I am finding that the choice of the spider web coil form is good for use to make a VFO at frequencies up to 30 MHz with the existing Colpitts oscillator of the SR 40 V6. The wisdom of this is in the 8x division that occurs in the clock circuit of the board.
I have used the MFJ-204B Antenna Bridge as a signal generator and sort of dip meter to analyze the coil with a frequency counter. Using a 75 pF tuning capacitor with wide plate spacings. It is easy to make the tuning circuit resonate in a predictable way and also trim the coil turns to adjust its tuning range as desired before mounting in the finished product. Likewise I do have a single coil tap to select if needed. I am using a range of tuning here of from 14 to 25 MHz with the coil tapped.
I am certain that this type of coil will allow one to design a Huff & Puff Stabilized VFO as high as 30 MHz. Likewise if the output is divided down 8x then the stabilized VFO will then appear to be extremely stable on frequency and no further need of schemes to control it will be needed. The effects of step jumping as well as jitter is divided along with the phase noise bandwidth being divided. The idea of adding CPU control doesn't even come into the picture here. Extreme control can be had with this idea. And minimal parts. Lest one wants to add the charge pump to the Huff & Puff and add a few more parts. But I am not certain that the charge pump is needed yet?
If one used the SR 40 V6 as I plan to do then the 8x frequency division of the VFO made from the existing on board Colpitts oscillator will divide the VFO down 8x and also reduce the drift and be somewhat stable for use in any i.f. in the 2 to 3.75 MHz range. If one makes a VFO that tunes around 3.64 MHz then when divided by 8x on the SR 40 board they will be able to tune back and forth across .455 MHz. Stability of the VFO after divided down like this should be good. {At this low down however one may need to experiment with the values of C5 and C6 if needed to effect feedback for oscillation. Then again not, only those who try will know.}
It appears that the best way to make the VFO is to remove the the 28 MHz crystal and then add the coil and capacitor in series with each other to the transistors base input in place of the crystal. I am not certain that the VFO that uses a parallel combination of the coil and tuning capacitor will work effectively in the existing circuit on the board: one may try however but I suspect that C5 and C6 have to be changed somewhat.
Make sure that the hook up wire to the tuned circuit is not subject to vibration meaning that it is stiff and not easily moved and this will minimize its effects due to thermal changes.
ka9rza
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