ZL2PD DDS Variable Frequency Oscillator (VFO)
HF DDS-based VFO with its compact graphics-capable LCD display
was published in Silicon Chip, the Australian-based electronics
magazine in March 2008. It uses an Analog Devices AD9850 DDS IC which
is controlled by an Atmel AT89C4051 microprocessor. All of the software
can be downloaded from here for free (Go to the bottom of this page)
No schematics are shown here. Copyright is
held by Silicon Chip.
Direct Digital Synthesis (DDS) HF
VFO is designed for use with amateur radio equipment in the 1.8 to 30
MHz high frequency (HF) bands. It uses an Atmel 89C4051
microprocessor to drive an Analog Devices AD9850 DDS
chip and a compact
graphics display. The display uses the monochrome LCD which is used in
entry-level Nokia 3310 cellular phones. The
software does not use any special features of the Atmel chip and it can
readily modified for use with other 8051 microprocessors.
The LCD displays a digital readout of the DDS
VFO frequency to a resolution of
10 Hz as well as an analog-type display which scrolls left and right as
the tuning control is rotated in either direction. The VFO supports
tuning at slow, medium and fast rates, two independent VFOs (VFO A and
VFO B), a
fixed transmit/receive frequency offset to cater for the usual IF
offset requirements, and receiver incremental
tuning (RIT). The fine tuning range of the RIT control is +/- 5 kHz. A
'lock' button is also provided to prevent accidental tuning of the VFO.
The original article can be found in the relevant issue of Silicon
Chip, and is available for download on the Silicon Chip
website, for a
This is the transceiver in which I use this
DDS VFO (Right
click with your mouse to get a closer view)
What's on this Page?
webpage contains software for the DDS VFO published in Silicon Chip
magazine. It also contains some answers to some frequently asked
questions. There is also a link provided at the foot of this page to a
PDF file which provides some additional information on modifications
for other IF offsets.
At the present time, I am not able to display the circuit diagram or
PCB layouts. Copyright for the circuit schematic, PCB layout and
related material is held by Silicon Chip and can be obtained from their
for the DDS VFO may be downloaded here. (See below) The free
software here includes HEX files (for direct programming of the
microprocessor), and the full source code, the latter for those wanting
to see how the software was written or for those wanting to modify the
software for specific applications.
Some additional project related information, such as a few extra photos
and drawings, have also been provided here, with the permission of the
editor of Silicon Chip magazine, to help those interested in building
Q1. How - precisely - did you mount your
VFO on the front panel?
The photo below provides another (top-down) view of how I mounted the
prototype VFO in my transceiver. While other methods no doubt exist,
this approach worked really well for me, and it permitted me to 'fine
tune' the spacing of the buttons from the plastic panel. Right click on
the photo to get a better view.
View from above the transceiver
showing the DDS VFO mounted behind
the black plastic front panel of the enclosure (The other transceiver
modules have not yet been fitted in the enclosure)
Q2. How did you cut the Nokia LCD
display away from the keyboard/display assembly?
Figure 2 shows how I trimmed my display. I've done this quite a few
times now. Use a sharp knife, a pair of sharp sidecutters and take your
time. The oddly shaped solid black lines on the left hand side of the
figure which show where to trim actually connect convenient holes in
the plastic. These make it easier to trim it to the required size. I
also used a small modeller's hacksaw to make the straight cut on the
right hand side.
Just make small cuts, or trim small pieces of plastic from the display,
little by little. Take care.
The Nokia LCD is supplied with this plastic
assembly and an integral speaker. Toss the speaker in your spare parts
bin. The black
lines show the desired trim lines.
Q3. How did you make the optical encoder?
This uses parts from an old computer mouse. Look for the ones with the
rubber ball inside. These contain an optical encoder wheel (a wheel
with tiny slots in it) along with a plastic shaft.
This type of mouse uses two infra-red LED/detector pairs to detect
movement. Each detector contains two IR light-sensitive transistors.
One LED/detector pair is required for the DDS optical encoder. (Sorry -
I don't have a method to use the increasingly common optical mouse.
However, many of these have a centre scroll wheel and this often uses
an infra-red encoder which can also be used for the DDS optical encoder)
Mount these parts so the IR light from the LED passes through the slots
onto the detector. The detector requires two resistors and the IR LED
requires one resistor. The resistors are shown in the circuit diagram
in the original article and are mounted on the DDS VFO PCB.
This photo shows the optical encoder
I built for the DDS VFO. The optocoupler and wheel from the mouse were
mounted on a scrap of brass and a connector from the same mouse was
used for the cable connection.
Note: Most web browsers will allow you to see these photos in more
detail if you click on the photos with the right hand button of your
Q4. Can you modify the software for
me? I want to use this for a different application
but I can't write 8051 software.
Q5. Can you build one of these DDS
VFOs for me?
No, sorry. I have a full time day job and I am working on some new
designs in my spare time. The source code available here should allow
experienced programmers to do any software modifications required.
There are kit builders who advertise in Silicon Chip who may be able to
help you if you cannot build this VFO yourself.
Q6. What assembler should I use to
modify the source code?
Almost any 8051 assembler can be used. I used the free Metalink 8051
assembler which can be downloaded from the web. You can find it via
Q7. Do you have a kit of parts
No, sorry. PCBs are available from the usual suppliers, and parts can
be obtained from usual component sources. The article provides more
details. In particular, check out the advertisers in Silicon Chip
Q8. Can you send me a copy of the
circuit diagram and PCB layout?
No, sorry. Copyright for the circuit schematic, PCB layout and related
material is held by Silicon Chip magazine. However, PCB artwork can be
downloaded from the magazine's website at no cost.
Q9. My receiver has an IF of 9 MHz.
What do I need to do to use your DDS VFO with my receiver?
See the PDF file at the foot of this page which describes DDS VFO
modifications. It has all of the details you need.
Files Available for (Free) Downloading
Intel HEX file - Clicking on this link will
allow you to download a zipped HEX format file (4 kB) for programming
an 89C4051 chip for use in this VFO
Source Code file - Clicking
on this link will allow you to download a zipped Metalink compatible
ASCII TEXT format source file (19 kB) for this VFO. This will allow the
code to be modified by experienced 8051 programmers who may wish to add
DDS VFO Modifications - Clicking
on this link will allow you to download a PDF file describing how to
modify the DDS VFO for other IF frequencies and for transceivers which
require a VFO with an IF offset applied during both transmit and
Listing - Click
here to download a zipped text file containing a listing of the
assembled code. Some folks have asked for this.
on this link will allow you to download a zipped file containing an
Excel spreadsheet I wrote to help calculate the various 32 bit words
needed for different IFs
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