> In fact are there *ANY* real crystals much above 40MHz, or are they
> mechanical overtones (confusingly not integer related to the
> "fundamental" mode frequency in the sense that frequency multiplier of
> an electronic frequency, or selecting a harmonic of a non-perfect
> sinewave is)?
>
> I think a 101.5MHz crystal I have will give approximately 1/5th
> frequency (but actually 1/5th) in a basic oscillator circuit rather that
> one designed for 5th overtone.

I think your confusion is aided by a lack of understanding of series and 
parallel modes of crystals, along with operation in overtone mode.

At the fundamental frequency there are two slightly different fundamental 
frequencies, one being the parallel mode, and the other series mode. Most 
microprocessors use the crystal in parallel mode, and crystals used in this 
mode always have a specified parallel capacitance (typically around 20-30pF, 
but may be lower or higher), at which the calibrated frequency is set.

The other mode is typically used in other oscillator modes, and has no 
capacitance specification. The original clock chip used for the 8080 
microprocessor used a series mode crystal IIRC, but this was not common 
among microprocessors.

Overtone mode always uses the series mode of the crystal, and requires some 
form of external frequency control circuit to stop the crystal operating at 
the incorrect overtone. This external control circuit is typically a series 
tuned circuit at the nominal frequency of the crystal at the selected 
overtone mode. Overtones are always an odd integer multiple of the 
fundamental frequency. This is a function of the way in which the crystal 
physically vibrates during oscillation.

It is possible to have a crystal oscillate at a non-integer multiple of its 
fundamental frequency, but the frequency is reliant on oddball modes of the 
crystal, and is not reliable. 

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