VK5TM


News, Updates and Other Minutiae

As a lot of people have pics of things unrelated to ham radio on their websites, I thought I would add a couple of my own (Scroll down for site updates). Banjo, our Red Heeler/Koolie cross at 4 months old. Banjo at 4 months And at 12 months old (Feb 2016) enjoying his birthday breakfast treat of banana on toast. Banjo at 4 months

May 2017

New project added - my version of the X-Phase noise canceller. PCB available and a kit of parts to follow soon

27 December 2016

Another addition to the Gallery section. This time, fitting a DDS module to a Philips PM5326 RF Generator to cure a bad drift problem.

26 November 2016

Additional software for the Simple DDS VFO added. This enables the pushbutton on the encoder to change step size (step sizes 10kHz, 1kHz and 10Hz). See the notes in the Download section of the Simple DDS VFO page for more info.

20 November 2016

I have had some interesting conversations with homebrewers over the last few years relating to experiments/modifications of the works published on this website. One of the things on the 'list of things to do', was to publish some of their pics' etc on my website, so along that line, the first one, a filter/leveller for DDS modules from G3OAG, is now on my site. This one and all the others I have available to show, will be posted under the 'Gallery' menu. Now that things are starting to return to somewhat normal around here, I will endeavour to get the others up as quickly as possible (with apologies to those that have been waiting to see their work in lights).

I have had a quantity of pcb's made commercially for this project which can also be used for the Xtal Substitute project. Cost is $10 AUD for one including postage (local or oversea's). Additional pcb's $6 AUD each for oversea's or $6.60 each in Australia. See contact page to get in touch with me. 22 Feb 2017 - PCB's for this project are all gone, if there is enough demand, I will get some more made.

DDS xtal substitute pcb DDS xtal substitute pcb

The software has been rewritten and updated. See the Software section of the page.

DDS Xtal Substitute

A Simple DDS based xtal substitution project

One of my transceivers on the to be repaired shelf is a Yaesu FT-7. As many will know, although it has a '10m' position on the band selector, it only covers a 500kHz portion of the band depending on which xtal is fitted. To cover all the 10m band, you need a series of xtals and a switching circuit to cover the whole band (The FT-7B at least has all positions on the band switch). With the xtals getting harder to find 'off the shelf' and the price of custom xtals becoming prohibitive, this project should be an economical alternative for this and possibly many other rigs. The circuit is basically the same as the Simple DDS VFO without the encoder. Click for larger version. Opens in new window.

DDS xtal substitute schematic

If you have one of the 3.3v modules, all that needs to be done is to replace the 7805 with a 3.3v regulator. The PIC will happily run at 3.3v and it doesn't control anything but the DDS module, so no other interfacing problems.

A few words about the schematic. You will see two possible ways of switching between frequencies, using switches or transistors. Which ever system you use, make sure that it stays set. IE, if you use switches, use on/off style switches, not momentary pushbuttons and if you use transistor switching, make sure the transistor stays 'on'. The reason is that the software in the 12F629 uses a feature called 'interupt on change' on the three inputs. So, regardless of whether an input goes Hi or Lo, the 12F629 recognises that there has been a change and sets the DDS output frequency according to the new input setting. As eluded to, once the frequency has been set, the 12F629 goes to sleep until any input changes. This should help on two fronts - reducing any possibilty of interference from the 12F629 and reducing current consumption. Now, just to upset the apple-cart, there are different requirements for switches used during calibration. SW-1 & SW-2 need to be momentary push buttons. SW-3 is still an on/off style switch. Their use will be explained in the paragraph on calibration. I have not included any sort of filtering, but have placed pads for both the filtered sine and unfiltered sine outputs of the DDS modules so either can be used. Bandpass filtering is strongly suggested for this application, expecially if you use frequencies above about 25MHz on the AD9850 module and about 40MHz for the '9851 module. (Note that the square wave outputs only work up to 1MHz apparently. Never tested them myself). It should be easy enough to arrange a filter on a piece of pcb material to do the job. A shielded box for the whole assenbly is probably a good idea as well. The pcb is only 2 inches (50.8mm) square. You can use either the AD9850 Module or the same format AD9851 module. Xtal substitute pcb

Software

A few (more) words about the software. You will need to input the values of and recompile the software for your required set of frequencies: Xtal_1_0...3, Xtal_2_0...3 and Xtal_3_0...3 in the asm file are the frequencies that the DDS will be set to when either of input 1,2 or 3 is taken low (or grounded). The default frequency, default_0...3, is the frequency that is sent to the DDS module if NO input is low (or grounded). This could be set to zero. 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 range of frequencies. How to calculate these is presented further down this page. If you use MPlab to compile the software, the above are between lines 124 to 165 in the asm file. In the file available to download below, the frequencies are 1 MHz, 1 MHz, 3 MHz, 4 MHz & 4.5 MHz in order as listed above. They are of no particular relevance, other than to be far enough apart so you know that the frequencies are changing during testing. There is no limit to the frequency range other than that of the AD9850 or AD9851.

