M1KTA’s QRP ham radio blog: PTO VFO

January 28, 2010 | Amateur Radio | By: Mark VandeWettering

If you go back through lots of amateur radio designs, you’ll find many, many circuits that use the nearly uniquitous 365pf air spaced variale capacitors that were nearly ubiquitous up until about 25 years ago. In the last couple of decades however, they have become like Avatar’s unobtanium, seemingly impossible (or at least expensive) to find. One solution to this problem is to use varactors controlled using variable resistors (which are still relatively easy to find) but another interesting technique is to build your own variable inductors. Hence, was born the PTO (permeability tuned oscillator), a nifty little homebrew circuit and gizmo that can provide a variable frequency oscillator. M1KTA talks about building one of his own:

M1KTA’s QRP ham radio blog: PTO VFO.

I’ve had this in the back of my head as an interesting project, so seeing notes on someone building one is inspiring.

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Comments

Comment from Dom
Time 3/20/2010 at 3:03 am

The callsign is M1KTA :-)

Comment from Randall
Time 3/30/2010 at 4:55 pm

Did not know that you can’t fine variable compactors any more.

In the past I used one to build a indoor am loop antenna.

Comment from Scott
Time 6/4/2011 at 1:05 pm

I stumbled upon this concept accidentally, built one, and just now realized it’s called a “PTO”! The concept should be used far more often than it’s given credit for on the net http://www.swharden.com/blog/2011-06-02-screwy-oscillator-idea/ I pulled a full MHz swing off a screw going in and out of wire wrapped around a McDonalds straw!

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