Test jig for DDS module and Dongle

AD9850 DDS module on breakout board with interface dongle

CH1 sine wave output, CH2 comparator output +DUT 25%

This post is about using those cheap DDS, ( Direct Digital Synthesizer ), modules based on the AD9850 or AD9851 DDS chips, and found on auction sites for only a few dollars. I have been programming and using them for several years, going back as far as August 2013 ( see post 27/08/2013 ). I am now using the interface dongle, ( USB-SPI v1.00 ), and software from www.spectecs.com which makes the modules extremely quick and easy to use; the tuning word is created and uploaded to the DDS over USB with just a few mouse clicks.
The modules are ideal as a signal source for testing amplifiers etc or embedded in projects which is what I have done recently with the phasing exciter ( 02/11/2017 ) using an AD9851 DDS module on that occasion.
I made a simple jig, ( break-out board ), for speed and convenience when connecting the module for use as a stand-alone signal source. The SPI and output connectors are extended out to pin-strip headers. I also provided LEDs for testing the two GPOs from the dongle.
The module has two sine wave outputs, ( OUT1 & OUT2 ), two square wave outputs ( VO_P & VO_N ). Sine output OUT2 is present on the disconnected black jumper. Referring to the top picture, I've set the jumpers as follows:
J1 green, connected = serial data upload, ( disconnected = parallel data upload ).
J2 blue, connected = enable square wave output, ( disconnected = no square wave output ).
J3 yellow, connected = digital to analogue converter ( DAC ) full scale current set internally, ( disconnected = DAC current set externally ).
I uploaded a tuning word ( hex 010624DD ) for the DDS to generate an output signal at 500KHz. The duty cycle of the square wave is adjusted using the trim-pot next to J2. I found the range of adjustment to be about 12%-87%. The lower picture shows the signal on OUT1 ( 1V peak to peak ), and VO_P ( 5V peak to peak ), which I adjusted for 25% duty cycle.
Although the top picture features the AD9850 DDS module, the AD9851 module is identical in appearance apart from the chip type of course.  

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.

My very low power transmissions on longwave

Last night I made successful radio test transmissions on 137.7KHz, 2190m band, using only 3.5W transmitter power. My signal was received, ( screen capture below ), at a distance of 17Km. The signal strength suggests that 2-way communication at this power level would be possible over a much longer distance. The vertical streaks are probably static crashes as a thunder storm was active in the vicinity.

My setup was my own-design PIC controlled DDS and the TDA2030 AF amplifier featured on 22 February. 

It is unfortunate that amateur radio activity on the 2190m band is so low, as it is possible to enjoy communicating on this band with a minimal setup, as I have just shown.  

Two generations

The early MKI ( lower ) and very recent MKII ( upper ) embedded control frequency synthesisers are both resting on top of my longwave transmitter. I will interface the MKII with the transmitter as that was always one of my intended applications. The MKI will now be used as an item of test equipment on the workbench to provide an lf signal source.

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.

Stepping up and stepping down

A few hours ago I took my frequency synthesiser project a significant step nearer to completion. I had a 'eureka' moment; not actually in the bath but instead while waiting to go Christmas shopping with my daughter.
A long time ago I had programmed the PIC microcontroller to show the frequency on the liquid crystal display. The number crunching sequences needed are binary to binary coded decimal ( bcd ) conversion, bcd to ascii, and finally, supression of any leading zeros to prepare the number for display.
For weeks I had been baffled why I couldn't achieve this repetitively in order to step the displayed frequency up or down in response to clockwise or counter-clockwise turning of the rotary encoder; the only difference being to include multiple byte addition and subtraction. This I have now managed to do. It's a great result for me and so good to see on the display. Now I can enjoy Christmas and not be distracted by searching for a solution any more.
This was one of the last two crucial issues to be resolved; the other being to increment / decrement the tuning word by the same step size for sending to the synthesiser chip, which should be straightforward. So I ought to be able to complete the prototype frequency synthesiser this month - famous last words !

On display

Seeing is believing ! I programmed the frequency synthesiser, which I am in the process of building, for an output signal on 137.700KHz. Both the display on the Thandar PFM200A frequency counter, ( connected to the output ), and the display on the synthesiser, ( connected to the microprocessor ), agree. What a relief.
The next task is to make it tuneable up and down, ( probably in 1 and 10Hz steps ), implement band changing and at the same time make sure the display updates correctly.

Frequency synthesiser

Having decided on all the additional functions I want, ( band change, tuning step selection, PTT and output filter ), and the switches and connections that will be needed, during the last few days I have progressed further with the frequency synthesiser project from the experimental and development stage, ( see 25th January ), to the first, ( boxed ), prototype. The green, back-lit, dot-matrix liquid crystal display gives a particularly pleasing appearance. The source code still needs some refining. The inset shows new code being uploaded via the 'in-circuit serial programming' interface, ICSP.

New design in progress

This is my current project which is still at the very early stages of development. I am hoping that eventually it will become a frequency synthesiser. For the first time I am using a programmable integrated circuit, PIC, which is basically a microprocessor using a reduced instruction set of around 40 commands. I am writing and testing the source code, burning the PIC, rewriting and retesting etc. Keep a look-out for future progress reports.