Favourite parts with off-label uses?

A timing ramp doesn't need to be especially linear. Polynomial calibration is easy.

It does need low jitter and low tempco and not ring much.

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard

12 GHz, but they're amazingly stable.

in a class with relays, but 1E5 times faster.

ors with ceramic output caps. (It's good to be able to disconnect the suppl ies during bring-up, and putting the jumper between the reg and the output cap has this additional benefit.

;)

o CMOS gates for power supplies, precision voltage switching, class-C r.f. power amps.

o Various semiconductor junctions as varicaps.

o I'm still searching for a way to use crappy ceramic caps as varactors. Tuning a WWVB loop antenna might finally be that chance.

o LEDs as detectors.

Cheers, James Arthur

Doesn't help--you need something to reduce the pressure drop across the seals. An aluminized rubber diaphragm inside the box, with one side vented, might work.

I made a calculation that 50 grams of 5A molecular sieve would keep the inside dry for about 10 years.

The board has a T/H sensor on it, so it can complain if the sieve gets saturated.

We're conformal-coating the boards, and the sensors are running at zero bias. Doing the latter is unusual for me, but the bandwidth only needs to be 200 Hz or so.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

While not a component...

Early in my career, it was demonstrated to me that you could make a reasonably accurate RF millivoltmeter out of an analog Simpson 260 voltmeter.

(Without modifying it, of course.)

George, I'm trying to understand this. What do you need 20 zeners for?

FWIW, I'm currently making a broadband noise source for testing filters up to 1.5GHz. Still scratching around for the best source to put before a string of ERA-3 MMIC amplifiers.

The BFR93A data sheet says abs max Vbe is 2V, but it doesn't zener at

5V. Although avalanche zeners produce much more noise, I'd rather not boost my 5V supply (though I might need to). I wonder how much reverse current a microwave Shottky diodes (say HSMS-286) would survive. Abs max peak reverse voltage is 4V, so it might withstand 5V anyway...

Any better suggestions for a device I might have in the drawer already?

Clifford Heath.

I meant that it would be vented to the outside. Little plastic tube or something.

I guess the seive would soak up humidity and reduce the pressure inside, so a tiny flow through the seals would introduce a little more humidity. Wouldn't that eventually get to zero humidity and zero pressure differential?

I guess atmospheric changes would still pump the system slightly.

Mothballs?

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard

Mo found this at some neighborhood junk sale and bought it for me. $3 or something.

It's probably a thermocouple.

How did that Simpson thing work?

I have some Spice models of an RF detector using an SMS7621 low-barrier schottly. It works around 50 mV RMS.

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard

If we start with a dry atmosphere inside, humidity by itself would not be a first-order forcing function.

Air pressure varies +- 7% or thereabouts on time scales of a few days. If you assume that the variations are the same inside the box, you can calculate how much water will get in.

Our stuff has to work in medium-extreme conditions. Harvesters are still working in January in some places--I've spent a little January time in West Texas installing and verifying installations, at temperatures well below freezing and with high winds. Simon has done about three times more of that stuff. It's difficult.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

onably accurate RF millivoltmeter out of an analog Simpson 260 voltmeter.

Yes, they are Thermocouple meters. Those were used to measure the curren t to an antenna.

Larger ones were used in Broadcasting to log transmitter output in the s tation's daily engineering logs for the FCC.

Simple RF probes. There are a lot of minor variations. I have a Fluke85R F. The link is for a two page manual, and schematic. It used a Germanium di ode, but modern diodes are usable. and have a better frequency response. Th e ARRL handbook has had a probe design in every issue that I've ever seen. The early ones used a dual diode tube like a 6AL5.

If you need better accuarcy at low levels, an old Boonton 92 series is hard to beat. Its sucessor, the 9200 is even better. It is digital and it has I EEE-488 interface as an option. Probes often cost more than the meters, and the cables are as well, but you can find them on Ebay. I've ought 'unteste d' 92 meters with a probe and cable cheap, because the seller is clueless. I will be building clones of some of their probes, now that I have a lathe. and suitable shielded cable. Wirepro still makes the origin Amphenol two p in microphone connectors, and I have a new, 250 foot spool of the cable.

Vented might be okay. Trouble is, the gas diffusion rate in elastomers is many orders of magnitude higher than in metal or glass, so it would need to be metallized. The metal film would have a lot of cracks, of course (*), but most of the surface would still be metal.

