Table of contents
- Pictures of the finished module
- Notes & comments
While working on a project I came across the DDS chips from Analog Devices. Especially the AD9835was very atractive to me because of its super simple handling and small size. Normaly you would use such a chip as a VFO or for other, similar functions but I decided to use it as a “Soundcard” for a project to do various digital RF modes with a PIC microcontroler. Some would say “I can do this with just the PIC and dont need a DDS chip”. That’s true but you would waste a lot of program memory for things like sine tables etc. With this solution you will have more memory available for your programs and you will get better signals than you could produce with just the PIC.
The AD9835 comes in a very smal TSSOP housing. So to start developing I needed a more bigger, better to handle form of the AD9835. I packed it with a 50 MHz osciliator module and some parts needed for propper operation on a small PCB with two connectors with standard pin sizes. This simplifies the development enormous. You now can use your standard breadboard to design systems with the AD9835.
After the hardware description you will find a code sample of a SSTV Testgenerator based on this module and a simple PIC 16F84. This shows that even uncommon digital modes can be done with a PIC. This Testgenerator produces a color bar screen using Martin-1 SSTV protocol and also sends the correct VIS code for Martin-1.
DISCLAIMER: THIS CONTENT IS PROVIDED TO INTERESTED PEOPLE "AS IS" FOR NON COMERCIAL PURPOSE ONLY. UNDER NO CIRCUMSTANCES I AM RESPONSIBLE FOR ANY DAMAGES TO YOUR RADIO, PC OR OTHER EQUIPMENT. THIS DEVICE WAS TESTED UNDER DIFFERENT CONDITIONS AND UNTIL NOW IT WORKS WITHOUT PROBLEMS.
This is the schematic of the AD9835 Module. It follows in some parts the schematic in the AD9835 reference manual. Other parts could be better. For example the power handling (Analog and Digital VDD and VSS) is not that optimal as it could be (just L1 to prevent some disturbances).
The PCB is single sided and except the 50 MHz oscillator module and the coil which are of normal size all other components are SMD size 1206. Special care must be taken soldering the AD9835. It comes as a TSSOP and unless you don’t have a soldering iron with a very, very fine tip and a magnifying glass you wont get happy builing this project. Be warned! This chip is hard to solder.
The red marked lines on the PCB are simple jumper wires which must be soldered from the non copper side of the board. For more details see the pictures.
For more detail: A DDS Module based on the AD9835 using PIC16F84