Re: [Amps] Alpha 87A troubles with fault 17

["Ian White, G3SEK" <G3SEK@ifwtech.co.uk>]

Paul Christensen, Esq. wrote:
> > > 850 ,870 ......They ALL have overshoot on the leading edge big time
> > > ......The Yaesu 1000MP series and FT-990 do not ..
>
> However, the FT-1000 genre of transceivers address the leading edge
> overshoot issue through the use of a fast ALC response time.  What's the
> negative side-effect?  CW clicks due to an unusually abrupt and fast leading
> edge wave-form.
The FT-1000 key clicks are not a side-effect of solving the ALC 
overshoot problem. When the overshoot problem was cured, there was still 
a *separate* key-click problem, that now has to be dealt with by 
aftermarket modifications.

> And the problem is not limited to CW, ALC overshoot is a problem on SSB,
> particularly at power levels at less than 100% of the radio's capabilities.
> Some radios allow the operator to independently adjust drive level, and this
> certainly helps but it is not a perfect solution.

The problem is that these so-called "drive" controls are actually 
operating through the ALC loop. The full drive capability is still 
there, so when you turn your 100W rig down to 5W, it's still capable of 
100W (or more). The rig is continuously relying on the ALC to hold the 
output level down, from each millisecond to the next... and it doesn't 
completely work.

This is not surprising, because ALC is a reactive system. There *has* to 
be some initial spike of excess power, because that's what the ALC 
reacts to. The ALC cannot begin to close the stable door until *after* 
it sees the horse's nose coming out!

A true drive level control should not be reactive. It should be a simple 
manual attenuator between the exciter stages and the PA - just like the 
RX audio gain control really. So when you turn down the drive level, 
full power output is completely impossible, not even for an instant; too 
much drive is simply not available.

Ideally a true drive level control should be directly calibrated in 
watts output, and that should be exactly the PEP output you get under 
all circumstances.

One of the difficulties with that system is that the overall gain 
through an exciter (and hence the available output power) will vary with 
production spreads of device gains, filter losses etc. In each 
individual rig the overall gain will continue to vary with factors like 
supply voltage, temperature, band changes and ageing.

The reason why transceiver manufacturers like ALC is that it's a cheap 
fix which largely compensates for all of those variations by using a 
closed control loop. Except that it doesn't quite work...

Modern transceivers *could* be smarter than that. Many already have 
digitally preset RX IF gain, so they should be capable of making preset 
adjustments to the TX IF gain as well. The initial goal is that the 
power output calibration of the drive control is correct on all bands 
and modes. That initial setup should be done at the factory and 
committed to memory - but not for all time. The rig should also be 
capable of checking itself afterwards, to compensate for long-term 
changes. In many transceivers, the necessary hardware is already there.

There is still a role for ALC, perhaps, but only as a backstop to catch 
any gross peaks of the input modulation on SSB and AM. The main drive 
level adjustment should always be done by a real manual gain control.



--
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                           Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek
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