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Permeability Tuned Oscillator (PTO) Working Nicely

My last entry described my accidental discovery of the PTO for QRP purposes. I breadboarded it and was amazed at the results! I went ahead and built this carefully in an enclosure and the output is wonderful. It’s strong, it’s stable, and it tunes effortlessly over the same range it did before (about 1MHz). The video describes details of the action, and demonstrates the stability of the oscillator by letting you hear it audibly on a nearby receiver.

The fundamental concept and hardware is straightforward. Two nuts are soldered into an Altoids tin providing much-needed grounding for the screw (reduces shift when it’s touched). Also the wire soldered over the screw is pinched firmly at the base to apply constant pressure to the screw to make it hard to turn and therefore more stable while turning. The inductor is a bunch of turns (no idea how many, about a meter of magnet wire) around a McDonalds straw.

Alltogether it’s a simple colpitts oscillator with a MPF102 JFET at its heart, using a 74hc240 CMOS buffer as an amplifier. There’s a voltage regulator in there too.DSCN1356

The result? Pretty darn stable (by CW QSO standards).  That’s without any regard to thermal isolation or temperature compensation. I’m quite pleased!  I look forward to MUCH more experimentation now that I’m starting to feel good about designing and building simple, tunable, stable oscillators. It’s always hard to nail all 3 in a single device!

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/2011-06-05-permeability-tuned-oscillator-pto-working-nicely/


  1. dave

    Nice work!

    If you do have stability problems, then you could move the voltage reg out of the case. Those linear regulators can run quite warm, having something dissipating a few hundred or more mW of heat right next to your oscillator is a good recipe for drift.

  2. Scott

    This was a dilemma for me because I was previously told to move the regulator INTO the case. The reason was to prevent stray capacitance and inductance causing drift. Previous designs of mine would drift just by waving my hand near the enclosure. I believe this was because the power wires entering the device acted as a bit of a capacitor with the case? I guess it’s possible this was cured with the all-metal case as well, so it’s not a fair decision to make this early…

  3. Dave

    If the supply is well decoupled, and fed into the VFO case though a feed-though capacitor then it really shouldn’t “pull” the VFO frequency. Ultimately it comes down to what works for you – if the VFO is stable enough for your liking, then there is no good reason to change the layout.

    Best of luck!


  4. AI4YC

    So…. are you bringing it to the next GARS meeting?

    Have you played with it on a receiver?

  5. ulimited hosting

    You could certainly see your enthusiasm in the article you write. The arena hopes for even more passionate writers such as you who are not afraid to say how they believe. Always follow your heart.|

  6. Charlie

    Scott, in fact you could use the voltage regulator to stabilize the temperature of the oscillator. I would stick the TO-220 under the oscillator’s PCB. I the power dissipated by the regulator is fairly constant the temperature of circuit should stabilize.

    Regards, Charlie.

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