PLEASE NOTE: Make sure that once installed into your rig/project or other, that input 3 is NOT HELD LOW when the power is first turned ON, otherwise it will enter calibrate mode.

UPDATE 6th June 2014: The software has been updated such that, once the calibration has been done, the above restriction does not apply

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. Calibration requires the use of momentary push buttons connected to inputs 1 & 2 and an on/off switch connected to input 3. Connect the frequency counter to the output of the unit and turn the switch connected to input 3 ON. Do not turn it off until calibration is complete. Apply power. The frequency counter should show a frequency somewhere in the vicinity of the calibration value put into the software. Go away and have a cup of coffee, maybe a biscuit or 2 or what ever your choice. Once you have left the unit to run for a minimum of fifteen minutes, then continue. Or in other words, give it time to warm up before doing any adjustments. Push either the UP or DOWN button as is required to change the frequency so that it becomes your calibration value. There is approximately a 1/3 second delay between each frequency change step. Release the button when you have reached the CAL frequency. You can now turn the CAL switch OFF. 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.

Calculating and entering different frequencies

For those with Windows systems, it is easier to use the upgraded scientific calculator available for download (in "Programmer" mode). Users of other operating system (or those that don't want to download the calculator), will need a decimal to hex converter program of some sort (unless you are lucky enough to be able to do it in your head). In the DDS_XTAL_SUB asm file, starting from line 124, are several 'EQU' statements for xtal_1_0..3, xtal_2_0..3, xtal_3_0..3, default frequency and calibrate frequency. I will use the value of 5MHz to show you how to get the numbers. The following sequence applies to all five of the 'EQU' sections (with different numbers of course). Firstly, enter the dec number 5000000 (5 million) into your dec to hex calculator/program (I'm not going to tell you how to work your calculator/program, you need to work that out). The HEX number will be 4C4B40 Now break this number down into groups of 2, starting from the right hand side:- 4C 4B 40 The right hand number is the least significant byte, so you enter 0x40 into the line that says, funnily enough, least significant byte. Enter 0x4B in the next line up and 0x4C into the line above that. The '0x' tells the MPlab compiler that the number is in HEX format. So, where is the most significant byte? Well, in this case, it is 00, because 5000000 only converts to the three bytes just shown. So, if your calculation only gives you three nice even groups of two numbers, the most significant byte will be 0x00. Lets do another one that doesn't give you three nice even groups of two numbers. Enter 20000000 (twenty million) into your calculator/program. It should come back with the HEX number of 1312D00. Breaking that down into groups of 2 as before gives you: 1 31 2D 00. Not so nice even groups of two numbers plus an odd one. To fix that, you put a '0' in front of the 1 to get 01 31 2D 00. Because nought is nought (or zero is zero), it has no effect on the calculated value, so you can enter the numbers as before, but the most significant byte will now be 01. Don't forget to put 0x in front of your numbers, otherwise, strange things will happen. Using the AD9850 or '51 modules, the most significant byte will not be any more than 03 hex (which puts you upwards of 50MHz).

Downloads

These files are provided free for personal use ONLY. I retain all copyright on all works published on this website. They may NOT be used in any commercial or profit making enterprise of any kind without the express WRITTEN permission of the copyright holder.

(Right click and 'Save as..' or what ever is required by your browser) VK5TM_DDS_XTAL_SUB_b.asmThe ASM file for the Xtal substitute project. It is set to AD9850 by default. Instructions are in the file to change it. I have removed the HEX files as they are not relevant. You need to change the frequencies to suit and recompile the software. If you really, really, really, need a HEX file with your custom frequencies, I will create it for a small donation. VK5TM_DDS_XTAL_SUB.lay6 The Sprint Layout 6 pcb file. See the PCB Info page for information on using this file.