VK5TM

Like my AD9850/51 based Simple DDS VFO, this is a VFO that can be used just as an ordinary transistor/fet VFO would be. This has been kept as simple as, any buffering, amplification or filtering is left up to you to build to suit your purposes. Why another DDS VFO and why the AD9833? Well, the AD9850/51 modules have become expensive and I've had a few requests over the last couple of years for a limited range device to use as a VFO, so I looked around and currently, the AD9833 modules (without all the extra's) are relatively cheap - around $6 AUD at the end of November 2022 - just watch the postage charge! Plus the other favourite, the Si5351 is in very short supply at the moment. First to note - this is a preliminary presentation of the project. The software is based on V2.4 of the 2017 Simple DDS VFO and features all the same functions but I have not fully tested that everything works as it should (mainly due to the current lack of a workshop), so if you find a bug, please let me know so I can correct it. Also note that the maximum frequency available can be set to 12MHz but I probably wouldn't top much more than 10MHz. This currently uses a 12F629 PIC, but I will update it to the 12F1840 in a little while.

Click picture for larger view. Click again to close.
Note the resistor from pin 5 of the PIC to GND

A note about the two sets of capacitors marked '1' & '2' in the original Simple DDS VFO project if you are using that pcb for this project - ignore the set marked a "2", ie the 4n7 ones - just fit the 100n ones in position "1"

Software

There have been a few cases of errors compiling files and it is caused by using an older version of MPlab. I use version 8.92 (which is the last of the old MPlab versions) and the problem relates to the 'include' file and the Config defines of those older versions. Specifically the FOSC_INTRCIO definition of the later version - change it to INTRC_OSC_NOCLKOUT for earlier versions of MPlab. A few words about the software. You will need to input the values of and recompile the software for your required VFO frequency: Limit - The upper limit frequency you want the VFO to reach. Limit_Low - The lower limit frequency you want the VFO to go down to. Default - The default start frequency - this will be overwritten during operation by your last used frequency, it is just the first time start up default (it must be within the VFO range above). Cal_freq - The Calibration frequency - This is the frequency the unit needs to be adjusted to during the calibration procedure. I suggest making it in the middle of your VFO range. In the file available to download below, the frequencies are 5.5MHz, 5.0MHz, 5.0MHz & 5.250MHz in order as listed above. There is no limit to the frequency range of the VFO other than that of the AD9833, so your limits could be 1Hz and 12MHz if that is what you wanted. At the first power-up, the step size is set to 10Hz and each subsequent push of the step switch advances the step size by 1 i.e. - to 1kHz and then 10kHz. Another push of the button steps back to the 10Hz step size. The step size set at power down will be restored the next time you turn the unit on. One point to note: Until the calibration is done, you need to make sure that the 'CAL/STEP' input is not held low when first turned on. Otherwise the unit will enter 'calibration' mode. Once the calibration has been done, this restriction does not apply. If you do not want to do a calibration, all you need to do is switch the AD9833 VFO into CAL mode and back out again without making any changes (see below). The software 'should' work with either mechanical or optical encoders. You need to remove the pullup resistors and associated caps on the encoder input lines to use an optical encoder (I have come across a couple of encoders that need the pull-up resistors left in place - check the datasheet of your particular encoder). You may notice at some point, regardless of encoder style used, that it appears to hesitate or not step the frequency. This is caused by the frequency save routine going off and doing it's thing. It is not a bug or fault, PIC's can only do one thing at a time, so it can't respond to the encoder while doing a frequency save. In the code below, the save delay is set to 2 seconds, increasing this to say 5 or 10 seconds may be beneficial.

Calibration

Once you have built the unit, it really should be calibrated before you fit it into anything. Calibration requires the use of an accurate frequency counter with 1Hz resolution and is used to adjust the "OSC" values in software to correct for off frequency Xtals on the DDS modules. Connect the frequency counter to the output of the unit and short the pads marked "CAL". The short must not be removed until calibration is complete. I would suggest you wire an on/off switch across the pads. With the pads shorted (or the switch "ON"), apply power. The frequency counter should show a frequency somewhere in the vicinity of the calibration value put into the software. Let the unit to run for a minimum of fifteen minutes before doing any adjustments and keep it out of any drafts. Turn the encoder in which ever direction is required to change the frequency so that it becomes your calibration value. Note that the oscillator calibration steps are very small, so you may have to turn the encoder a fair number of turns to reach your calibration frequency. Without turning the encoder further, remove the short (or turn the switch connected to the CAL input OFF) and then turn the encoder again (direction does not matter). This will save the new computed "OSC" values into EEPROM and the unit will now be functional. At this point, you can remove power and fit it into what ever it is you are going to use it in.

Calibration flag reset modification

This info is on the 2017 Simple DDS VFO project page, but I will repeat it here:- In older versions of the software for the older Simple DDS VFO project, it was necessary to re-program the PIC if you wanted to do another calibration. This newer software does need a minor pcb mod though and the AD9833 module needs to be socketed so it can be removed for part of the procedure. Firstly the pcb mod - you need to fit a 10k resistor between pins 8 & 5 of the PIC, see below. It can easily be done under the pcb.

Next, you will need to make up a little wire jumper lead to connect between the two points shown below in orange (this not a permanent jumper, it is only used during the reset procedure).

Program your PIC with the software in the downloads section. OK, now on to doing the reset procedure:- 1: With everything powered off, remove the DDS module. 2: Fit the temporary jumper as in the pic above. 3: Power up, leave for a couple of seconds and power down. 4: Remove the temporary jumper, refit the DDS module and carry on as per normal. You can now do another calibration run - no re-programming required. Each time you want to do another calibration, all you need to do is repeat the above procedure. Further information on modifying the software can be had from the 2017 Simple DDS VFO project as it is basically the same software although the line numbers mentioned on that page will be different. The "Step LED" add-on is also catered for in this project.

Downloads

These files are provided free for personal use ONLY. I retain copyright on all works published on this website (unless otherwise specified). These files, or any derivative of them, may NOT be used in any commercial or profit making enterprise of any kind. Updated August 2025 - new AD9833 initialisation procedure and remove some unnecessary calls.

(Right click and 'Save as..' or what ever is required by your browser) AD9833_VFO_V0-5b.asm The ASM file for the VFO. Default VFO range 5 - 5.5MHz. AD9833_VFO_V0-5b.hex AD9833 VFO hex file. Below are the 12F1840 files. There have been some changes - the rotary encoder routine has been changed and there is a major change to the upper and lower frequency limits checking routine. As before, these files have not been comprehensively checked for errors, so if you find any, please let me know so I can correct them. AD9833_VFO_V0-6b.asm The ASM file for the VFO. Default VFO range 5 - 5.5MHz. AD9833_VFO_V0-6b.hex AD9833 VFO hex file.