VCO Design Considerations

It depends a lot on what you want it for. If you're trying to build your own cordless phone for grins, I'd build a resonator out of a piece of coax with a varactor at the other end, and use Colpitts-style feedback. You can set the operating frequency with diagonal cutters, which is a plus. The downside is that you'll have to worry about how to suppress higher overtones of the coaxial resonator (a small inductor in series with one of the feedback divider capacitors is a good method).

If you want more than a couple, you should do a board. I leave that to the RF product designers in the group, since I haven't designed a commercial RF product in 20 years.

Cheers,

Phil "dikes forever" Hobbs

Why not go to a coaxial tank circuit to improve stability...

John

A 1/4 wavelength resonator is only 6mm long (assuming a 75% velocity factor). That's pretty small; if you shorten it down to 1/8 wavelength and call it a coil it's smaller yet. I suspect that even if you just used plain-jane RG-174 the "coil" Q would be rediculously higher than the loaded Q of the resonator, but if you were worried about that you could use mini hardline.

I read an article years ago about using coax resonators that were plated onto ceramic forms using a fairly low-loss but very high dielectric constant ceramic. The idea was that the velocity factor would be significantly smaller than 1 and the resonator would be quite short.

I have no idea if this idea was ever carried through commercially, or if was if you can get the resonators in lots of less than 100000 at a time.

Oops. Slipped a digit -- I should have caught that. OK, 60mm isn't "small" unless it's sitting next to a 4CX150 or a klystron or something.

But really, 9GHz is much more cool than 900MHz anyway.

------------------------------------------- Tim Wescott Wescott Design Services

.... coil the tank, it won't care... you should be able to get it small enough for just about anything but a watch...

John

Hello Redhat,

Just pick an oscillator circuit and calculate the LC, pick a varicap that can do the small adjustment here, and it's done. I never use much more than my HP11C calculator. When nobody is there to make fun of me I take the slide rule calculator.

The L will be the big variable here because such small inductance values can vary a lot with minor (and sometimes unintended) geometry changes. You may even get away with tuning the L to center by squishing/expanding the coil.

Regards, Joerg

No, wait dammit! I was saying 6mm and visualizing a bit over 2 inches (i.e. 6cm). Even stretched out straight that's not that dang big, unless you're building radio collars for butterflies, which was my whole @#$% point!

------------------------------------------- Tim Wescott Wescott Design Services

So you're one of them fundamentalists we keep reading about in the news, then?

... chuck in a buffer/filter, you are good to go...

John

We a coaxial ceramic resonator in one of our products, at about 600 MHz. The res is a standard part made by Trans-Tech, about $4.50 in 250 pc quantities. Several other people make them. They have very high Qs and are very temperature stable, but are typically about a 10 ohm line so can be a little tricky.

It's the rectangular silvery thing, upper-left.

John

Hello John,

Yes, you could. It depends on how small this circuit has to be.

Regards, Joerg

Hello Tim,

At 900MHz?

We played similar RF tricks on hybrids but that was >15 years ago. Now you only see that in the hi-rel world. Design costs weren't that much higher than doing a circuit board but laser programming and rig-up was kind of expensive. Especially if you had to do active trim. Our qties used to be north of 35k/year so it didn't contribute much to a unit.

Regards, Joerg

Hello Tim,

It can be cool but I never got much fun out of very high frequencies. You achieve a nice link, then somebody moves a cabinet or a car and the SNR goes to zilch.

Regards, Joerg

Hello John,

And before turning it on tell the oscillator to please start on the fundamental and not a harmonic...

Regards, Joerg

In message , John Larkin writes

No-one asked the ssb noise/stabiliy questions. To minimise noise floor the levels got to be high which can self bias varicaps etc. Close in noise can be the loop characteristic to determine. Is it a spec critical application?

Hello Tim,

ROFL!

Only when it comes to circuit designs. Old school and all that sort of things. Oh, and with cooking. None of this modern shrink wrap stuff or fast food.

Regards, Joerg

aplication note an1777 for the sa601/620 comms ic goes into a lot of detail about VCO at 900mhz

Colin =^.^=