Switching tiny small capacitors to a VCO

tend to

Teledyne relais is the only thing I can think of in that situation... They have them TO-99 (or sort of) sized, I think polarised exist as well, and IIRC they specified 0.3 pF capacitance - perhaps that would be OK for you. I also found a much cheaper version - and somewhat larger - but I don't have the capacitance figure at hand, it may have been 10x the Teledyne one, and then may be not. I can find out if you want me to, I'll have do some digging.

Dimiter

------------------------------------------------------ Dimiter Popoff Transgalactic Instruments

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Joel Kolstad wrote:

Why don't you just use a hyperabrupt varactor with a wider range?

Because the varactors in question are inside the VCO IC that I have no way of changing. :-)

Ah, a #120 drill bit and a Buck Roger laser cutter with a big magnifier glass should do it!

--
"I\'m never wrong, once i thought i was, but was mistaken"
Real Programmers Do things like this.
http://webpages.charter.net/jamie_5

Use...

a. the drain of a small-geometry gaasfet

b. one or two schottky diodes

c. one low-capacitance PIN diode, with adequate off drive. Skyworks?

d. A MAX1474 with a series padder cap. Maybe.

e. A real switch, one of the Bourns things that looks like a trimpot.

Actually, I think that a typical PIN diode doesn't need back-bias. It's so slow that, at 400 MHz, it won't even rectify. Backbias would lower the off capacitance, of course.

John

Thanks John, those are all great ideas!

Or

(f.) a GaAs MMIC T/R switch like a Hittite HMC226 in a SOT-26 package

inductor

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ground

They don't specify "off" capacitances, or any capacitances actually, and it doesn't look like they can be calculated from the datasheet. Most RF type parts are shockingly underspecified for their actual electrical behavior.

I used one Hittite part, an 8:1 GaAs mux, that was specified "DC-4 GHz" in big bold text. Turns out that they consider "DC" to be 200 MHz and up, and there are in fact series capacitors inside. So during our conference call, trying to find a workaround, I asked the designer the value of the series caps. He said, smugly, that he couldn't tell me because it was proprietary.

Hey, this is interesting:

| | | +---------------+ | | | | | | C1 L | | | | +-----+ | | | | C2 / SW | | / | | | | | | | +-----+---------+----GND

Where C2 is the parasitic switch capacitance. L makes the whole mess look infinite when the switch is open. Some algebra required.

John

Join the club... we were bitten by the same problem with Hittite HMC349 switches, also specified as "DC-4GHz" but also containing series capacitors on the internal terminations (something that *is* shown in the tiny schematic drawing of the part, if you squint just right). We redesigned the board to use Macom crossover switches and external terminations instead! The quantity was ~1,000, so probably not much of a sting to Hittite, but we certainly let their reps know our displeasure.

That being said, in general Hittite makes some good stuff. Maybe it's just their marketing weasels who play these specsmanship games...

---Joel

re your original question...

consider switching some additional inductance in series with the low side of the inductor...

Mark

Thanks, but there's a similar problem with that approach: A 22nH inductor at

400MHz is 55 ohms reactive. If you hang a switch off the end of that inductor, a capacitance of 7.2pF is -55 ohms and the two resonate. So while I think that's a workable approach, you'd presumably still want a low capacitance switch, somewhere in the ballpark of

I was going to suggest: ' +------+ ' +-------------> >----o----| | ' | | | VCO | ' --- C1 ( | | ' --- 2pF ( +------+ ' | ( ' o----------+ | L2 ' SW | | | ' ...|....... | === ' . o---+ . | GND (?) ' . | | . | ' . --- | . ( ' . --- | . ( L1 ' . | / . ( ' . | / . | ' . | | . | ' . +---+ . | ' ...|....... | ' === === ' GND GND

where L1 is chosen to resonate Cstray of SW, no algebra required.

This technique (either version) allows use of a heftier, lower resistance PIN switch, improving circuit Q.

James Arthur

Macom (now part of Tyco) also has some very interesting PIN switches.

Cheers

PeteS

...

Good suggestion James... I had thought of that but had dismissed it since I was thinking L1 would be too small, plus I didn't want to have to start dealing with the problem of inductors only being available in fixed sizes and having to worry about the variations in switch capacitance from one unit to the next. Upon reflection, however, it seems I must have calculated L1 incorrectly... checking it now, typically it would be some tens of nH, which is reasonable.

---Joel

Joel, it works well if you use a diode to short out part of the inductor. Microdyne used this method for 20 years in their 1100AR ,1200

1400 and 700 series receivers at up to 510 MHz. Reverse bias the diode to turn it off, and forward bias it with about 10 mA to shunt the end of the inductor to ground. I have a pile of scrap modules, but none of the VCO boards. We switched it to three diffrent lengths instead of two, to reduce phase noise problems at the ends of the band segments. I did a lot of work with these, but I don't have any schematics or other documentataion.

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

Joel,

Why not build your own uhf vco and make have the voc tuning sensitivity you need? If this is a caommmercial endeavor, I woul dbe delighted to assisit, having many years of experience and several patent dealing with wide band and LINEAR tuning VCO's! snipped-for-privacy@att.net for further discussion.

Best regards

Marc Popek

inductor

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Most

the

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This all is nice theoretically :) But in real world swept frequency synthesizer designs there's usually a tight switching speed specification. That's the amount of time it takes for the frequency synthesizer to switch from the lowest frequency to the highest frequency. BTW, I think Joel is using the AD4360-7 VCO/PLL combo chip.

Well, you never heard of "DC" frequencies? Some even consider FM as being in the DC frequency range.

Which chip are you using? Is it a differential (symmetrical tank) oscillator or a single-ended one? Is there any deliberate off-chip capacitance across the inductor that you could absorb into the off-capacitance of a switched capacitor bank?

Chris