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.
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.
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.
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.
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.
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.
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.
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.
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