"Phil Allison" schreef in bericht news: snipped-for-privacy@individual.net...
So you seem to think. If you'd had any intellectual credibility left, that assertion would have blown away any remaining shreds.
The BBY40 has roughly the same capacitance as the OP's NTE613 at about 4V of reverse bias, but it seems to have a C2/C30 range of closer to 7:1 where the NTE613 only offers 2.9:1 (typical).
For the 0V to 26V tuning voltage swing used by the OP the BBY40 would offer about 45pF of capacitance change. Let us assume that the NTE613 does as well. We knw that the OP can tune over a frequency range of 88 to 96MHz.
This imples a 50nH inductance and a stray capacitance of 50pF. If adding another 4V of reverse bias across the diode reduce the diode capacitance from 5pf to 4pF, the LC product would change by 2% and the output frequency by 1%, jacking up the top of the range to 97MHz, which is 1MHz, not several. The OP wants to get to 108MHz, so getting even several MHz in this way would be wasting time and effort.
The actual capacitance swing available from the NTE613 is probably a bit smaller, so the real inductance and stray capacitance wil be appreciably smaller, not that it makes much difference to the arguement.
As everybody has pointed out, the OP needs to concentrate on reducing that "50pF" of stray capacitance - probably by replacing the inductor in use with a self-supporting coil wound with a gap between each turn equal to the wire diameter - five turns of 22 swg (0.7mm OD) wire on a 0.63mm OD (0.25") former is traditional.
Note the 4 to 44pF tuning capacitor and the nominal tuning capacitance of
12.5pF for 108MHz.
I'm still planning to - and have had the bits for some thirty years now. Sadly, I started earning enough money to be able buy decent commercial FM tuners at about the same time that I acquired a wife and house, which put paid to investing spare time in things that I could buy.
I do seemed to have learned more in the proces of not building an FM tuner than you've learned in the process of completing one - which isn't really surprising, granting the learning capacity you've exhibited on this user-group.