Peter Lawton G7IXH
Huff & Puff overview
The first design for the Huff & Puff stabiliser was published in 1973 by the late Klaas Spaargaren, PA0KSB. However, all but the most stable of VFOs were beyond the stabilising powers of this first H&P and it was only in 1996 that Klaas described the first practical Huff and Puff circuit.
The third step was in 1998 when QEX magazine published my article, giving details of a new design which I called the 'Fast' Huff & Puff stabiliser (thanks to Klaas who tested and endorsed it and to Pat Hawker, who first published the circuit and an outline of its operation in 'Technical Topics', for their encouragement).
The fast H&P addresses two weaknesses in earlier designs, i.e. (firstly) slow response to VFO frequency changes, and (secondly) unnecessary over-complication. It has a much quicker response to any VFO frequency fluctuations and hence confers the ability to counteract a far greater drift rate, and has fewer components. If you are building a stabiliser, the Fast H&P is the only circuit to use. Previous designs are of interest only.
A brief description of the fast H&P is as follows...
The heart of the device consists of only two ICs: -
1) A (High Frequency first stage) shift register. The VFO is applied to the input. I tend to use a separate D-type latch for the first stage. This allows the use of a many stage (but low frequency capable) shift register IC for the rest of the stages. The more stages the better. In the diagram beneath the "IN" point is from the output of the D type latch (or from the output of the first stage of the shift register).
2) An XOR gate. The two inputs to which are a) from the first stage output of the shift register, and b) from the last stage output of the shift register.
The XOR output will be a stream of pulses (not in general a square wave), but the interesting (and useful) thing about it is that the average voltage of the output is a function of its input frequency. You can test this with an old-fashioned voltmeter at the XOR output which will repetitively cycle between 0 and 5 volts and back to 0 volts as you gradually and evenly increase the frequency of the input to the data latch. It would have to be an old fashioned voltmeter with a heavy coil and pointer in order to show the AVERAGE voltage - remember that the output from the XOR consists of a stream of pulses, so at any particular instant it will either be high (5V) or it will be low (0V).
3) Lastly, an integrator with a standing bias of 2.5 volts. The output of the integrator controls a varicap placed across the VFO coil.
The effect of all of the above is that the vfo hovers around the frequency which produces an average XOR output of 2.5 volts. It is important to realise (I'll say it again!) that the XOR output at an instant can only be either 0 volts or 5 volts. An average of 2.5 volts means that it is a square wave at that point (half the time "high", half the time "low"). Note finally that the "step" (distance between lock points or "hover points", in Hz) is a function (among other things) of the delay time between the two data streams in the frequency to voltage convertor. It is best to achieve the required delay for your chosen "step" by using a fast clock with many delay stages (use a shift register, say) rather than a single delay stage and a slower clock. The faster the clock, the more powerful is the stabiliser, simply because it will catch any drift sooner and correct it before it can accumulate.
And a final, final note - The above describes a "type 1" stabiliser (uses the reference frequency derived from a crystal oscillator for the two "clock" inputs to the stabiliser. The "clock" and the "signal" can be swapped over to produce a "type 2" stabiliser, which will have somewhat different characteristics (in particular a dependence of the "step" on the signal frequency).
A few numbers: -
The formula for the VFO step for a type 1 stabiliser is
If the signal is used for the clock, the formula for the VFO step is
Applications for the H&P:
The fast H&P has a powerful lock if the parameters are adjusted
correctly - BUT it is wise to realise that if the lock of a H&P is broken, there
is no mechanism for it to return to its previous frequency.
Because of this, it is absolutely necessary to isolate it and its
associated VFO from mechanical and electrical transients.This may be more difficult to accomplish in
valve (tube) radios. In such circumstances one might be better off
using Ron Taylor's
which I believe retains a memory of the frequency, and will try to
return to it, as long as it hasn't been thrown too far away from it by
1) Fast Huff & Puff simulator program
1) Fast Huff & Puff simulator program
2) Visual Aid 1