Hi Richard:
Great Info.
THe Drake PTO coil is wound in the way you describe. It also has a core
stack that is made from multiple pcs attached to the brass rod. I
always thought that by altering the spacing of the cores on the shaft,
that linearity could be tweaked. I have never attempted to verify this
but think it might have been a possible solution. The pictures of the
original Drake Mfg test fixture where the PTO's were adjusted is
interesting but it was never actually described as to how they were
adjusted. Perhaps just moving the windings around.
Would be an interesting weekend I think :)
Curt
KU8L
Richard Knoppow wrote:
----- Original Message ----- From: "K9sqg" <[email protected]>
To: <[email protected]>; <[email protected]>
Cc: <[email protected]>
Sent: Friday, February 18, 2011 3:56 PM
Subject: Re: [Drakelist] PTO/VFO
Typically, a PTO refers to a permeability tuned oscillator. In other
words, it tunes the inductor with a core that moves in and out of the
coil. A VFO typically refers to an oscillator that varies a capacitor
to vary the frequency. However, VFO is sometimes used to refer to a
traditional VFO, a PTO, and even VCO.
Collins also seems to use both terms although theirs _is_ a PTO.
I've also seen other confusing terms used, I think Hallicrafters
called theirs a LMO (Linear Master Oscillator) or something of the sort.
The advantage of a permeability tuned oscillator is that its fairly
easy to get a linear frequency scale by shaping the coil so that the
inductance has a square-law relation to linear movment of the core.
Its possible to shape the plates of a variable air capacitor to
achieve the same thing but the plates become extreme in shape and are
hard to make. Such "straight-line-frequency" capacitors were offered
by Cardwell and Hammarlund, maybe also National, in the early 1930's
but I think they proved to cause more problems than they solved.
There have also been oscillators with simultaneous variation of
inductance and capacitance, mostly for VHF/UHF applications. This has
the advantage of more linear dial calibration and probably also higher
Q. General Radio used this in one or more of its instruments and I
think had a patent on it.
A problem with the PTO is that its difficult to get the coils to be
exactly square-law so some means of correcting them is needed if the
dial calibration is to be accurate without individual calibration.
Collins used a "corrector stack" consisting of a series of thin
washers clamped by a bolt. The washers could be slid to make a cam
surface according to the correction needed. The surface was followed
by a roller attached to a lever which could move the position of the
nut driving the core just a little. the result was that the exact
frequency could be adjusted continuously along the working length of
the coil. Other manufacturers made similar arrangements that
accomplished the same end without infringing on the Collins patent. I
don't think Drake uses any such arrangement.
--
Richard Knoppow
Los Angeles
WB6KBL
[email protected]
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