Digital RF Capacitors

Has anybody here had contact/experience with WiSpry parts?

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Grizzly H.
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mixed nuts
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No. I've used the Maxim "flecap" digital capacitor, which worked well, but it's EOL now. Never buy Maxim!

I'm designing a 600 MHz ceramic resonator oscillator that I plan to coarse tune with a 32-step Peregrine part, probably the PE64907.

I have looked at that WiSpry quad part. It would be interesting for making, say, a tunable 4-pole bandpass filter.

The WiSpry parts seem to use MEMS switches. They are pretty vague about how it works. MEMS switches have a pretty bad reliability record. Are the WiSpry parts actually available? I didn't see a data sheet on their web site.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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John Larkin

It's interesting that there are no good continuously-electronically-variable components.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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John Larkin

Well, saturable inductors aren't too bad, and then there are servomotor-driven variable caps. ;)

Making something accurately bilinear is fairly hard. If you could still get decent OTAs like the late lamented VA713 from VTC, it would be simpler, at least below 20 MHz or so.

Varactors wired in series opposing are reasonably decent, for not-too-demanding definitions of "decent", and PIN diodes wired the same way are pretty good attenuators for high enough frequencies.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Phil Hobbs

None of those things are easy, or linear. Photoresistors are at least linearish and isolated, but they are terrible resistors. Varicaps are pretty bad capacitors. Saturable inductors are awful. PIN diodes are terrible resistors, useless at DC.

An RTD or a thermistor, controlled by a heater, is at least an isolated linear resistor, over a limited resistance range. I guess you could make a flexing membrane MEMS variable capacitor, which would double as a microphone.

It's unfortunate that most optofet SSRs have essentially Schmitt gate drivers, not linear gate drive.

We are designing a new isolated, programmable, multichannel resistor simulator, mostly for simulating RTDs and thermistors. It's impressively difficult. The competition does it with a board full of resistors and relays.

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John Larkin         Highland Technology, Inc 

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John Larkin

Now you're moving the goal posts. ;)

Depends. I'm working with a company in Israel on a new varactor technology that might be streets better.

PINs make pretty good RF attenuators, though. Not much help to you time-domain dudes, though, I agree.

And a limited range of dissipation.

But it would probably be electrostatically actuated, and so also nonlinear.

For the jet engine folks?

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Phil Hobbs

I guess that is the only solution for any decent power application (over 100 W) filters.

Apparently the WiSpry is intended for some short range wireless applications, so the power levels are in the order of 1 W or less.

The OP seems to be building just an oscillator, so the power levels are significantly lower, so the anti-parallel varactors should be more than adequate.

Reply to
upsidedown

Cool. Send me some prototypes and I'll abuse them for you.

I hope you can keep the tempcos down. Varicaps have horrible TCs that change with capacitance!

Mostly. They need lots of channels of smoothly programmable RTD simulation. The relay things glitch when you change the value, and engines shut down, so a good sim is worth it to them.

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John Larkin         Highland Technology, Inc 

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John Larkin

,

can't do some switched capacitor trickery?

or

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;)

Reply to
Lasse Langwadt Christensen

That might be an issue with free running VCOs at least a half centur4y ago.

A few decades ago, the VCO was included into the PLL, so the TC has not been an issue since then.

Reply to
upsidedown

Well, for PLLs it's certainly less of an issue, but not necessarily a total non-issue, especially for time-domain things.

JL sometimes uses initially-free-running oscillators to provide instant-on timing references, i.e. their zero phase is set when the trigger pulse arrives, and disciplining them with a slower PLL eventually. Being able to adjust the free running frequency accurately in an open-loop fashion is important.

I sometimes use open-loop VCOs driven by DACs to run things like acousto-optic deflectors. Every so often I do a secant-method search through the codespace of the DAC to find evenly spaced frequencies, and then I just blast the list out to the DAC on every scan line. (Did that just last year in fact, in two dimensions.)

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Phil Hobbs

It's the usual NDA situation - one must be worthy of attention. They are MEMS devices but with measures taken to avoid sticking. I haven't researched the patent landscape yet so I don't know if alternate sources would be possible. WiSpry seems to be looking for very high volume customers - which I am not (10k-ish).

I have a very low power consumption requirement which makes PIN devices somewhat impractical. The RF power during the transmit pulse is 10s of watts so varactors and the MOS switched parts are out as well. For RFI immunity and physics details, I'm hoping to do a balanced tuning circuit so ground isolation is very desirable.

And I'm working between ~10 MHz and ~200 MHz in a high Q environment so very low loss and capacitance in the 0.5 pF step --> 20 pF are desired.

Motor(s) possible but space and power are severely limited. Latching relays might be the best option (relevant anecdotal experience appreciated).

