Two Tone Audio Generator

This is a device that generates two stable audio tones.  It is handy for testing the intermodulation of a SSB transmitter. 

YouTube episodes can be found at these links:

Episode #1

Episode #2

Before building your version of this project, you need to download the ARRL test procedures manual from the ARRL website.  This project starts on page 140.   There’s some technical information in their writeup that I am not repeating here.

See below for my version of the schematic.  All symbol references made below refer to it.

The ARRL design specified using polystyrene or mylar caps in the filter networks, likely for temperature stability.  This would be for capacitors C1 through C14.  I used C0G/NP0 grade MLCC’s instead.  The C0G material is plenty stable in this application.

I could not find the 25ma 12V lamps they specify, so I went with 40mA 12V lamps.  I added spots for series resistors R1 and R2.  As it turned out, they weren’t needed so I populated those spots with zero ohm jumpers.  I did have to adjust the two trim pots RV1 and RV2 to get 1 VRMS at pin #7 of the oscillator outputs, not 0.5 VRMS, to get stable outputs.

U1 is the lower frequency (~700Hz) oscillator, U2 is the higher frequency (~1.8kHz) oscillator.  They are on (working) unless nodes H1 and H2 are grounded.

H3 (LOW_OUT) and H8 (LOW_IN) are connected using a 5k potentiometer.  This control adjusts the balance between the two oscillators.

H4 and H5 are connected using the 1k potentiometer.  This control adjusts the output level.

H6 and H7 are connected using the attenuator.  See the ARRL schematic for its construction.

Here are a few special components that I used (along with DigiKey part numbers):

  • 12V 40mA lamps (289-1216-ND)
  • 500 ohm trimmer pot (3362P-501LF-ND)
  • TL082 in SOIC8 package (296-1284-1-ND)
  • 5kOhm balance potentiometer (PDB181-E420K-502B)
  • 1kOhm level potentiometer (PDB181-E420K-102B)
  • Single pole, 6 throw attenuator rotary switch (CKN11064-ND)

Lastly, I am also including the 3D .stl model of the dual 9V battery holder that I created.  You will need to 3D print two of these.

Here are links to the project files.  Enjoy!

Schematic Diagram

KiCAD Project Files

Dual 9V Battery .stl File

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