A DDS Module based on the AD9835  

Table of contents

Purpose

While working on a project I came across the DDS chips from Analog Devices. Especially the AD9835 was 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.

Schematics

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).

AD9835 Module Schematic
Image 1: AD9835 Module Schematic

The PCB

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.

The PCB
Image 2: The PCB Module

  Component positions
Image 3: Component positions

You can download a PDF File with the schematic and the PCB here.
If you dont have the Adobe Acrobat Reader installed on your machine you can find it here

Pictures of the finished Module

Here you can see how the module looks during different stages of the building process and how it looks when it's finished. A picture says more than thousand words.

Developed PCB
Image 4: The exposed and developed PCB

  Finished PCB
Image 5: The finished PCB with all components

Module on the BreadBoard
Image 6: Module used on a Breadboard ....

PIC Development system
Image 7: ... and with the PIC development system

Notes & Comments

I have tested this module as a VFO and also as a "SoundCard". With this "SoundCard" you can reproduce nearly every digital mode like RTTY, Packet/APRS and even SSTV. I have connected the output of the module to a PC's soundcard and run some audio test software. You can see the output of a 1000 Hz tone on the folowing images.

Usable software to for analysing audio signals are:

1000 Hz Wave
Image 8: 1000 Hz measured testsignal

  1000 Hz Wave
Image 9: 1000 Hz sine wave

1000 Hz Spectrum
Image 10: Spectrum of the 1000 Hz testsignal

Spectrum display
Image 11: Spectrum of the testsignal

Click here to see details about the SSTV testgenerator based on this project.

Credits

This project is based on some Infos found on the Internet eg. Analog Devices and modified by me OE1RIB, Richard
If you have any comments or sugestions just drop me a line.

73, OE1RIB