Wien Bridge Oscillator Design

Wien Bridge Oscillator  HPV testing for THD
Wien Bridge Oscillator HPV test­ing for THD

I found myself in need of a very low-dis­tor­tion 1,000 Hz sine wave
gen­er­a­tor for an upcom­ing project.
A quick search on Ebay found sev­er­al used pieces of test equip­ment that met or exceed­ed my require­ments, but were priced at well over $1,000 USD.
I did also find some low-cost China sourced 1 kHz gen­er­a­tor boards, but they had a dis­tor­tion of 0.1% which was way above my 0.005% lim­it.
My cur­rent test gear includes a Keithley 2015 THD Multimeter with a low dis­tor­tion sine wave gen­er­a­tor, but it is only capa­ble of 0.03% THD at 1 kHz.
It was time to see if I could build some­thing for under $100 that would meet my needs.

After break­ing out some of my old audio design books, I noticed a design for a Wien Bridge oscil­la­tor that used diodes to con­trol the sig­nal ampli­tude. After a bit more research I found a myr­i­ad of ways to con­trol ampli­tude and reduce dis­tor­tion.
A method described by Larned A. Meacham in 1937, using a fil­a­ment lamp for auto­mat­ic gain con­trol was used by William Hewlett, and David Packard, and found its way into the HP200A audio sig­nal gen­er­a­tor, the first prod­uct of Hewlett-Packard.
So I decid­ed to go down the Wien Bridge Oscillator rab­bit hole.

My first bread­board pro­to­type used a dual low-noise JFET-Input TL072 oper­a­tional ampli­fi­er and a 12 volt auto­mo­tive light bulb. The THD was around 0.75% and the bread­board lay­out was con­tribut­ing to some of that. A print­ed cir­cuit board for the TL072 along with some dif­fer­ent pas­sive com­po­nents and a 14 volt fil­a­ment bulb, got me down under 0.18% THD.

Prototype boards using the Texas Instruments OPA16XX series oper­a­tional ampli­fiers

While research­ing low-noise op-amps, I stum­bled across the Texas Instruments OPA16XX series of SoundPlus oper­a­tional ampli­fiers. These op-amps had some very impres­sive spec­i­fi­ca­tions, includ­ing a low of 1.1‑nV/√Hz noise den­si­ty with a dis­tor­tion of 0.000015% at 1 kHz for the OPA1612.
I designed and ordered a set of boards that would accom­mo­date some dif­fer­ent sized pas­sive com­po­nents, along with an OPA1612A and a cou­ple of OPA1662 op-amps.

Circuit board test­ing using dif­fer­ent pas­sive com­po­nents and bulb con­fig­u­ra­tions

After exper­i­ment­ing with sev­er­al com­bi­na­tions of op-amps, lamps, and pas­sive com­po­nents, the board with a com­bi­na­tion of the OPA1612A along with low-noise pre­ci­sion thin-film resis­tors, and film capac­i­tors pro­duced the low­est THD read­ings. They were so low that they exceed­ed the capa­bil­i­ties of my Keithley 2015 THD Multimeter.

0.000 THD reading on Keithley 2015 THD Multimeter
0.000 THD read­ing on Keithley 2015 THD Multimeter. upper har­mon­ics 3, fil­ter 30 read­ings

The cir­cuit uses the series and par­al­lel RC pairs along with two 14 volt fil­a­ment light bulbs in L1 and L2 posi­tions. I includ­ed trim­mers in both the series and par­al­lel cir­cuits and anoth­er trim­mer for the neg­a­tive feed­back adjust­ment.
The sec­ond sec­tion of the op-amp is set for a gain of 10, and the out­put ampli­tude is adjust­ed using VR4 a 10K ohm vari­able resis­tor that can be exter­nal­ly mount­ed.
The cir­cuit uses an exter­nal low noise pow­er sup­ply of +15 and ‑15 volts, and includes addi­tion­al fil­ter and de-cou­pling capac­i­tors on board.

Wien Bridge Oscillator HPV 1.1 schemat­ic with oscil­la­tor fre­quen­cy cal­cu­la­tion for­mu­la, and notes

The two 14 volt fil­a­ment bulbs are con­nect­ed in series to effec­tive­ly make a 28 volt bulb with a large fil­a­ment mass. I was ini­tial­ly wor­ried that dou­bling the bulbs would cre­ate more micro­phon­ics from vibra­tion of the fil­a­ments, but the effects of micro­phon­ics are almost non exis­tent.

The cir­cuit worked very well for my project, and I am now work­ing on a 400 Hz ver­sion of the board.
Eagle CAD V7.7 Oscillator HPV 1.1 board and schemat­ic ZIP file
BOM Parts list for 1 kHz HPV 1.1

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