DDS SYNTHESIZER KIT

The Original DDS PC board design through to the current boards are compatable with the Original software from the NJQRP groups Website. This means that all Pin functions on the PIC16F84 Micro have been kept the same. The Software allows the DDS to be used anywhere between 0 & 30MHz, & had bandswitching to jump between the various HF Amateur bands. It also allows the DDS to be used as a signal generator, or a local oscillator for Direct Conversion receiver, or transmitters. The software does not allow for display offset to DDS frequency for Intermediate Frequency type Transmitters or Receivers. The original software although very good, was designed to use a Optical Type Rotary Encoder for tuning. These are very expensive in Australia > $100 Aus. The Mechanical types were tried & do work, but can cause the DDS to jump large frequencys due to contact bounce.

I was looking for Software that would allow the DDS output frequency to LCD display frequency offsets, & IF frequency offsets to be able to be programmed to make it more useable for building equipment for the VHF to Microwave bands. I was also looking at designing a basic SSB/CW transceiver for 144MHz similar to a FT290R. The New Software required the following functions.

Use the original PCB, modified to gain access to RA2..RA4 pins. Using diodes to provide the number of switch inputs required;
DDS frequency output, 0Hz to 40MHz;
TX/RX switching. By grounding RA4 to go to TX mode;
Frequency readout can be offset by 1Hz to 11GHz;
No requirement for band switching, but able to program upper and lower limits;
Memorise the last frequency on power up;
Use low cost mechanical rotary encoder instead of the optical type;
Variable rate tuning not required;
By grounding RA2, (PIC pin 1) then turning the rotary encoder, be able to set the step size required. eg 100Hz, 1KHz, 10KHz etc;
Use 16x2 LCD with or without R/W pin. (use delays rather than busy checks);
Software fine tuning of the XTAL oscillator frequency still required;
RIT available in RX mode by grounding, via diodes, both RA3 & RA4;
Still require the option of programming the software for different XTAL oscillators. eg 66MHz, 100MHz, 125MHz

Further Information on programming the setup screen using the calibrate button when powering up can be read in the Synth.ASM file. For quicker programming of the DDS software the Synth.DAT file can be modified to suit your application.

Thanks goes to Steven Jones for Re-writing the new Software. The software includes the SYNTH.ASM, SYNTH.DAT, & SYNTH.HEX files. It is being supplied free for all Amateurs.

The New software now allows either a PIC16F84 or 628, or either a AD9850 or 9851 to be used. A x1 or x6 Multiplier can also be selected when using the AD9851. There is now an option for a Repeater offset function on TX.

The control for the EME85 DDS requires a number of diodes, switches, & an ALPs EC16B-24 Rotary Encoder. The prototype is shown built on a surplus PC board from a Sony HIFI unit. The EME85C DDS board has 6 connections, Switch A/B/C, Encoder A/B, & Gnd. The Older type EME85A board will need modifying to add an extra diode to the RA4 connection on the 16F84.

For more Data on Pin connections for the ALPs EC16B Rotary Encoder, have a look here.

The Software for the EME85C Kit has been written to suit Standard Hitachi HD44780 16x2 LCD displays. Displays that either have R/W pins or not, can be used with the EME85C Software. A delay routine as been added in the software to accomodate this. Both the LCD2 & LCD3 Displays are compatible. The Vo ( LCD Contrast ) adjustment is not used on the LCD3 displays as they have resistors to preset the contrast on the display.

Additional DDS notes

It is suggested that the DDS board is mounted inside a metal box as it radiates quite a lot of spurious RF. Recently an interference problem on channel 9 VHF was tracked down to RF radiating from the DDS board. All leads in & out of the box should be kept as short as possible, & use ferrites over the leads if possible.
The maximum DDS output frequency that should be used is 22MHz, which is 1/3 of the 66MHz crystal oscillator frequency. There is quite a lot of spurious produced above 22MHz so a 20MHz low pass filter has been used to filter the spurious & 66MHz clock frequencies from appearing on the output. For higher output frequencies, you will have to increase the crystal frequency, & change the low pass filter to suit.

The highest possible frequency recommended for the AD9850 is around 40MHz using a 125MHz crystal oscillator. To produce an even cleaner output, it is suggested that the DDS output is mixed with another frequency to produce a new output & then filtered. The frequency display can be offset to accommodate this.

New circuit version without rotary encoder (with optical encoder from mouse)


ORIGINAL VERSION EME85A	CURRENT VERSION EME85C


DISPLAY IN RX MODE	DISPLAY IN TX MODE