----- 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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