Varactor-tuned oscillator

"Dirk Bruere"

....... Phil

On Mon, 20 Mar 2006 22:54:48 +1100, "Phil Allison" wrote some boring bullshit:

YAWN......

The simplest way for a newbie to extend the range of his/her frequency tuning is to use multiplication. This will allow you to divert your confusion from oscillator design to filter design. Go with a cheap frequency tripler in your band and a simple filter. Your tuning ranging is automatically tripled and the spectrum will be cleaner too.

In article , John Fields wrote: [...]

I don't think he meant *that* much forward current.

In article , John Fields wrote: [...]

Just hook the Cathode to Vcc.

Doesn't the problem remain the same, to get x% of frequency pull range? The only advantage I can see of multiplying is that possibly the required lower-frequency oscillator is less affected by stray capacitance. But the required deltaC/totalC is the same.

But really, octave VCOs are common, so covering the FM band should be trivial without resorting to tricks. My Walgreens has a varicap-tuned pocket AM-FM radio, with ear buds, for $3.

John

There are special HYPERABRUPT varactors intended for very wide frequency range shifts.

No, I pointed out that it wss wasted effort. If the OP had got rid of the stray capacitance, the generous 0V to 26V tuning voltage range already available should have been more than enough for a 88-108MHz tuning range.

On the contrary - everybody agress that there was too much stray capacitance in the tank circuit.

He may wel have done. We've made it obvious enough what it was.

-- Bill Sloman, Nijmegen

Phil, Yeh, it's the only parallel cap - must be strays. But even if I eliminate strays, assuming they're parallel strays, wouldn't that just decrease the whole capacitance range and shift the frequency range up? I don't see how that would wider the range. I guess I should do it anyway - good rf practice. The varacter has a max reverse bias of 30 volts. I can get 26 volts regulated with three 9v batteries hooked to a variable voltage regulator. So you think I should try it with 30 volts (or maybe 29 to be safe)? I'd really like to keep this thing to three 9v batteries if possible. Any way to do it with two varacters?

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I think someone mentioned them in an earlier post.

John

Thanks John.

That does seem to be the concensus amid all the name-calling. I can certainly reduce strays here and there. Seems to me that series strays would be the more serious offender in this case, but eliminating all strays is always a good rf goal. Wonder if I could simplify things a bit with a different varactor? One of the replies included a link to spec sheets on Zetex varactors. They have some low-voltage, wide-tuning diodes. Thanks for your reply.

Allen

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If you can find one that hits higher capacitance at llow bias and has a wider capacitance ratio over your bias range, and you can reduct the inductance it resonateswith a bit, you may get there that way. But, first, I think you should think about every place your arrangement and coil structure is adding capacitance across the inductance and try the simple things that might reduce it.

Yes, I see that Zetex carries some. Thanks Don.

Allen

Yes, I mentioned in another post that I thought series strays might be even more serious that parallel. The transistor is a high-frequency (ft = 200mhz) NPN bipolar. The circuit you show is pretty much what I have. Not much load on the oscillator. The circuit is stiffly biased and wired for temp stability and variations in Hfe. Thanks for your reply.

Allen

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The resonant frequency of your tank circuit is defined by

(2.pi.f)^2= 1/L.C

Your varactor must be giving you at least 20pF of capacitance change, and this gets your from 88MHz to 96MHz. We can rearrange the equation as

L.C = 1/(2.pi.f)^2

and your resutls correspond to

L.C0v = 3.27E-18

L. (COv - 20pF) = 2.75E-18

subtract one from the other and you get

L. 20pF = 0.52E-18

whence

L = 26nH

and your C at 0V = 126pF of which about 100pF would be stray capacitance.

If you are getting a bigger capacitance swing out of your varactor - which is quite likely - this gives a smaller inductance. The first time I worked this out I used 45pF, and should have got an inductance of

If you can get rid of the bulk of your stary capacitnace, which ought to be possible, since everybody else tunes their FM tuners with something like 20 to 30pF of capacitance variation, and push up your inductance to compensate fro the lower capacitance, you should solve your problem.

I first guess would be that you are usuign a conventional close-wound commercial inductor as your tuning inductance.

Take a look at the sort of inductor normally used in these circuits

and wind yourself something like it - it has pretty much the optimum inductance capacitance ratio.

Phil does. I think you'd be wasting time better spent on finding where your exta capacitance is comiing from.

That should be possible. Commercial transistor radios do it with one. You might want a better varactor diode - Phil suggesed the BBY40 which you can buy from Farnell and seems to offer about twice the capacitance swing.

Not without getting rid of your stray capacitance, and once you have done that you won't need a second varacitor.

A ft=200(I assume Mega)Hz NPN may not be good enough. You may want to switch to a more high tech part like the 2N2222. :)

The collector current of the transistor you are using could be seriously lagging the E-B voltage. This will crunch down the top of the range just like a stray capacitance. If you are using a resistor for "gm" stabilization in the emitter leg, you may be able to infer the true phase with some careful scope measurements. You need a low capacitance probe but lucky for you the signal is large so you can afford to lose some in the probe circuit.

Since it is only the phase relationship you are after, you could do this:

Trigger the scope from the output. Grip a 5K-10K resistor in the probe tip. Touch the far end of the resistor to the two points and take your measurements. The absolute phase and amplitude will be way wrong but the ratio will be right.

"John Fields"

His post indicated an FM band transmitter or test oscillator.

.......... Phil