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Re: [Amps] drive power

To: "Win" <>, <>
Subject: Re: [Amps] drive power
From: "Tom W8JI" <>
Date: Tue, 20 Mar 2007 00:51:51 -0400
List-post: <>
>I hate to come back to this subject, but I am not sure I 
>understand the
> problem.  Is the overshoot a product of the ALC outputted 
> from the exciter
> to the amp, or is it caused within the transceiver at 
> significantly reduced
> power settings.

It's largely an internal function of the radio. All radios 
have a finite time before ALC activates. Nearly all radios 
use the ALC as a power control. Some radios are so slow the 
ALC cannot respond faster than the RF output waveform rises.

Think of it like this. The ALC is derrived from the very 
output of the radio, right where the SWR is and power output 
is measured. It takes time for a signal to get through the 
transmitter to the antenna port. It also takes time to 
charge the capacitors in the ALC detector, and then that 
voltage has to exceed the input of a circuit that compares 
the detected voltage to a reference voltage. That 
comparision is what determines the power output limit, or 
when the ALC kicks in.

When the ALC does kick in, it has to rbe applied to an 
earlier stage where gain is reduced. Again it has to charge 
up a few capacitors to start the gain reduction process.

It's easy to see the actual start of gain reduction by ALC 
is always slightly behind the leading edge of the rising 

If the ALC is fast enough compared to the rise time there 
will be almost no overshoot. All it has to do is be able to 
apply the control faster than the time of the envelope rise. 
With a 3000Hz wide SSB filter that could be faster than 
1/6000 of a second. This is because the rising edge is what 
matters, and the rising edge is only half of the full 
sinewave period. The ALC has to do its control thing faster 
than .000167 seconds.

What screws up many (if not most) radios is they run wide 
open gain and use the ALC exclusively to limit power by 
reducing gain. So the radio has a short period of full gain. 
If you set the power level at 100w it has full gain, and 
some stage saturates to limit power. This might be at 150 
watts on a 100W radio. Now if you turn the power back to 30 
watts the radio still goes up almost to the same peak power, 
then it very quickly folds back to 30 watts.

Some radios even take a millisecond or more to respond.

If the radio has a manual transmitter IF section gain 
control you can reduce drive manually so the ALC is just 
barely limiting power. Then you will barely have any 
overshoot because even without ALC the gain is low and the 
power output is low.

> In either case I should be able to see the overshoot if I 
> put a scope across
> the meter in my power meter.  Is that correct?.

Not unless you externally trigger the scope and have a 
storage scope. It is a common misconception that we can use 
a scope to catch that peak, and ideed we can sometimes if it 
is a repeating waveform and we trigger the scope at the 
correct time on a repeating waveform with a long gap between 
starts (so the ALC falls back to zero). But in nearly all 
cases a regular scope will miss the transient. If the 
transient is 0.5 ms long you would only have 1/2000th of a 
second to catch the peak. The detecting device would have to 
hold that waveform so you could actually see it.

This is why a very fast peak hold meter is best.

> I currently reduce my FT-1000MKV from 200 to 50 watts, 
> when driving my old
> SB-220 amp on CW.

That probably isn't the best idea. While the 1000MKV has a 
pretty fast ALC, it still messes up the leading edge of the 
CW waveform by abruptly squaring it off. What I do with my 
MKV FT1000MP when driving a single 8877  is reduce the TX IF 
gain in the hidden menu so I'm just able to show full bar 
ALC at 75 watts.  Then when I close the key the waveform 
leading edge isn't squared off so much and key closure 
clicks aren't so bad. On SSB you can just reduce the mic 
gain as well as the power setting.

The bottom of this page tells you how to get into that menu:

You have to set the IF gain band by band, so it's a PITA if 
you drive high drive and low drive amps with the same rig. 
You'd have to slightly modify the procedure to set the power 
control at maximum drive for the SB220, say 100 watts.

Lots of the blame for "parasitics" actually comes from these 
transients, as does some of the blame for adjacent frequency 
pops and spits on SSB.

73 Tom 

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