Sporadic Reflections

19 December 2006

Twin-T Oscillator

The Twin-T Oscillator is a form of Sine Wave Oscillator with
an interesting topology. The Twin-T Oscillator uses two
"T" RC circuits operated in parallel. One circuit is an R-C-R "T"
which acts as a low-pass filter. The second circuit is a C-R-C "T"
which operates as a high-pass filter. The result is a much cleaner
sine wave than the typical 555 based relaxation oscillator.

Some typical circuits:
A Phase Shift Code Practice Oscillator from qrp.pops.net
The MorseBox from VK3YE
Discover Circuits Twin-T Oscillator PDF
Twin-T Audio Oscillator from G0XAN
Twin-T Audio Sine Wave Oscillator from Art's Theremin Page

An Audio Oscillator for Transmitting
MCW
is a project that uses a Twin-T oscillator for modulating
a 2 Meter HT with Modulated CW. This enables Technician
license holders to practice Morse Code even if they do not have
an HF rig.

There are at least two commercial kits that use a Twin-T oscillator:
The Jackson Harbor Shaped CPO kit - Schematic PDF
The Morse Express MX T-Tone Code Practice Oscillator Kit

The Morse Express page for the kit has two nice Morse samples of the CPO output.

18 December 2006

The Antikythera Mechanism

The Antikythera Mechanism is a 2000 year old
device discovered by sponge divers near a Greek island.
The best guess is that it is a mechanical
analog computer that can calculate the position
of the sun, moon and planets
.

By a weird synchronicity, I first learned of it
on Serendipita, a blog I occasionally visit.
The next day it was mentioned on SolderSmoke #44.

08 December 2006

Wien Bridge (HP 200a)

While doing research on the Twin-T oscillator, I came across
this fascinating tidbit. Hewlett-Packard's first product was a
sine-wave audio oscillator (the HP200A) based on William Hewlett's
1939 Stanford University master's degree thesis.

Hewlett's inovation was to use an incadescent bulb as
a variable resistor to stabilize the output level.

ARRL Code Practice Oscillator and Distribution Amplifier
using a Wien Bridge [PDF]

WikiPedia Wien Bridge article
HP Virtual Museum: Model 200A audio oscillator
Art's theremin Page: The Wien Bridge theremin