Ian:
Your sequencing is logically correct, and in fact is exactly how most
commercial/military gear is designed. However, this is executed without
ALC. Instead, an TX Inhibit signal is used. They are not one in the
same. An ALC has very fast attack, and therefore can be used to mute RF
output from the exciter. Problem with ALC is that the recovery back to
transmit in most ALC ckts is very slow, and takes several seconds. It;'s
not even fast enough for casual VOX on SSB.
An inhibit signal in fast in both make/break conditions, and usually
involves gain loop circuitry from the exciter to amplifier, as a unified
system.
All the best,
Vince
WA2PKE
-----------------------------------
Ian White, G3SEK wrote:
>
> W8JI wrote:
> >
> >ALC will not cure the problem, because ALC is traditionally too slow
> >to limit leading edges of waveforms.
> >
> >Also be sure the radio actually keys the PA before RF is applied, and
> >removes RF before the keying signal is removed. I've seen radios
> >output RF long after the PA is unkeyed!
> >
>
> As a side-issue, has anybody experimented with TX/RX sequencing that
> sends a TX inhibit signal back to the exciter ALC input?
>
> The sequence would be something like this.
>
> RX>>>TX:
>
> 1. RX state: PA is unkeyed, and is sending a negative TX-inhibit voltage
> to exciter ALC input
>
> 2. Exciter keys PA: TX inhibit voltage maintained while PA antenna
> relays are changing over.
>
> 3. PA antenna relays switched and settled: PA removes inhibit voltage
> from ALC line and exciter can now deliver RF. (Normal ALC operation is
> now possible if desired.)
>
> TX>>>RX:
>
> 1. TX unkeys PA, but PA antenna relays hold in. PA is quickly biased to
> cutoff, and quickly sends inhibit signal back to TX.
>
> 2. PA antanna relays release. System is now in RX state.
>
> I realise this won't be exactly QSK-speed, but is the idea workable -
> or did I just reinvent the square wheel?
>
> 73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
> 'In Practice' columnist for RadCom (RSGB)
>
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