Simon's the seal expert at this point. A sufficiently stiff box, with enough screws holding the lid on, and hermetic connectors, ought to be able to stay sealed pretty well. The pressure changes are nontrivial though--our box is about 3 x 5 inches, so a 7% pressure change amounts to about 15 pounds over the surface of the lid. They're also fairly slow, so it doesn't take much of a leak rate to equalize the pressure.

We're using a cable gland rather than a hermetic connector, primarily for cost reasons. I suspect that enough air will flow inside the cable to manage the vent job, but we'll probably have to measure that to find out.

;) If we get critters in there, we'll certainly have other problems first.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

:

s:

und 12 GHz, but they're amazingly stable.

st in a class with relays, but 1E5 times faster.

lators with ceramic output caps. (It's good to be able to disconnect the su pplies during bring-up, and putting the jumper between the reg and the outp ut cap has this additional benefit.

s s

Arghh! typo. sorry a 20V zener! Run near the knee you get these big avalanche spikes, with ~1us rise/ fall times.

1 GHz noise sounds hard. What about a spectrum analyzer and tracking generator?

George H.

Phil, I'm not sure I understand the problem. I wonder if you could say a bit more? (I do dream of someday maybe using a sealed box...) You've got a sealed box. But pressure differences can cause the seals to fail. So you put a vent in it. but now that will pump water into your box. (Because you have molecular sieve in there and it will reduce the vapor pressure of water in the box.) So you need a long vent such that the diffusion time for H2O is long....

Is that close to stating the problem correctly?

Some sort of long thin tube?

George H.

12 GHz, but they're amazingly stable.

in a class with relays, but 1E5 times faster.

ors with ceramic output caps. (It's good to be able to disconnect the suppl ies during bring-up, and putting the jumper between the reg and the output cap has this additional benefit.

;)

all the same. I do not try to be anonymous, I just try to make myself anon ymous to the first level of google search.

:

nd 12 GHz, but they're amazingly stable.

t in a class with relays, but 1E5 times faster.

ators with ceramic output caps. (It's good to be able to disconnect the sup plies during bring-up, and putting the jumper between the reg and the outpu t cap has this additional benefit.

ls ;)

Hi James, I spent part of the weekend trying to think about crappy ceramic caps as part of a parametric oscillator. (After re-reading Pippard's "Physics of Vibration" the oscillator seems like an easier first step.) But I have no idea how to couple two different signals into/ through the capacitor. Well maybe a bridge.

Do you have any clever ideas?

George H.

NoiseCom sells noise zeners that go to 110 GHz.

You get spikes at low zener current. As current increases, it approaches symmetric gaussian noise.

Here is my extensive scientific research on zener noise:

But to characterize a filter, a sweep generator sounds better than noise. Noise is too, well, noisy.

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard

The ones I've seen are waveguides around a gas-filled discharge tube. Recombination noise just couples to the ports, like holding a conch shell to your ear.

Rigol has a ~1 GHz SA with TG option. ~$3k? Maybe a used one? Or a boat anchor.

George H.

Air pressure varies by about +-7% (~1 psi) over time scales of a day or two. Our enclosures have a volume of about 300 ml, and will be mounted on top of a harvester. It will be painted a famous dark green colour, so its albedo will be low. Thus on sunny days, it will experience temperature fluctuations of as much as 70C over time scales of hours. That will lead to fluctuations on the order of +-2 psi in a sealed enclosure.

This isn't a whole lot of force, but there are all sorts of white papers from Parker and other companies that say that pumping liquid water past O-rings is a significant problem in outdoor equipment. One reason is that the pressure variations basically centre on zero, and O-ring seals really really like a single sign of pressure difference, which causes the O-rings to stay seated.

In a vented 300 ml enclosure with 50g of 5A molecular sieve, we can get about 24000 cycles of 20% air exchange at a dew point of 20C before the sieve saturates, so that's many years.

Liquid getting in is much worse.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

Rube Goldberg (or someone bored and tired of staring at the walls :-)) would suggest a spring-loaded piston with a solenoid release. Compress spring and "arm" solenoid release, seal in box with oring seals and hermetic wire feedthroughs, trigger solenoid so piston releases and raises the pressure in the box, and voila, happy orings :-). If you have to open the box for service, just rearm and fire the piston again once the box is resealed.

--
Regards, 
Carl Ijames