Until the advent of synthesizer IC's capable of working at 130 MHz and all electric tuning schemes, all automotive radios used PTO's, along with Permeability Tuned RF stages. There were typically three P-tuned coils per band, one for the LO, the other two the RF stages. They could easily be set up to track across the AM or FM band. The production tuning was actually done by machines in the later years when the capability became available.
The other reason for the PTO was that, especially in a mobile environment, there were fewer pressure based connections (think variable capacitors and trimmers) to become corroded and noisy. Microphonics are typically lower as well, so bumpy roads would not appear to affect the radio. Another benefit was much smoother tuning than could be accomplished with a rotary capacitor, and simple mechanicals, unless you had one of the signal seeker jobs! What does this have to do with Drake's? My guess is that Drake, Collins, et all, appreciated the same benefits, as they were also interested in mobile applications. Tom Holmes, N8ZM Tipp City, OH EM79 > -----Original Message----- > From: [email protected] [mailto:[email protected]] > On Behalf Of Richard Knoppow > Sent: Friday, February 18, 2011 9:08 PM > To: K9sqg > Cc: [email protected] > Subject: Re: [Drakelist] PTO/VFO > > > ----- 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] > > > _______________________________________________ > Drakelist mailing list > [email protected] > http://mailman.zerobeat.net/mailman/listinfo/drakelist _______________________________________________ Drakelist mailing list [email protected] http://mailman.zerobeat.net/mailman/listinfo/drakelist
