Colin wrote:
>
>Question: If the G2DAF circuit distorts because it draws too much grid
>current, then how could it a 2 tone test?
>
"Too much grid current" seems to be a minor (or minority) aspect of the
doubts about the G2DAF. The major doubt is the varying screen voltage,
leading to gain that varies with drive level.
It would be interesting if Hans could produce a table of output power
versus input power, across the whole range of drive level but
concentrating the detail in the top and bottom ends.
>Assuming it passes two tone testing, that would mean that even if the
>G2DAF draws substantial grid current, it would not matter. Thus,
>passing a 2 tone test would mean that the design is not flawed because
>of excessive grid current.
>
>The same argument about 2 tone testing addressed to the G2DAF circuit
>applies to all amplifiers - it is simply one test which does not tell
>the whole story.
>
Two tone testing conventionally takes place at maximum rated PEP,
because with ordinary amplifiers you expect the linearity to improve at
lower drive levels. The suspected problems with the DAF are that the
linearity gets worse at low drive levels, so a full-power two tone test
doesn't tell the whole story.
(Please note: I keep saying "doubts" and "suspected", because I'm not
sure how distortion at low drive levels affects the practical adjacent-
channel IMD performance with speech signals.)
>Once you argue that the cause for distortion is excessive grid current,
>then you must accept succesful plots of 2 tone tests as proof that
>either the G2DAF circuitry does not have excessive grid current or that
>it does not matter.
The control grid of the DAF design thinks it's in class B, zero bias -
starts to draw grid current at any drive level. In itself, that needn't
be a major problem for linearity. The real complications come when you
add in the effects of varying screen voltage.
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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