Rap 'n Tap
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From: Gabe Velez
Date: 9/3/99
Time: 9:55:02 AM
Remote Name: 205.219.233.54
Hi there.
1) Rectification occurs on the razor blade because there is a coating of some kind of silicon or other crystalline material. The rectification occurs when the reverse flow adds to the "barrier" set up by the point contact of metal to crystal (the negative half of the RF AC). I can get deeper into this but I don't think that there will be enough room. The flow of current in the "forward" condition does not add to the barrier so there is conduction, hence the rectification.
2) I don't know much about toroids to reply.
3) The reason that the table radio is sharper has more to do with the local oscillator than the loading of the tuned circuit. A bipolar transistor *will* load down a tuned circuit, hence why most BJT radios have a low impedance tap on the tuning and IF coils. The reason a table radio is more selective is because the local oscillator causes the tuned circuit to have a higher Q (Notice most LO circuits are one and the same as the first RF amp). This is also why regenerative radio circuits are higher in selectivity. The feedback causes the coil to have a higher Q. Q is defined as the ratio of power out to power dissipated by the coil. If we add more signal to the coil by means of feedback more signal will come out as opposed to what is being put in by the antenna. Higher Q.
In the superhet, the LO adds a very strong (relative to incoming signal) local signal to the tuned circuit. Any incoming signal will cause the LO to modulate with it. The stronger LO signal makes the tuned circuit have a higher Q. (Incidentally, an oscillator can behave like an inductor).
If the original poster really wants to get selectivity, he could use an FET or triode as a detector in grid leak operation. This way he will not load down the tuned circuit and get better selectivity. But then it will no longer be a crystal set.
Hope this helps some.
Gabe gvelez@trigon.com
From: Michael Cathcart
Date: 3/18/99
Time: 9:02:38 AM
Remote Name: 205.188.193.172
I will try that, thanks Paul. Am I to take it then that a 1.5v "AA" battery is too much? :-) Mike