Once upon a time the fairly obvious, cheap, and widely
available answer to your question was to use an integrated
circuit function generator / waveform generator such
as the ICL8038, XR2206, MAX038, or a handful of similar
devices that would output a fairly good sine wave for any
frequency between DC and around 100-200kHz for some
of them.
Nowdays, they've mostly been discontinued, though if
you search you can probably find some small electronics
distributor with a few in stock, or find some old kit
project that includes one for sale.
There is a NTE864 chip which is apparently available now
from Mouser for $30 or so per chip (maybe it's less elsewhere); apparently it's about the same as the old
ICL8038 chip. $30 seems like highway robbery to me,
since in the good old days the original parts of identical
function were under $3 in quantity one.
Your misunderstanding is that common oscillators oscillate
with sinusoidal oscillators and then convert that to a square
wave. Actually they usually oscillate with either a
triangle wave, saw tooth, or exponentially rising pulse
then convert that to a square wave based on the crossing
of a logic level threshold by that timing waveform. It's
uncommon to have access to anything BUT a square wave
for most oscillators. The 555 timer IC hooked up as an
astable multivibrator is a common example of using an
exponential RC time constant charging waveform followed
by a logic level comparator et. al. to generate an
square wave oscillator, though that doesn't help you.
I'd probably suggest going with the approach of
using a microcontroller and 10-12 bit DAC and
just outputting samples of a 100kHz sine wave with a
sample rate of something like 3 MHz - 10 Mhz for good
waveform fidelity and easy filtration. Then filter the
DAC output with a couple of stages of RC or LC lowpass
filtering. Use a dual op-amp so you can drive the first
op-amp with the DAC's RC filtered output, filter the
signal more on the output of that 1st op-amp, then use
the 2nd stage op-amp to generate the right output
amplitude / impedance / power you may want.
Even with all the cost of the uC, DAC, Op-Amp, PCB, you'll
spend less than the $30 one NTE chip will cost, and it'll
be a higher fidelity signal that you can adjust trivially
easily in frequency / waveform or whatever by just
reprogramming the sample table (use a simply in circuit
reprogrammable flash programmable uC like a PIC/AVR
chip of some appropriate kind).
I do recall that there are a couple of companies besides
NTE still offering integrated circuit function generator /
waveform generator parts which would make 100kHz
sine waves easily. However the parts and company names
don't come easily to mind for me now; if you search for
a while you could probably find one. Though you'll
still have to find a distributor with those in stock that will
sell a low quantity for a good price, and that's a bit harder
with more obscure parts than with mainstream ones.
Alternatively if you just need a fixed frequency without
such easily tunable options as a digital waveform
synthesizer will give you, and you can tolerate a bit of
temperature / mechanical instability of the frequency,
you could just build one of the op-amp based sine wave
oscillator circuits that've been discussed.
If you can settle for a frequency under say 20kHz you
could always just program a PC sound board to emit the
tone and get the signal from the headphone jack. :)
