Ultra-Low-THD Sine Wave Generator

Ultra-Low THD Sine Wave Generator
Ultra-Low THD Sine Wave Generator front panel

This is a con­tin­u­a­tion of my Wien Bridge Oscillator mod­ule design project. It hous­es all the parts need­ed for a portable Ultra-Low THD dual-fre­quen­cy sine wave generator.

I had already designed and test­ed the oscil­la­tor mod­ules, but need­ed a way to pow­er and mount them. I had already decid­ed that I want­ed to pow­er the unit from bat­ter­ies so that it would be portable and iso­lat­ed from ground.

Dual out­put reg­u­la­tor board designed in EagleCAD V7.7

I designed a board based on the LT3032 Dual 150mA Positive/Negative Low Noise Low Dropout Linear Regulator. It sup­plies +/- 15 volts at up to 150mA with a typ­i­cal 300mV dropout volt­age. Another key fea­ture of the LT3032 is low out­put noise. It also has shut­down pins for turn­ing off each of the out­put reg­u­la­tors with a shut­down cur­rent of less than 3µA which is per­fect for bat­tery operation.

Testing the Dual reg­u­la­tor pow­er sup­ply board at 60 mA per output

For the sup­ply I decid­ed to use 24 AA bat­ter­ies (12 pos­i­tive and 12 neg­a­tive) which gave me 18.7 volts for a fresh set of cells, and a low volt­age cut­off of 1.27 volts per cell.
The bat­ter­ies are con­nect­ed direct­ly to the reg­u­la­tor board, and the boards out­puts are turned on and off using the shut­down pins.

Front pan­el of the Ultra-Low-THD Sine Wave Generator from Front Panel Express

The next step was to design the front pan­el.
I used the same style Hammond 1455N1601 alu­minum box that I had used pre­vi­ous­ly, but this time in an anodized black fin­ish. With 24 AA bat­ter­ies I was not going to have a lot of spare room, so I used some obso­lete Bournes Knobpot’s that I had a quan­ti­ty of in my spares cab­i­net. Knobpot’s are an inter­est­ing poten­tiome­ter in that all of the mechan­i­cal com­po­nents are locat­ed out­side the pan­el, with only three ter­mi­nals and the mount­ing thread and nut on the back side sav­ing con­sid­er­able room.

Boards mount­ed in Ultra-Low-THD Sine Wave Generator box and ready for wiring

For my needs I used a fixed fre­quen­cy 1,000 Hz and a 400 Hz sine wave board for my project. Power is rout­ed to the board in use by a three pole dou­ble throw switch with cen­ter off posi­tion C&K # 7303K2ZQE.
In the switch­es cen­ter posi­tion the enable sig­nal is removed from the reg­u­la­tor board dis­abling the pow­er sup­ply.
I also tied a LED to the indi­vid­ual boards neg­a­tive sup­ply to indi­cate which out­put is active on the front panel.

Test mount­ing the front pan­el with unat­tached wires and bat­ter­ies installed

I indi­vid­u­al­ly shield­ed the knobpot poten­tiome­ter cables from the front pan­el to the cir­cuit board con­nec­tor, which most like­ly was­n’t nec­es­sary with the short run and met­al box. Probably the hard­est part was con­nect­ing the indi­vid­ual wires to the board con­nec­tion pads. Pre-wiring the switch made the instal­la­tion a bit eas­i­er.
I used some nylon stand­of­f’s to keep the bat­ter­ies from shift­ing posi­tion dur­ing use, which still allows the bat­tery packs to be removed from the back panel.

All com­po­nents mount­ed with cables attached and ready to test

Originally I had planned on using some high qual­i­ty RCA audio jacks, but I opt­ed for 3/4″ spaced banana jacks on the front pan­el.
Another option would be BNC jacks, which would also work well. My thought in going with the banana jacks was that I already had a var­ied col­lec­tion of banana jack adapters to con­vert over to oth­er connectors.

Ultra-Low-THD Sine Wave Generator with slid­ing top cov­er removed

Calibration of both gen­er­a­tor boards was next.
With the “V Adjust” knob set to 10, I adjust­ed the “Negative Feedback” trim-pot R1 on each board for an out­put of 10 volts on the banana jack con­nec­tors.
Next I adjust­ed the fre­quen­cy of each board after a warm-up peri­od using R2 and R3. If I need­ed to adjust R2 clock­wise 1/2 turn, then I would also adjust R3 the same 1/2 turn except in the counter-clock­wise direc­tion. This keeps the resis­tance of R2 and R3 near­ly the same and in balance.

Finished Ultra-Low-THD Sine Wave Generator

I am con­sid­er­ing build­ing an expand­ed ver­sion of this “Sine Wave Generator” at some future time that will house 8 to 10 oscil­la­tor boards, and use a rotary switch to patch each board to a com­mon out­put connector. 

OSHpark board fab­ri­ca­tion project page

EagleCAD V7.7 Dual Output Regulator board and schemat­ic ZIP file

Eagle CAD V7.7 Oscillator HPV 400 Hz board and schemat­ic ZIP file

2 Replies to “Ultra-Low-THD Sine Wave Generator”

  1. You are describ­ing this design as ultra low THD but you don’t actu­al­ly list mea­sured THD any­where in the descrip­tion. So, what’s the point?

    1. Hi esk3578,
      The mea­sured THD was less than the capa­bil­i­ties of my Keithley 2015 THD Multimeter, which is around 0.004% or –87 dB.
      I would need a bet­ter mea­sure­ment device to actu­al­ly give a spe­cif­ic mea­sured THD value.
      Wien Bridge Oscillator
      If some­one would like to loan me (or gift me) a piece of test equip­ment, I would be more than hap­py to spec­i­fy a mea­sure­ment value.

      Greg (Barbouri)

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