Analog Devices AD9850 frequency synthesiser

AD9850 evaluation board, 45mm x 26mm

Connected to MYDEV2 PIC MSSP module for programming

SINA and QP outputs

Another visit to an online auction site and another electronics purchase. This time I spent $9 on an evaluation board for the AD9850 frequency synthesiser chip. Surely the 125MHz 'can' oscillator and the chip itself are individually worth more than that. However it was made in China.
I mounted it on a larger piece of experimenter board and connected its programming inputs to a microcontroller PIC18F4550 on my MYDEV2 PIC development board. Before the AD9850 will produce an output signal it has to be programmed.
So I wrote a few lines of code to use the PIC's Master Synchronous Serial Peripheral ,( MSSP ), interface module to send the 40 bits of frequency, phase and control data to the AD9850.
The resultant output signals are a sine wave ( CH1 yellow trace ) of 1.04V peak-peak directly from the chip's digital-analogue convertor, ( DAC ), and a variable pulse-width square wave ( CH2 blue trace ) of 5V peak-peak via the chip's comparator for use as an external clock.
I have intentionally allowed plenty of space on the experimenter board to fit a dedicated PIC later; probably the PIC18F14K22 as I already have one.
The AD9850 will be a useful signal source and clock generator upto about 40MHz.

DDS #2 on test

Today I continued with the development of the second prototype frequency synthesiser. I have added the facility of changing the tuning step size in real time.
It is shown being tested on the work bench alongside "MyDev2", ( PIC18F4550 microcontroller ), and "ICD2", ( debugger and programmer ).
The output voltage at 137.7KHz is a 1.32v peak to peak pure sine wave across 300 Ohm. There is no output low-pass filter and I think this is the reason the output level is about twice that of prototype #1, which incorporates such a filter, probably introducing significant residual capacitive reactance.

DDS deadline met

I have completed the first prototype of the dual-band frequency synthesiser, ( just before my self imposed deadline of the end of the decade ), by writing the last piece of the software to tune the output frequency up and down in 1Hz or 10Hz steps. This function works perfectly; whatever the actual output frequency, it precisely matches the frequency indicated on the display, as of course it should do. So 2010 will end on that happy note. But what about the future ?
This first prototype is what I would consider as a ' concept and technology demonstrator'. For the second prototype synthesiser I have decided to change to a more sophisticated PIC microcontroller, and one that has not been in production all that long, e.g., the Microchip PIC18F4550, which will allow me to implement more features. In fact I have already completed about 25% of this prototype , based on "MyDev2"; see posting on 2nd November.
Concerning software, until now I have been programming in Microchip's assembler language. Using 'C' language is probably a better choice in future. I have absolutely no prior experience of 'C'; but what I do know is that its command syntax is more descriptive than assembler, and the code is not type specific to any one manufacturer's microprocessor families. Some of the extended functionality I want for the second prototype is better suited to 'C' and would be extremely difficult to program in assembler for a novice programmer as myself. The PIC18F4550 can be programmed in both assembler and 'C'.
Back to the present, and the first prototype is shown in the upper picture placed on top of my longwave transmitter with which it will be used to gain some operational experience which may suggest improvements or further development. The middle picture is the post-filter rf output voltage displayed on an oscilloscope, showing a 656mV peak-peak pure sine wave. The lower picture is the output frequency of the synthesiser as I tune it from 137700Hz +/- 50Hz in 10Hz steps.