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From: Charlie Cotterman
Date: 8/23/00
Time: 9:41:45 AM
Remote Name: 140.106.192.1
I know that most of this stuff has been spoken of before somewhere, but being short on consecutive brain cells today I can't find it.
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Head/earphone bypass cap: How much (200 pf or so seems to ring a bell...)? Is it needed at all? Is it needed with the little Mouser transformer setup and 'phones Scott sold me (500 or so ohms DC resistance)?
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Various variometer thingies (and thanks to all who recommended the below sites - the images were a major league help)
http://connix.com/~harry/drcrystl.htm This site shows two variocoupler designs, one very interesting. It needs 180 degrees of shaft revolution to fully 'mesh' ; this sounds to me a better setup, as it's easier to tweak in on a specific station when you're running that razor selectivity. (See "Coupler, Variocoupler Type" and follow the link "shown"). I like this setup, for both the above reason and that you don't have the coils interrupted in their winding by the rotor shaft. Looks to be easily built -- just wind it on your standard forms, but cut the stator form on a 45 degree angle and go from there.
There's also a table ("Variometer, Coupling With", follow link to "table") that shows the max/min inductances a variometer needs with a specific fixed capacitance to tune the BCB (I think). Gordon commented on putting a varicap with his variometer -- the more I look at that, I'm of the opinion that it's not necessarily a bad thing. Maybe a midrange 350pF for lower total inductance and a 100pF for something wound with a bit more accuracy. That way, if you miss on the inductance a tad, you can bring the circuit back into resonance with a tweak of the cap. On the other hand, see the next section.
http://www.radio.mcmail.com/vario-xtal.html shows a neat little variometer-only crystal receiver. Note here, there's no fixed cap in the circuit. Does this setup just depend on the capacitance between the windings, or did I miss something? Also, as I understand it these beasties aren't quite as sensitive as a coil/varicap setup. That being a given for the argument, with the appropriate RF-pre-detection-amplification or AF post-detection amplification, would a pair of these variometer setups (in series) be anything resembling a very selective circuit? I'm assuming they'd be wound identically. Ideas for form sizes and clearances would be appreciated.
To quote connix, "... the combined inductances will be more than double that of either coil alone..." when referring to two coils wound correctly for variometer use. The individual coils can be designed with any of the common coil design programs. Is the final result always 2+2=5 or is this a case of "wind it and see"? The varicap here would change the resonance if by coils alone things are too high. (Gordon, this may be the solution to the problem you had -- your overall inductance was more than you thought it would be, throwing your coverage calculations off.)
Where can I get the scoop on the variometer design used in the XS-104 (form sizes/wire sizes/turns/etc)? If it's goood enuff to go in a Society kit, it's good enough for me to twiddle with...
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I do believe I've caused enough brain cramps for one post...
Charlie