Tuesday, November 18, 2014

Stable Vackar oscillator

Greetings everyone,

I love playing around with different oscillator designs. From the SuperVXO to trusty Hartley, both crystal and LC oscillators are fascinating to build and study. Some designs stand out as working better than others, but it often depends on the application. One oscillator design that stands out to me is the Vackar and I would like to share a simple but stable version of it with you.

AmQRP produces an excellent magazine called Homebrewer. You will find various articles & excerpts from homebrewer as PDFs at various corners of the web. The Handyman's Guide to homebrew construction has been popularly shared amongst the online community.

The circuit I am going to share with you today comes from an article from issue number 7. The article is titled Better Crystal Set and was written by Phil deCaire, WB7AEI. Because I have not asked for permission to share the original schematic image from the article, I have redrawn the schematic using ExpressSCH. I should note that it is issues of copyright that will likely pose a large obstacle in a timely sharing of my favorite designs, as I will have to redraw every schematic that I wish to share. I am open to suggestions and/or influence regarding this matter.

I was impressed with the frequency range, stability and simplicity of this design. We all know that creating a stable VFO* can be a very difficult task. Often times we opt for the rock for this very reason. But even ceramic resonator based oscillators or SuperVXO with a great amount of "pull" can't come close to the range of this oscillator. I have not used this as a VFO/LO but I've used it as a BFO & as a general piece of test gear (sig gen) on the bench with good results.

* click Stable VFO  for link to hanssummers' incredible Huff & Puff!

Originally by WB7AEI from AmQRP Homebrewer #7
7-14 MHz Vackar Oscillator.
Click to enlarge

I would like to hear from anyone else who has used the Vackar design, whether a simple one like this or a more complex version (like G3PDM's Vackar VFO from ARRL HB 2004 - p14.18 Fig 14.15). How does the vackar fail in comparison to other oscillator designs and in what applications does it succeed? Want to see pictures of my build (heads up, I build ugly circuits)? Let me know your thoughts.

I could also get some pictures of the output of this circuit, as well as different results from varying the L element of the circuit. For L, an air-wound coil can be used, but a powdered iron toroid may produce better results. If you would like to see a more in depth look at this circuit, please let me know and I will spend some time making the results of experimentation available here. Otherwise, I will continue to move forward with more blog posts on other topics.

Thank you for taking time to read this. This is the first time I've ever attempted to share homebrew technical info like this. I appreciate any kind feedback and even criticisms (I am human, so please be gentle).

If you are reading this than you are part of the community and your voice is absolutely welcome here, so feel free to share your thoughts on the Vackar or other oscillator designs.

Keep your iron hot, stay fluxy and 73!