One of the major complaints about LC VFOs is that they allegedly cause "non-linear tuning." Essentially, this complaint claims that you will inevitably end up with your frequencies all "bunched up" at one end of the tuning range, with frequencies greatly "spread out" at the other end. BUT WE HAVE FOUND THAT THIS IS NOT NECESARILY TRUE. With a bit of careful design work, you can avoid the dreaded "bunching up." I have used the calculator in Bob's Electron Bunker to DESIGN VFOs that do not "bunch up" the frequencies.
Recently, Mike WU2D built a version of the Simple X Super receiver. It has a VFO in the 5 - 6 MHz range. And guess what? There is NO bunching up of frequencies. As proof, I present the tuning dial that Mike made for his completed receiver. See above. Note the nice linear tuning.
So you see, success in this area is possible -- all it takes is some careful design work.
Similar results are possible with the other major allegation about LC VFOs: That they are inherently and irreparbly unstable. Proponents of this theory would have you believe that only by the use of an Si5351 (or something similarly digital) is stability possible. Again, NOT TRUE! It is possible to make LC oscillators that are stable. All it takes is careful design and good construction practices.
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But of course you can get perfectly acceptable stability from the LC VFO's! Good design and choice of stable, Hi-Q components give excellent results. Sturdy construction is key. The type (who it's named after) matters little, if anything at all. And sure, the synthesizers work as well.
ReplyDeleteSet a stability criteria. Some publications have proffered 100 Hz/Hour after a 10 minute stabilzation. There are others. Then do a stability run. I automated mine, so I can run it and leave. See https://www.qsl.net/wn2a/MOUSEFET_11.pdf
pages 18-19. On 30M CW, I've been told "you must be running an XTAL rig". That was after a 1 hour QSO.
Just use what works for you and Get On The Air. 73!
Mike! Your dial is a thing of beauty!
ReplyDeleteThe Simplex Super is such a cool design someone (maybe me,) has to make one Solid State!!! I would
ReplyDeletepesteri Mike, but he is burnt out on it
Ok Soldersmokers, it really was not an attempt to linearize anything, it's just what happens when you have a heavily capacitor loaded tuned circuit and only tune a small segment like 500 kHz.
ReplyDeleteThe benefit of spreading the 40m out was due to the added padding needed for the 2 MHz
2 MHz IF rather than the original article's 1.7 MHz.
ReplyDeleteHaving built about fifteen solid state VFOs since 1979, my hands-down favorite for stability up to at least 7 MHz is Farhan's Daylight Again design, using a 9 volt regulator, otherwise similar to his schematic. There is only one critical capacitor affecting drift to worry about and that is C31.
ReplyDeleteLinearity is quite good also. I have also built a version with varactor diode tuning, with the coil screw in a fixed position.