Wien-Bridge Theremin Joseph Ballerini Matthew Newberg Jason Wright - - PowerPoint PPT Presentation

Wien-Bridge Theremin

Joseph Ballerini Matthew Newberg Jason Wright

Overview

• A theremin is an early

electronic musical instrument invented by Russian inventor Léon Theremin in 1928.

• The performer moves his or

her hands next to antennae to change the volume and/or frequency of the output signal.

• Most commercially available

theremins cost upwards of $400.

• A Wien bridge theremin is

easier to manufacture, and does not use inductors

Challenging Specifications

• Theremin is a very precise instrument
• Important specifications

– Effective calibration without affecting oscillation stability – Audible output frequencies – Low signal distortion – Playable with minimal effort (antenna works well)

Top Level Diagram

• Wien-bridge oscillators change frequency based on

calibration, hand position

• Mixer/filter uses heterodyning to output the

frequency difference between the oscillators

Top Level Analysis

• Oscillation frequency is

determined by component values, which antenna changes

• Heterodyning produces
• utput signal with

frequency components relating to sum and difference of inputs – higher frequency is filtered out 𝑔 = 1 2𝜌𝑆𝐷

sin 𝜄 sin 𝜒 = 1 2 cos 𝜄 − 𝜒 − 1 2 cos⁡(𝜄 + 𝜒)

Wien-Bridge Oscillator

• Noise is amplified by

the op-amp

• Loop gain stabilizes

at unity

• Key is automatic gain

control – originally done by thermistor, but many methods

𝑔 = 1 2𝜌𝑆𝐷 𝑆𝑔 𝑆𝑚 = 2

Theremin Version

• This example uses a

JFET and half-wave detector

• Presence of antenna

introduces hand- antenna capacitance (1-2 pF) and antenna-ground capacitance (~15 pF)

• Can also stabilize

with diodes

Mixer/Filter

Top-Level Circuit

1000 2000 3000 4000 5000 6000 7000 8000 10 11 12 13 14 15 Output Frequency (Hz) Capacitance (pF)

Audio_out vs. Antenna Capacitance

Performance Summary

• Simulation demonstrates stable oscillation across a

range of capacitor values

• Quality sinusoids were the largest challenge

– Mixer needs to be able to handle differences in amplitude – Minor problems in oscillation due to circuit noise requirements – Start-up time

Performance Summary

13 pF 15 pF 5.5 kHz 7.5 kHz

Layout

Mixer/Filter

(1400x900)

Variable Oscillator

(550x790)

Local Oscillator

(790x550)

Large capacitors for low- pass filtering Small transistors

Conclusions

• Wien bridge oscillators provide stable

sinusoids with low distortion, without the need to use an inductor.

• Transistor-based heterodyning can produce

reasonable, audible output given reasonable antenna capacitances.