PTOs better than CTOs. Why?

[K6JEK]

PTO's are better than Capacitor Tuned Oscillators aren't they, more stable, more linear. I realize I don't know why. Is it because the components are less affected by heat? Is it because they are smaller and can be sealed up? Is it because the motion is linear so can be fancified with clever mechanical compensation cams and such?

I just don't know.

[Opcom]

The fine threads moving the slug in and out inside the Collins-type PTOs seem to help with repeatability and granularity.

[WQ9E]

It is easier to achieve a linearity in frequency change versus motion with a PTO via core shape and variable winding pitch compared to a capacitor tuning setup which provided a great advantage with readout prior to electronic digital counters.  The variable element is also easier to make more compact and rugged providing packaging and mechanical stability advantages.

[W4EWH]

PTO's are better than Capacitor Tuned Oscillators aren't they, more stable, more linear. I realize I don't know why.

When a VFO has a variable capacitor, it's the actual capacitor which is being changed to shift the frequency, so there's a compromise between mechanical rigidity and usability, which makes them prone to microphonics, backlash, and vibration.

A PTO doesn't change the inductor; it moves a slug in the middle of the inductor, which changes the inductance in a much more predictable way. However, it also slows down the rate of change to the point where the VFO has a more limited range, such as the 200 KHz end-to-end spread of a Collins 75S-3, 32S-3, or KWM-2. It's not as much of a PITA for military operations, where frequencies don't change that often, but it's a challenge for contesters.

HTH.

Bill, W1AC

[nq5t]

It's true that a (good) PTO has inherently greater mechanical stability that a CTO, and that it's easier to linearize it (screw pitch, winding pitch) , though in a PTO with a wide frequency range that comes at the price of significantly increased mechanical complexity as in the 1Mhz PTO's from Collins and others in the 390 series, 51J series, 75A4, etc.

There is a downside in older PTOs.  The characteristics of the ferrite change over time, and that can require periodic surgery to get things tweaked into reasonable shape.  It's a major PITA when restoring some old Collins gear, and even after adjustment, they can slowly drift out of wack again over time.

On the other hand, there are some pretty good CTOs out there, too, such as the TRW (?) CTO in the SB-102.  The CTO in the Hallicrafters SX-115 is also surprisingly good — very linear, and stable enough.

[Jim, W5JO]

On the other hand, there are some pretty good CTOs out there, too, such as the TRW (?) CTO in the SB-102.  The CTO in the Hallicrafters SX-115 is also surprisingly good — very linear, and stable enough.

There were a couple of other Halli receivers that attacked the problem of the oscillator section drifting as the variable expanded as it heated.  I seem to recall that section had plates made of some material that would not expand very much at all as they heated leading to the stability of the oscillator.  Also the B&W 5100 had a very stable oscillator.  it ran down around 1.7 meg and had similar plates in the variable.

[K6JEK]

Thank you everyone for the answers.

[w3jn]

One other point - a PTO has no moving electrical connection.  Variable caps have end bearings and wipers which both contribute heartily to instability and resetability - hence linearity.

[K1JJ]

As proof to what has been said here:   The Collins and Drake PTO's have stood the test of time for stability and general performance.  These manufacturers had a choice and decided to do it right in all aspects.

Compare their FB rigs to an Eico 753, Galaxy V, Swan 350 -  all  using variable capacitor VFOs.

(Did he say Eico 753?)  

T