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.
These files include:
- Upgraded ATtiny85 program software, and
- PCB Gerber files and a PDF and JPG image file of the SINGLE-SIDED PCB layout
By downloading these files, you agree that these files are for your personal non-commercial use.
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