A power sampler head cabled back to the exciter).

There is also a potential issue with tube amplifiers here in that they can quite happily match into loads quite a long way from 50 ohms, where the power sampler would probably assume a 50 ohm match.

The takeaway from this is that if ALC is employed, the external amplifier should have as much ALC bandwidth as possible, so as to have minimum effect on the loop dynamics implemented by the exciter, certainly the ALC output should have a bandwidth at least or the same order as the modulation bandwidth.

Better would be for the amplifiers to provide a sample of the internal directional coupler output at a standard level on a pair of BNC or similar, if everything provided samples at say 0dBm = full rated power (or maximum rated reflected power), then the rig could use these samples to actually close the loop in a sane way with more or less fully known loop dynamics. 

73, Dan (Who is designing a homebrew radio using these methods, hence having the plots available from my development simulations).
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