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Re: Follow-up Antuino question

 

The H and P works only with an already stable vfo that has a long term drift under the locking range. It cant fix a badly made VFO.
That said, I should repeat that a stable VFO is so not merely by design but construction and choice of components.? For instance, if you use the standard disc ceramic caps, it is bound to drift. If you keep it open without a shield it will drift too. At times, the heat from a regulator can move the frequency.
A stable vfo is made with a few rules of thumb:
1. Build the entire VFO circuit using point to point wiring directly soldering component leads to each other. Keep the leads short but dont stuff them very close together.
2. Use polystyrene capacitors if you can. These are transparent capactors with silver colored body cylindrical bodies. They are available from mouser. You will need high value (about 100pf caps paralled up) for resonating at the design frequency and low value (2.2pf) for coupling to the JFET.
3. Use either a T50-6 toroid or an air core inductor wound on a relatively stable thermal expansion. A teflon rod is a good alternative..so is a test tube. A drinking straw is not! Even free standing copper inductors change their shape as copper expands the wiring. It has to be held in place with clear nail polish lacquer.
4. Avoid the polyvaricons, they are noisy, rough tuning and drifty. Hunt around for the old, all metal broadcast tuning capacitors. Mount the VFO board on the tuning capacitor or mount the tuning capacitor on the VFO board. No loose wires!
5. Mount the regulator at least 2 inches away from the VFO's capacitors and the inductor.
6. For low noise operation, a JFET is the best choice. You have to bypass the voltage regulator at the input too, as well as the output. Mosfets make noise VFOs due to their increasing flicker noise at lower frequencies.
7. Avoid using varactor diodes, they are meant for PLLs where drift of the VCO is a given. A varactor that is used as fine tuning over a few khz is acceptable in parallel to a tuning capacitors. The drift will be proportionally less. A combination of a variable tuning capacitor and a varactor over a small range is a good, backlash free alternative to a good slow motion drive.
With these guidelines, it is possible to consistently produce VFOs that are stable enough for CW and SSB work. Even PSK31 and FT8 are possible with effort. An interesting idea is to cancel the thermal drift by using a diode like the 1N914 to sense the increased ambient temperature and provide correction offset DC to the varactor.
I am worried that we are losing the skill to produce VFOs. It is fundamental to our persuasion. Generating stable and clean RF is the start of any radio project.

On Mon 16 Nov, 2020, 6:21 AM Arv Evans, <arvid.evans@...> wrote:
Tom

You got it!? analog VFOs used to drift all over the band.? Then came the Vackar design?
which was very stable.? Next seems to have been H&P which made even a? Vackar design?
better (cheaper components and less stringent builds) stabilized by H&P).? With PIC?
and AVR/Arduino?came really good and inexpensive FLL designs.? PLLs were always?
available, but seen by many as too complex and expensive (not really true but still hard?
to dispel?the myth, mystique, hype, and traditions).

H&P was seen as an easy and quick fix for old equipment with drifty VFOs.

Arv
_._


On Sun, Nov 15, 2020 at 5:24 PM Tom, wb6b <wb6b@...> wrote:
On Sun, Nov 15, 2020 at 03:57 PM, Arv Evans wrote:
It tries to hover around one particular frequency.?
So in following the part of this thread that is strictly related to Huff and Puff, was the original reason for the Huff and Puff circuit to create a manual VFO that would jump and hold in accurate increments rather than be free to drift around uniformly. A way to allow operators to dial up drift-free discrete frequencies before digital frequency synthesizers became the primary way to set the frequency of a radio?

The non-drifting VFO settings were more the goal than the actual circuit details of Huff and Puff, itself. Huff and Puff was likely chosen because it was easily implemented with a simple type-d flip-flop, in the pre microprocessors everywhere days?

Tom, wb6b

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