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Grizzly H.
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mixed nuts

Our delay generators measure time using an oscillator that is quiescently stopped. When we get a trigger, we start the oscillator and use it to measure time. It only gets servoed to the XO, and the TC corrected, when it's running. So if a user waits a long time, and then triggers the gadget, any temperature drift will make the oscillator start a bit off-frequency. So we don't want the open-loop frequency to drift. So we temperature compensate the whole thing, which isn't much fun.

The HP5359A time synthesizer ran and servoed the oscillator all the time. When they got a trigger, they quenched it for about 75 ns and then restarted it, in phase with the trigger, and then later re-phase-locked it to the XO. That solves the temperature drift problem, at the expense of insertion delay and trigger rate. Their XO lock algorithm was slow, so they couldn't tolerate much drift.

I need your new varicap gadget.

DDS?

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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John Larkin

h

that

That was going to be version 2.0, but I can cruft together pretty snazzy RF things dead bug style in a hurry, and I was in one. ;)

Cheers

Phil Hobbs

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Phil Hobbs

We use the cute little Fujitsu FTR-B3G relays a lot, both the latching and non-latch versions. You need a 5 or 6-bit C-dac, which needs a relay per bit. There might be a clever way to exploit the DPDT contacts to economize.

I'm just now laying out another of my multi-hack experiment boards, which includes a couple of tests to see how those relays work at GHz.

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John Larkin         Highland Technology, Inc 

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John Larkin

Rob has some nice FPGA DDS cores, if they might help. He did me a

64-bit version recently, aHz resolution.

You might train one of your numerous offspring to code VHDL.

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John Larkin         Highland Technology, Inc 

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John Larkin

Thanks, I'll keep that in mind. The project unfortunately went away after a very successful proof of concept, because Intel pulled the development mon ey due to the meltdown in the spring of 2015. I may have the opportunity to revisit it--it's a scheme for increasing microscope resolution by a clear factor of 2, to the equivalent of NA 6.4 at 532nm. That's a lateral resolut ion of about 28 nm.

Dashing Firmware Hunchback has spent the last 18 months writing Verilog cod e for the data acq and discriminator modules of the DEAP dark matter experi ment at TRIUMF/SNOLAB, so he knows a good deal more about that than I do. I 've only done one Verilog project myself.

He's going to be in charge of the digital part of the auto-tweaked noise ca nceller, and also wants to learn high performance analogue.

Fun.

Cheers

Phil Hobbs

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Phil Hobbs

arch

d

id that

RF things dead bug style in a hurry, and I was in one. ;)

isn't rocket surgery ;)

tell core generator how many bits you want and what resolution etc. and ins tantiate

-Lasse

Reply to
Lasse Langwadt Christensen

You might want to take a look at the Elecraft T1 automatic antenna tuner, which uses magnetically latched relays and runs off a 9V battery: The schematic is on Pg 12 in the owner manual: The Elecraft T1 is in the approximate power range (20 watts), but the frequency range of roughly 1.6 to 54MHz might require some changes to operated from 10 to 200Mhz.

Another product worth investigating are the various LDG antenna tuners: Operation is similar to the Elecraft T1.

My experience with antenna tuners is in marine HF radios (2-30 MHz) at much higher power levels. The trick is to not switch any relays with RF power applied.

The MEMS switched capacitor antenna tuners that you're investigating were made for much lower power cell phones, where a single (or dual diversity for LTE) antenna needs to operate from 700 to 2600 MHz and deal with antenna detuning due to hand/body capacitance. An example is the HTC One handset: which I believe (not sure) uses two MEMS switched antenna tuners. Anyway, you probably won't find MEMS switched capacitors large enough to work at 20 watts and 10 MHz. My guess(tm) is that you'll need silver mica caps in order to obtain the required Q and high voltage ratings. If high cost can be toleraged, look into porcelain caps. Step size will be determined by the range of impedances (or VSWR range) that you're trying to match, which you haven't disclosed.

You might want to play with some antenna simulators:

Good luck.

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Jeff Liebermann

Hand held < 1 kg - large cell phone scale. Pulses are microsecond scale with duty cycle < 1% so voltage matters but not pdiss. Capacitors are always porcelain NPO or sapphire.

Match is to a high Q LC resonator (20 db or better return loss) for transmit, noise match to a preamp for receive. Nothing special or difficult but reproducibility after power up and self tune/calibrate is critical. Users won't have the tools or knowledge to do the set up themselves.

I'm a kilowatt/megawatt sort already - radar and whatnot. Now they're asking me to scale something that fills a room to pocket size. Fun.

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Grizzly H.
Reply to
mixed nuts

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