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Screwy Oscillator Idea

> > > UPDATE!

Can you believe it’s been almost 3 months since my last post? A lot’s been going on since then, namely the national board dental exam. I’m happy to report I prepared for it and performed above and beyond my expectations on the exam, and I’m quite satisfied. The last few weeks were quite a strain on my life in my aspects, and during that time I realized that I didn’t appreciate the little things (such as free time) that I would have loved to experience instead of studying. I guess it’s the feeling you have when you’re really sick and think to yourself “remember this moment so that when you’re well again, you can appreciate feeling well”. Now that it’s all behind me, what do I do? I sit at my work station, play some light music, grab an adult beverage, turn on the soldering iron, and make something special.

Update: read the bottom of the post for reflections about the concept discussed below...

I’m resuming work on my simple transmitter/receiver projects, but I’m working at the heart of the device and experimenting with oscillator designs. I built various Colpitts, Hartley, Clapp, and other oscillator designs, and I think I landed on a design I’m most comfortable with replicating. I’m actually creating a voltage controlled oscillator (VCO or VFO), with a frequency that can be adjusted by rotating a dial or two. It’s always a balance between stability and tunability for me. I don’t want to use polyvaricon variable capacitors (expensive!), and LED-based varactor diode configurations only give me a swing of about 20pf. What did I come up with?

I had tremendous success using a variable inductor for coarse tuning! The inductor is nothing more than a screw entering and exiting the center of an air core inductor. I can’t claim all the credit, because I got the idea from this photo on one of the coolest websites on the planet, Alan Yates’ Lab. It looks like Alan got the idea from this page… This is so useful! Is this common HAM knowledge? Why am I, someone who’s been into RF circuitry for a couple of years now, JUST learning about this? I’m documenting it because I haven’t seen it out there on the web, and I feel it should be represented more! Here’s a video of it in action:

This is the circuit I was using: DSCN1334

This is what it looked like before the glue or screw: DSCN1307

Here’s the variable inductor enveloped in hot glue before it cooled and turned white: DSCN1316

At the end of the day, it looks nice! DSCN1339

Band changes can be accomplished by swapping the capacitor between the inductor and ground. It couldn’t be any easier! I’ll see if I can build this in a more compact manner…

UPDATE (2 days later): Apparently this is called a “Permeability Tuned Oscillator”, or PTO. It’s an early design for radios (earlier than variable capacitors) and I guess therefore not described often on the internet. Knowing it’s official title, searching yielded a few pages describing this action:

Dave, G7UVW did some analytical measurements using a mercury core!
The Tin Ear uses a PTO as its primary tuning method (also McDonalds straw?)
This guy made a PTO out of PVC with a nice screw handle!
This PTO kit seems to be used in many projects.
The Century 21′s VFO is a PTO! I love that rig and had no idea it tuned like that…
This guy used a PTO in his MMR-40 radio.
This guy uses a straw too!
Someone on Hackaday recommended This ARRL Challenge winner with an almost identical design as mine!

I guess this bright idea was so bright, it was thought of by many people long ago…

About the author

Scott W Harden

Scott Harden has had a lifelong passion for computer programming and electrical engineering, and recently has become interested in its relationship with biomolecular sciences. He has run a personal website since he was 15, which has changed names from HardenTechnologies.com, to KnightHacker.com, to ScottIsHot.com, to its current SWHarden.com. Scott has been in college for 10 years, with 3 more years to go. He has an AA in Biology (Valencia College), BS in Cell Biology (Union University), MS in Molecular Biology and Microbiology (University of Central Florida), and is currently in a combined DMD (doctor of dental medicine) / PhD (neuroscience) program through the collaboration of the College of Dentistry and College of Medicine (Interdisciplinary Program in Biomedical Science, IDP) at the University of Florida in Gainesville, Florida. In his spare time Scott builds small electrical devices (with an emphasis on radio frequency) and enjoys writing cross-platform open-source software.

Permanent link to this article: http://www.SWHarden.com/blog/2011-06-02-screwy-oscillator-idea/


  1. dave

    PTOs are quite an old idea, they were very common in older car radios for example. I think the ease of getting (getting harder now) decent variable caps pushed out the use of PTOs. I did dig up some references for space-winding the coil on a PTO to linearise the tuning, it’s something I must revisit in my experiments.

    My one written up experiment with a PTO is here : http://webshed.org/wiki/Mercury_PTO


    Dave – G7UVW

  2. Leong, 9W2LC

    Thanks for an interesting article!

    When I started designing car radios for Blaupunkt in the early 90s, the frequency was changed by driving ferrite rods into the coils. There were three sets of rods and coils for the oscillator and the two ganged input bandpass filters. The rods were moved in unison by a screw mechanism so that the pass-band and oscillator frequencies could track correctly. Several years later, the PTO era to an end when PLL tuning eliminated the tuning knob.

    If you are interested in changing the frequency in both directions, you may substitute with a brass screw. Iron based material lowers the frequency, but brass and copper will raise the frequency.

    The reason for commercial products using ferrite instead of an iron screw is to prevent Q degradation. Ferrite material has lower eddy current loss than iron because it is an insulator. This is the reason behind the progressively lowering of resonance amplitude with Hg insertion % as reported by Dave G7UVW. Hg like iron is a good conductor – therefore it increases the coil eddy loss when it is fully inserted :-). Hence you can never find an iron slug in Toko tunable coils.

    Leong, 9W2LC

  3. eddy current

    I think the ease of getting (getting harder now) decent variable caps pushed out the use of PTOs.

  4. Brian Burns

    Hello Scott,

    Nice to see that you have gotten a full 1 mHz shift with your arrangement! I’ve been trying to come up with a VFO that will allow coverage of the ham bands that are oddball multiples. Gone are the days when they were all integral multiples!

    One idea I’ve had for extending the range is to use a ferrite slug in tandem with a brass one. With the ferrite slug fully inside the coil the oscillation frequency would be at minimum. As you backed it, out and the brass slug began taking its place, the inductance would be decreased below its air core value, and the frequency should reach a higher maximum.


    Brian Burns

  5. hassan

    can you pls redraw diagram for this pto clear or by use any program because i think some differences between diagram and real pcb circuit
    tnx you so much

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