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Jan 3, 2025:
Revision: v1.1

An Accurate Two-tone Digital Audio Oscillator

My "DiTA" digital 1kHz sinewave audio tone generator was published in the July 2024 issue of UK's Practical Wireless magazine. This latest version, described in February 2025 issue of Practical Wireless, adds three selectable audio tone pairs for two-tone transmitter testing.

Introduction

Shortly after completing the precise 1kHz tone generator (Practical Wireless June 2024), my friend Keith (ZL4JA) asked me if I had an audio two-tone oscillator he could borrow. A two-tone audio oscillator (or two audio oscillators and an audio combiner) is used to test the linearity of SSB transmitters.

In this type of test equipment, a pair of oscillators output two equal amplitude non-harmonically related audio frequencies. These are combined and then used to modulate the SSB transmitter.

I did not have such an oscillator. I typically just grabbed a pair of audio oscillators off my test equipment shelf instead. However, after some thought, I found I was able to squeeze this additional software functionality into the original 1kHz oscillator design hardware. A zero-cost upgrade. What's not to like about that?!

The full background and technical description of this design is available in the February 2025 issue of Practical Wireless magazine.

Summary of the Changes

Using a single 8-pin microcontroller chip and a few passive parts, this compact oscillator produces 1kHz or any one of three pairs of two-tone outputs. It measures just 1” x 1.33” (25 x 34mm). The tones are very accurate and have minimal distortion. The unit draws less than 20mA from a 3 – 5VDC supply.

The schematic for this expanded audio test oscillator is identical to that of the original June 2024 version. One jumper now selects either the single 1kHz tone of the original design or a 665/2250Hz two-tone output. Adding a second jumper to the pads available on the original PCB allows the selection of either of two further two-tone pairs (700/1900 or 700/2300Hz) if desired.

The February 2025 article also described an (optional) low pass filter and an audio output connector which may be preferred by some when the oscillator output is directly connected to the transmitter or transceiver. A new PCB was designed for those wanting to use this slightly revised circuit. However, the original PCB is 100% compatible with the new software.

Building the new Prototype

I could (and did) just use the upgraded software with the original oscillator hardware for quite some time. After two or three months, however, I decided to built a new prototype for the revised oscillator in a new box. You can see it in Figure 2. I then loaded the original 'DiTA' software back into the original June 2024 oscillator hardware. Why? Well, that 1kHz oscillator is used for a whole raft of general testing around the bench. I found it really handy to just reach for one or the other. And having two oscillators is always better than just one!

To minimise the size of the upgraded oscillator with its integrated audio output connector, I provided access to the two tone selection jumpers through a hole in the front panel. In hindsight, this was probably not ideal. I think a pair of DIP switches might have been a better arrangement.

I also used a single AAA battery to power this tiny device. I designed an integrated battery holder into the new 3D-printed box, and then added a tiny SMD boost converter to power the prototype PCB at 5V. I added a tiny slide switch and an LED so I could see when power was turned on. The original oscillator didn't need this because you can always hear it while it's running!


Figure 2 : Although I originally just upgraded my 'DiTA' prototype with the new software, I later built a new version for this oscillator software. I designed a tiny 3D-printed box for the PCB, an AAA alkaline battery, and a tiny boost converter.

All of the details of the design can be found in June 2024 and February 2025 issues of Practical Wireless. For those interested in duplicating the design, the software and PCB details are provided below for download.

Downloads

All of the following material is contained in a single ZIP file which you can download by clicking this text.

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