Hi all
Huff & Puff was a passion of mine. I was licensed in 1994, but for various personal reasons did not go on air. I did keep reading RadCom (monthly journal of the RSGB) and was fascinated by the Huff Puff articles which appeared in Pat G3VA (SK)'s monthly "Tech Topics" column. I collected everything I could find about Huff Puff and started making my Huff Puff library, eventually putting it online on my website that I created, initially just for this purpose, in 1999. So... everything grew from these roots!
In late 2001 David WN5Y emailed me and we started discussing Huff & Puff, he was using it in his "Electroluminescent Receiver" which he still to this day sells as a kit see??- and I actually finally purchased one of his kits a year ago but still have not finished building it! Anyway - as a result of that correspondence, I decided to also build my first Huff Puff circuit. It was also the first time I had built ANYTHING at all for 8 years. I wanted to try the magnetic field method of varying inductor core permeability that David used. The result was my stabilizer??and I built a 14MHz VFO that was stabilized by this circuit.?
That then became, with a mixture of other circuits also from Pat G3VA (RIP)'s column - such as the Tayloe detector (Quadrature Sampling Detector) and Polyphase networks - my first amateur radio receiver which you can read about here: .?
Then I built my 1-valve (tube) CW transmitter which was also from G3VA's column.?
My ATU was built around 1984 and was also a RadCom project at some point see ?
That was the station, in March 2002, that I used for my first ever QSO see??
So. This Huff Puff stuff is a very important part of my personal radio history. Later, the website was expanded to include lots of old projects, and I started adding new ones... so really 2002 was the birth of my amateur radio life.?
Anyway I wanted to say a few things about Huff Puff.?
1. PLL vs FLL is a debate that raged on sometimes (PLL = Phase Locked Loop, FLL = Frequency Locked Loop). A Huff Puff circuit is not exactly like either. You can make a reasonably convincing argument for why it is not a PLL or why it is not a FLL. However if the assumption, from your proof that it is not a PLL, is that therefore it must be a FLL... or vice versa - then you probably start falling into difficulty. I think not everything is as simply classifiable into one of two categories.?
2. A Huff Puff circuit doesn't exactly lock a VFO precisely on any particular frequency. What it does is compare pulse edges and try to line up the pulse edges. One is a divided down timebase, the other is the VFO. Actually which one is divided down can be swapped. It is continually hunting, around a target frequency. There are multiple stable target frequencies, separated by typically 10, 20, 30Hz etc depending on how your circuit is designed. The Huff Puff circuit will try to steer the frequency to the nearest target frequency.?
3. One critical aspect of Huff Puff is that the correction power of the circuit must be sufficient to correct any drift occurring, but not so aggressive that it overcompensates or that it does not allow you to tune the VFO normally. This is quite hard to set up properly. If the correction capability of the Huff Puff is too weak, then the VFO my drift far enough before being corrected, that the Huff Puff circuit "jumps" to trying to move it to the next stable target frequency. If the correction capability is too strong then it would make it hard for you to manually tune the VFO; additionally it makes for large excursions around the target frequency (which if extreme, could also make it "jump" to the next target frequency.?
4. If correctly set up, the Huff Puff circuit does not add much in the way of any objectionable sidebands or phase noise. In the days when PLL synthesizers were often not well designed, leading to quite high levels of phase noise, the way a Huff Puff circuit retains the cleanliness of the underlying VFO was said to be one of the advantages. That and the inherent simplicity (low parts count, at least).?
5. As Allison KB1GMX said, and what Pat G3VA said before, something like: "A Huff Puff circuit does not turn a bad VFO into a good VFO; it turns a good VFO into a better VFO". Every effort should be made, to make the VFO as good as possible before trying to add a Huff Puff circuit to make it really perfect!?
6. The "Fast" style Huff Puff circuit developed initially by Peter G7IXH, is effectively like an array of ordinary stabilizers all acting in parallel on the same VFO. What it allows you to do, is
a) stabilize a worse VFO?
b) stabilize a good VFO better
Using it for a) is not a good idea, according to the make-the-VFO-as-good-as-possible-first principle. Stabilizing a good VFO better, means that you can make the Huff Puff corrections much smaller, it means that the frequency excursions become a lot smaller. The circuit is also a lot more forgiving when setting it up.?
7. Some practical and simple designs I worked on are here:??, which are targeted towards minimalist implementations. The 1-chip version??was an experiment to really see how far a Huff Puff stabilizer can be minimized. It is very inefficient and not easy to set up. I don't recommend it for other than curiosity value. The 2-chip "Fast" type??with discrete-component XOR gate (though an ordinary XOR gate could also be used) is a good circuit, it works reliably and efficiently and isn't fussy to set up.?
Overall Huff & Puff circuits are a bit dated now but who cares, they are really fascinating to play with and very educational.?
73 Hans G0UPL
