Lars Harlin wrote:
>
>AG6K:
>> ? not according to DAF recommendations.
>
>SM3BDZ:
>And exactly WHAT is the DAF recommendations??
>
Only what G2DAF himself wrote (copy posted on my web site). All the rest
has come from other people, and G2DAF is not around to pass comment.
Actually, that's probably just as well for some people, because G2DAF
was a totally ruthless critic of SSB signal quality. Anyone who checked
into his 80m SSB Experimenters net "for a signal report" could expect no
mercy!
Earlier, Rich served an ace with his point that a linear amplifier
demands constant gain at all signal levels. Because the gain depends on
the 3/2 power of the screen voltage, as Rich said, the transfer
characteristic of the G2DAF-type amplifier cannot be a straight line. It
has to be S-shaped - starting with low gain at low signal levels (and
therefore low screen voltage), increasing for medium levels, and then
flattening off like any amplifier does if over-driven.
This might explain why the two-tone results from DAF-type amps can look
reasonable at high (but not excessive) drive levels. At lower drive
levels, the IMD relative to the output level could be significantly
worse. An oscilloscope pattern of the two-tone signal would presumably
show significant "pinching" of the crossovers as the gain drops... but
the output level is lower, so maybe it matters less? What this means for
a real-life speech signal is hard to guess.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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