VK5TM

This is a mod using a DDS module to repair a drifty Philips PM 5326 RF generator. Stefan has done a marvellous job, not only of the mod, but also in providing the write-up and pictures presented below. Please see notes at the bottom re software and using the Simple DDS VFO project for this mod.

Starting point for this project was a constant frequency drift of my Philips PM 5326 RF generator. Even after hours of warming up it doesn't come to a full stop. Furthermore tuning to a specific frequency was a touchy thing because no fine tuning was possible with the build in tuning capacitor. The RF section of the generator is based on the PLL principle. The reference frequency is generated by a VFO, comprising a simple transistor LC oscillator. This reference frequency determines the frequency of a VCO as the active element of the generator. It generates the RF frequency between 100 KHz and 125 MHz via several range coils and divider chains for the phase comparator. Every slight deviation in frequency of the VFO results in a relatively higher deviation of the VCO and therefore of the RF frequency. So, the weak point was the VFO, which was state of the art in the late 70's, early 80's. It was as good as was possible in those days. So, I decided to replace the "analog" VFO with a modern DDS-VFO. An additional requirement was, that the generator shoud remain in its original state. I never wanted to drill any holes to the case for adding any switches etc. The modification should not be seen from outside and it should be reversible. Browsing through the internet I found the "Simple DDS VFO" from Terry (VK5TM) which fits all my needs, except for the step size switch. But Terry thankfully updated the software for the microcontroller, so that the step size could be toggled with a push switch on a suitable encoder. The following pictures show how the project was realised:

Frequencies now are absolutely stable and precise (tolerances of only a few hertz). A huge improvement to the former state ! The modification was worth all the efforts. I am very grateful to Terry for all his support and help making this project a great success.

Notes

Normally I have been emailing the PIC software for this mod to those requesting it. However, despite the schematic at the top of this page, some confusion has ensued when I mention you can use the Simple VFO pcb to build this mod. This confusion arises because the software was written for the older Simple VFO project and uses different connections for the encoder and switch. I have updated the software so that it is now directly compatible with the updated Simple DDS VFO project schematic and pcb, i.e. - you follow the Simple DDS VFO 2017 update project details and use the software posted at the bottom of that page for the PM5326 mod. Also, the pcb may be slightly too wide depending on how you fit it, so you may need to shave a couple of mm off two of the opposite sides or build it on a piece of strip board as in Stefans build above.