'TT', July 1973, included some notes from Dick Rollema, PA0SE, translating into English an outline of an ingenious 'Crystal-Stabilised VFO' drift-correction system that had originally been described by Klaas Spargaren, PA0KSB, in the Dutch VERON journal Electron, April 1973. The system seemed to offer the promise of the stability of a frequency synthesiser without incurring the excessive spurii and phase noise of the early PLL synthesisers. The system was intended to provide small but stable incremental frequency steps when used with an existing reasonably-stable VFO.
At the time, I added a note: "This VFO-stabilising technique would seem to be along comparable lines to the 'Racalator' [a commercial unit designed for Racal by Keith Thrower that he had demonstrated to me a few months earlier and reported in Electronics Weekly]. PA0KSB's system appears to be basically similar, but with a degree of tolerance that makes it highly suitable for amateur operation. Since one is using a DC control system in conjunction with a fundamental [free-running] oscillator, there will clearly be none of the problems of spurious mixer products associated with some types of frequency synthesis. In fact, it seems to represent a VFO technique that could be extremely useful for either HF or VHF operation. I would be interested to hear from anyone who gives the idea a try." My plea produced an informative and useful letter from a former Hanslope Park colleague and indefatigable trier-out of new ideas - the late Joe Cropper, G3BY. He reported trying out a slightly modified form of PA0KSB stabiliser. He wrote: "So far it looks as though PA0KSB has come up with the answer to the VFO problem. When tuning, the frequency varies normally with virtually no sign of 'steps' (the steps can just be heard if tuning is fairly fast but, at normal careful search speed, cannot be heard at all), and then when left stays there. It is not a cure for a bad VFO, but the system makes a good one very good." His mixer-VFO with the VFO in the 5 to 7MHz range was found to have a drift of 50OHz in half an hour without the controller; with it the output stayed within a few hertz for 45min. He pointed out that the July notes covered only the barest outline and added some further explanatory notes on "this most promising technique."
G3BY added that watching the control voltage pulses on a scope reminded him of the old 'hit and miss' gas engine governor where this lifts one of the valves to reduce speed resulting in a series of "thump, thump, gasp, thump, thump, gasp, gasp." This encouraged me to name the system as 'the huff and puff VFO', a name which has stuck, although I don't think that PA0KSB ever really approved of it. The system gradually became known in amateur circles I the world. A CMOS version was described by PA0KSB in Ham Radio, December, 1977, summarised in 'TT', April, 1978 and later in the 'Building Blocks' chapter of the sixth Edition of Radio Communication Handbook. ARRL's QEX (February, 1995), published a detailed explanatory article 'Frequency Stabilisation of L-C Oscillators', which I noted in 'TT' July 1996, and supplied photocopies to several interested readers. That year G3DXZ, G3GKG and G7IXH came up with further modified forms, some of which were endorsed by PA0KSB who also prepared a Dutch-text version of his QEX article (Electron, 1996). Sadly, in 'TT' December 1999, 1 announced that PA0KSB had become a silent key at the early age of 62 years.
So much for the history. My excuse for including this month a current version of a Huff & Puff stabiliser (Fig 1) stems from finding one described by an old friend of 'TT' - F H V Geerligs, PA0FRI -'VFO-stabilisator Getest en Toegepast' (Electron, May 2004, pp215/216) in conjunction with PA0CMU. While my interpretation of the Dutch text is abysmal, it would seem that this stabiliser, although based on PA0KSB's 1995 CMOS design, does have some added features that should ensure its suitability for use with any VFO (capacitance- or permeability-tuned) within the popular range between 5 - 9MHz. It can be assembled on a relatively-small PCB. Selection of a overtone crystal unit within the range 25 - 66MHz provides small incremental steps of 4 - 16Hz. Clearly with these small steps, the basic VFO must already have good stability (such transceivers as the Yaesu FT-7, FT-401, Ten-Tec Corsair II and a good home-built VFO are mentioned). When built on a 5.5 x 6.5cm. board, it can be accommodated within the transceiver. The 2.2pF capacitor (marked MKM) in the integrator circuit should be a low-leakage type (polycarbonate or polystyrene). If you need further information - and particularly if your Dutch is better than mine - print, photos and more information can be downloaded from PA0CMU's site: ????
Circuit diagram of the Huff and Puff VFO-stabiliser as implemented by PA0FRI/PA0CMU.