>
>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.
>
? good point, Ian. Therein lies the sticky wicket. Hows about using a
100F screen bypass C in the DAF amp? Whistle it up and talk for ten
minutes.
>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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>
- Rich..., 805.386.3734, www.vcnet.com/measures.
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