Re: Phil Allison can't do FM design calculations.


"Phil Allison" <philallison@xxxxxxxxxx> schreef in bericht
news:4844u5Fi8fb5U1@xxxxxxxxxxxxxxxxx

<bill.sloman@xxxxxxxx> = a criminal LIAR


( snip piles of UTERLY irrelevant *** )


Unfortunately fig 2a of the application note I cited shows exactly what
I claimed.


** It completely contradicts your claim -.

So you seem to think. If you'd had any intellectual credibility left, that
assertion would have blown away any remaining shreds.

This one:

" .... increasing the voltage across the OP's diode from 26V to 30V won't
produce anything like a 30:26 change in capacitance, and would be a
complete waste of time, "

The published data for the diodes like the BBY40 shows that the varactor
diode will drop by a significant amount the high end of the voltage range.

In the OP's application, a reduction of *1 pF * will shift the 88 - 100
MHz oscillator frequency several MHz higher.

Going from 26 to 30 volts does just that.

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.

http://www.boondog.com/tutorials/rfTransmitter/rfTransmitter.htm

Note the 4 to 44pF tuning capacitor and the nominal tuning capacitance of
12.5pF for 108MHz.

Try building a voltage tuned FM tuner some day -

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.

--
Bill Sloman, Nijmegen



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