🚀 go-pugleaf

In article <827@wrs.wrs.com>, roger@yuba.wrs.com (Roger Rohrbach) writes:
> I am building a Theremin (an electronic musical instrument) from a
> schematic published in a very old (1967) issue of "Popular Electronics".
> The schematic calls for "50-300 uH RF coils".

That *is* a very wide spread; I've usually seen ranges of like 2:1.
Considering the purpose, I'd guess that the article really meant "two variable
RF coils, maxing out somewhere between 50-300 uH"; you're not trying to tune
a particular wide range, you're trying to get two oscillators to match.
(Hmm, unless you're going to use a portable radio as a detector rather than
building a mixer, in which case you still don't need that wide a range)

> Does anyone know of a source,  or know
> enough about the topic to suggest a substitution (I have to tune two RF
> oscillators so that their difference frequency is audible)?

Is the detector part of the circuit, or do you use a separate radio?  If
so, what frequency, and how much capacitance is in the tuned circuit?

> While I'm at it, the design also used "polystyrene capacitors", rated in
> picofarads.  Can I substitute polypropylene, for which I have found a
> source?

Maybe yes, maybe no.  Polystyrene is fairly temperature stable, so as the
room warms and cools you don't have to re-tweak the coils.  Polypropylene
isn't (generally) stable, so the oscillators are going to drift a little,
and probably not together.  Other stable capacitor types are NP0 ceramic
(not just any ceramic, NP0 is a special type) and silver mica.  DigiKey
carries some NP0 ceramic disks.
-- 
John Woods, Charles River Data Systems, Framingham MA, (508) 626-1101
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