Some DDS circuits (updated April 15, 2017)

Direct Digital Synthesis (DDS) is an attractive method to generate stable signals having good spectral purity and fast frequency settling time.  

You'll find a lot of information on the DDS theory and implementation in an article published by my son Lionel in the "IEEE Signal Processing Magazine" you can download from his WEB page :
http://lionel.cordesses.free.fr/gpages/electronics.html

- More information on the subject is also available in the chapter 21 of the book  "Streamlining Digital Processing" Click here
-
Furthermore, have a look at the Wikipedia page devoted to the DDS.

At the beginning of the 1990s, Lionel and I built from scratch a home made DDS (picture below) creating signals in the audio range.  

I have used many different DDS chips since then, mainly from Analog Devices, to build signal generators for different purposes and I present some of them below.  

1. AD9834 board.

The AD9834 is a low cost, low-power DDS ( a few mA at 5V) that can be clocked up to 50 MHz. It is housed in a 20 lead TSSOP SMD package.
I selected this chip  for the Function Generator I describe elsewhere :   (more informations)
More recently I designed a PC board that can be used as a simple DDS development tool when associated with a PIC 16F873 microcontroler card described elsewhere.  

- Click here to get the schematic diagram of the DDS unit
- Update May 22, 2008 :  The PCB artwork of the lower side (solder side)  is available as a PDF file by clicking here.
The upper side of the PCB remains unetched : it is used as a ground plane to which all ground connexions are soldered. The AD9834 chip is soldered on the lower side, the other components being on the upper side.
It is necessary to remove the unetched copper around the components holes not connected to ground. I use  a 6 to 8 mm drill to do that.
One can see some of these "insulated holes" on the picture below showing  the complete DDS card .

Some remarks :
- Any clock oscillator up to 50 MHz can be used.
- There are two 5 V regulators on the card , one for the digital power supply and the other for the analog  supply. You can use only one if high spectral purity is not a matter of great concern.
- The output relay allowing selection of sine/triangle or square waveform can be omitted if  not necessary.
- The inductor on the left of  the PCB (picture below) is a part of the elliptic low-pass filter rejecting the DDS clock frequency. Such filters can be designed with the help of  filter tables in the Zverev's "Handbook of Filter Synthesis" (or in the much less expensive "ARRL Handbook") or with one of the many softwares available.
- The SCLK DDS pin is connected to port B5 of the PIC, the SDAT to  line B4 and FSYNC to line B2.
- Pins PhSEL and FSEL are grounded if not used.

The PIC software controlling the DDS is written in C language using a CCS compiler.


New (January 29, 2010) : A demo program (in CCS C language) showing how to send  words from the PIC to the DDS registers is available : it generates a 18.1125 MHz signal but may be modified to create any other frequency inside the AD9834 range. Click here to get this file.
==> Have a look at the Analog Devices Web site to download the AD9834 data sheet and/or to use the tool for calculating codewords to program the DDS : 

http://www.analog.com/designtools/en/simdds/#/?part=AD9834&fin=75M&mult=1&ftw=&rso=111111&harmonicDB=-50&useFilters=0

The picture on the left below shows the DDS board connected to the PIC board and on the right is a bottom view, the DDS being the small SMD chip .

                              

Click here to get the schematic diagram of the PIC card.

2.AD9854 board.

Some years ago I needed an RF source and selected the AD9854 because of its high upper frequency limit. In contrast to the AD9834 it is a power hungry device ( # 500 mA at 3.3 V) and the heat sink is a must. This chip is a large one (80 pins LQFP SMD package) and is not so easy to solder on the PCB!

The schematic is available here (200 kbytes PDF)

Remarks :
- The AD9854 is under control of a PIC 18F2525 included on the PCB. The control interface I select is the 3 wires serial SPI.
- A rotary encoder connected to pins  B6 and B7 of the PIC allows  frequency tuning and a  3 position switch controls frequency step.
- There are 2 output lines, one in phase and the other in quadrature, each connected to the DDS output pins through elliptic low-pass filters.
- Two clock modes are available : direct or multiplied. While a lower clock oscillator can be used when the second mode is selected, the output spectrum is not as clean :  I thus used the first method.
- DDS supply voltage is 3.3 V and the PIC one 5 V : resistive voltage dividers take care of  the control lines going from the PIC to the DDS.
- Prog 2 terminal (connected to pin B5)  is used only during the software development cycle.
- The PIC code was developed with the aforementioned CCS C Compiler.

Below are two pictures of the card, the left one showing the top side and the right one the bottom side. The large black heat sink on the right of the picture is the power dissipation device for the AD9854 hidden under from sight.
Note : This method allows proper cooling of the DDS chip under operating conditions in use here : 100 MHz clock,  no Sinc and  no Digital Multiplier engaged. Be careful in case of  harder operating modes!!!

                             

                         

The PCB shown on these pictures is the first one I designed and some changes or improvements have been introduced during the tests.

Update June 8, 2008 : the new PCB artwork and part placement files are now available. Click here to download.
This new design is slightly different from the one shown on the above pictures.



 

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