Vic K2VCO wrote:
>KB0NLY wrote:
>
>> ""Make up a resistance value close to this number with low inductance
>>resistors
>
>Make sure they are *really* low-inductance. Many modern film-type
>resistors are made by cutting a spiral groove in a conductive film.
A small amount of inductance won't matter in this situation, because the
resistance value (1-2k) will be at least 100x higher than the inductive
reactance.
Film resistors are manufactured by depositing a continuous cylinder of
carbon or metal film on a ceramic body. A range of different resistance
values is then created by cutting a spiral path around this cylinder.
More turns give a longer, narrower path and a higher resistance.
When the process has reached the maximum number of turns that can be cut
to give an accurate resistance value, the manufacturer will switch to a
higher-resistivity film material and start again from only 1-2 turns.
Thus you'll see a series of breakpoints between highest and lowest
inductance values, though even the maximum is not very high.
It is quite easy to make a good estimate of the inductance by scraping
the paint off, counting the turns, measuring the length and diameter of
the spiral, and then plugging those numbers into the standard handbook
formula. Small 0.25W resistors rarely use more than about 10 turns, so
the maximum possible inductance is only a few tens of nanohenries.
Unfortunately the locations of the breakpoints will depend on the
manufacturing process, so you can't depend on any particular resistance
value always having a very low inductance.
--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek
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