Rich Measures wrote:
>>GW4FRX's measurements quoted in my article (October QEX) show that a
>>zener string emphatically does NOT "work best" from the viewpoint of
>>IMD!
>
>Volts are volts. It matters not what type of regulator they come from,
>unless the regulator is performing poorly.
You surely don't mean that, Rich. Under conditions of varying current
demand, some types of voltage regulators obviously have a lower output
impedance than others.
If you look at the datasheets, you will see that a string of high-
voltage zeners cannot have a output impedance less than a few hundred
ohms - a load pull will confirm this. A few hundred ohms is big enough
to have a very noticeable effect on the IMD.
For the benefit of people who haven't seen the QEX article, the diagram
(which is also on my web pages) shows three sets of IMD plots. The two-
tone measurements were made by GW4FRX using a pair of 4CX250Rs at 500W
PEP output. The best IMD was obtained using an electronically regulated
screen supply with a very low output impedance. Second-best IMD was with
the same screen supply, but with its output impedance deliberately
raised by a 150-ohm series resistor. You can see a clear difference in
IMD levels.
Worst was a string of zeners, and that is exactly what you'd expect from
the known output impedance. VR tubes would be very similar.
>>
>>> For larger tetrodes, a 723-based positive
>>>floating series pass regulator has advantages. You can find both types at:
>>>http://www.vcnet.com/measures
>>
>>But note also that a series regulator cannot handle reverse screen
>>current, which many tetrodes can generate. You MUST use a shunt resistor
>>from screen to cathode, that permanently passes the maximum negative
>>screen current that the tetrode can generate. ...snip...
>
>True indeed, Mr. White. But note that essentially the same information
>can be found in *Amplifiers* at the above URL.
(Yes, but in a different file. You might add the bleeder resistor to the
schematic as well.)
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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