This is the DC receiver that I built back in 2017-2018. I had used a ceramic resonator in the VFO. That receiver was on the cover of SPRAT magazine. It may not have deserved the honor -- recently Dean KK4DAS and I discovered that the ceramic resonator VFO drifted rather badly. So Dean and I are now building real LC analog VFOs. This is kind of an aside to a Virginia Wireless Society -- Maker Group project. This video shows my receiver working yesterday on 40 using the VFO that was recently thrown together.
More details on the original project (that used the ceramic resonator) here:
The VFO circuit comes largely from W1FB's Design Notebook page 36. I followed most of the conventional tribal wisdom on VFOs: NP0 caps, often many of them in parallel. Air core coil (in my case wound on a cardboard coat hanger tube).
For C1 I used a big variable cap (with anti-backlash gears) that Pete N6QW advised me to buy on e-bay. Thanks Pete. L1 is on the cardboard tube. I only built the oscillator and the buffer -- I did not need the Q3 amplifier. (The water stain in the upper left is the result of a heavy rain in the Azores around 2002 -- water came pouting into the shack.)
I think the VFO is more stable than the Ceramic Resonator circuit. But I want to go back and give the ceramic resonator circuit another chance... Miguel PY2OHH has some really interesting ceramic resonator circuits on his site. Scroll down for the English translation: https://www.qsl.net/py2ohh/trx/vxo40e80/vxo40e80.htm
Dean KK4DAS commented that VFO construction is as much an art as a science. I agree -- there is a lot of cut and try, a lot of fitting the components you have on hand into the device you want to end up with. You have move both the frequency of the VFO AND the tuning range of the VFO. Mechanics (in the form of reduction drives) is often involved. And, of course you have to apply lots of tribal knowledge to get the thing stable. You could, of course, avoid all of this by using an Si5351, but I think that moves you away from the physics of the device, and is just less satisfying.
So, JOVO! LC JOVO! The Joy of VARIABLE Oscillation!
Nice work, Bill. The water stain on DeMaw's notebook adds soul to the project. For a receiver, an LC VFO is a cool project, sort of like hand-making whipped cream with a whisk (in a proper copper bowl). It tastes a lot better than the stuff out of a can--or seems to, at least. Copper is better than stainless steel, though, and the slightest trace of oil or fat in the bowl or on the whisk will kill the process, and the cream has to be within a narrow temperature range, and you might need to add cream-of-tartar, and . . . and it's a hell of a lot of work.
ReplyDeleteThat's okay, because sometimes you just want the real stuff. Other times, though, all you want is to spray some canned whipped cream on top of your otherwise-made-from-scratch strawberry shortcake. After all, you already picked, hulled, sliced, and macerated the strawberries, and you sifted flour and baking powder, cut-in the butter (not margarine!), and baked the shortcake. Surely no one could begrudge a little Reddi-wip. Could they?
There is more W1FB stuff on archive.org! https://archive.org/details/qrp_notebook_1st_edition
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