Varactor-tuned oscillator

"fourblackcats"

Is the varactor the only parallel capacitor being used?

IME - volts above 26 do a lot to diminish C.

.......... Phil

Sounds like it would help if you could reduce the stray capacitance of the inductor and connections. Can you see any possibilities for this?

The input capacitance of the device maintaining the oscillation is too large. Try tapping it down the coil. But you are probably wasting your time.

Varactor diode capacitance decreases with increasing reverse voltage, so increasing the reverse voltage to more than 26V won't make much difference to the capacitance.

unless you go above 30V and get into the region where you can destroy the diode.

Eliminating stray capacitance does happen to be a good idea, even if Phil has chosen to espouse it - he's not always wrong.

Others have spoken about parallel capacitance. Don't ignore series capacitance and the speed of the amplifying device.

Series capacitances that are effectively is series with the variable capacitor will shift the whole band up and compress the bottom of the range.

Slow amplifing devices add a phase shift that looks like someone added a capacitor to the system. They tend to shift the band down and compress the top of the range.

If you are using a single transistor, a semi-isolated design helps to keep you from pulling the tuned circuit with the load. Vcc ! [R] ! +------ Load * !/c -[L]-+----! ! ! !\\e [C] [L] ! ! ! [Z]* GND ------+ ! [L] ! GND

Biasing not shown.

• These parts are semi-optional.

= Da Slow Man F****it

What a MORON.

......... Phil

Have a look at the web site I posted, and you snipped

and notice that the capacitance changes most rapidly wth voltage when the voltage across the vactor is low - 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, as anybody who understood the various varactor diode characteristics would appreciate without having to read an application note.

Only to you. You don't seem to be able to follow anything too complicated.

I've looked at varactor-diode-tuned oscillators from time to time since the 1970's - one that I designed for the Cambridge Instruments electron beam tester ended up in the Cambridge Instruments S.300 electron microscope.

I was distinctly proud of that . Not because the engineering in the S.300 was all that marvellous, but because Jerry - the engineer who grabbed my design - was very good, and I'd been rather hoping that he'd polish the crcuit a bit.

I had to settle for the ego boost ... You're the one who is presenting as clueless and inexperienced here

If you say so Phil - I'm sure this opinion is just as reliable as the other opinions you manage to conjure up.

On 18 Mar 2006 16:58:53 -0800, snipped-for-privacy@ieee.org Gave us:

Zetex makes REALLY good transistors and FETs. I am sure that Phil has shot himself in the head, much less the foot. I also note that he doesn't retort with technical argument. More like lame, pathetic jack-jawed baby bullshit.

"Roy L. Fuchs"

........ Phil

On Sun, 19 Mar 2006 13:45:56 +1100, "Phil Allison" Gave us:

Yes, you are just that.

"Roy L. F****it"

....... Phil

On Sun, 19 Mar 2006 13:56:33 +1100, "Phil Allison" Gave us:

You are as mature as a freshly laid stool pile, Phil. Is your entire family that way? Was Grandpa a murderer or a rapist?

On 18 Mar 2006 19:13:57 -0800, snipped-for-privacy@ieee.org Gave us:

Don't be so sure. He comes from criminal roots.

= a criminal LIAR

( snip piles of UTERLY irrelevant S**TE )

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.

Try building a voltage tuned FM tuner some day - f****it.

....... Phil

"Roy L. F****it "

Just some anonymous customers of a $5 w**re.

...... Phil

On Sun, 19 Mar 2006 21:39:13 +1100, "Phil Allison" Gave us:

I was asking you about yours, you Aussie fucktard!

I didn't expect you to have any aptitude for reading comprehension however.

"Roy L. F****it "

Just some anonymous customers of a pox ridden, $5 w**re ........

...... Phil

--- You must be related then. ;)

Joke:

The king was out one day, surveying his kingdom, when he happened upon a peasant who looked exactly like him.

"How extraordinary!", remarked the king, "Did your mother ever work at court?"

"No, your majesty," replied the peasant, "my father did."

-- John Fields Professional Circuit Designer

Phil may be Australian, but so am I. Admittedly, his social skills suggest that some of his ancestors might have been anti-social, but not all Australians can trace their ancestry back to a transported criminal

- none of my great-grandparents were born in Australia, and none of them were transported. The one grandparent who wasn't born in Australia wasn't transported either (though he did emigrate to get away from a failed marriage).

My wife's Australian ancestry goes back a little further (her great-grandfather signed Ned Kelly's death warrant) and none of them were transported either.

This is a small sample, but enough to demonstrate that Phil Allison doesn't have to come from criminal roots - persuasive though the hypothesis might be. And he doesn't have to be criminal - or even stupid - to have difficulty undertanding the difference between log/log graphs and graphs with linear axes.

"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

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.