Hi, all--I'm looking for a curve tracer. Ideally it would be USB-powered, and come with software that would make plots and generate data files. Nothing too fancy, just I_C and I_B vs V_CE and V_BE, and the equivalent for FETs. A plus would be automatic computation of beta, V_GSth, Early voltage, and even some of the DC SPICE parameters like emitter and base resistance.
It would be especially nice if it could measure gate current with a resolution of a picoamp or less.
You could probably simply import the data points as tables into PSpice and PSpice would do the computations... and I'd guess LTspice could do the same, though it's not as robust with macros as PSpice.
USB powered? If 1 Amp maximum would work. I'd be more inclined to use external power and the USB only for control and data.
Only with lots of time to wait >:-}
(I'd be interested as well. I've always coveted having my own curve tracer ;-)
...Jim Thompson
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| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
Curve tracers can go reasonably fast, so a big cap could provide the oomph for a brief excursion to higher power. I'm reasonably agnostic about having a wall wart too.
Could be reasonably fast if they integrate the gate current. A minute for a set of 5 or 10 curves would be OK at low levels.
I found one at Photonics West for $169, distributed by Ana Tek. (For Canadians with long memories, this isn't the power supply manufacturer from Vancouver.)
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Doesn't seem to do gate leakage, though. The optimal combination might be one of these plus a bandaid box made of op amps, to increase the measured current by, say, 1E7 times to make the FET look more like a BJT. That would confuse its built-in functions completely, I expect.
What I'm using now is a cast-aluminum Bud box with op amps in it, that generates a nice ramp for V_GS, has a knob for V_DS, and has TIAs for I_G and I_D. Works well but is a bit of a pain, and has no data acq.
I could put something like that on the front of a Labjack (Joerg's fave). That would be a bit more dough, but I'll be needing to do a half dozen protos with data acquisition in the next few weeks anyway, so I can amortize the s/w labour over my gizmo and the client's work. (I hate charging clients for building libraries.)
Capacitance would be nice, especially if it were done at medium frequency a la Boonton, so that you could put in whatever ferrite beads and stuff you needed to keep the DUT from oscillating.
For my biochip front ends, I measure input capacitance by finding the gain with a 1-pF input cap and a 100-pF, then looking at the change in gain to find the voltage divider ratio. Not 100% kosher unless the feedback capacitance is super low, but it has to be if one wants
Hey, doesn't one of your kids want to get his hands dirty with some software work? If you charge the client a lower rate for that work he could still sock away a lot of money towards his first souped-up TransAm with mag wheels and all that :-)
If it's any comfort I'll have to design some radio circuitry soon and build (yuck ...) several prototypes. Will use Labjacks again, mainly because those come with a copy of a SCADA software to make life a bit easier for SW-challenged guys like me. I just hope the garage ain't so cold anymore once I get to do the boxes. But I don't envy you guys out there right now, had a teleconference this morning and one guy is on Long Island, snowed in.
Brr. My son is _not_ "going to drive me to drinkin'" if I can help it, but anyway that wouldn't be his style. He's busy writing Arm Cortex M3 firmware at the moment, but since he's learning C at the same time, he isn't fast at it yet. (He did get a task switcher working, using a linked list of tasks handled by the systick interrupt handler.)
My wife and I spent a very pleasant hour or so on Saturday, digging out from 18-24 inches of pretty light snow. It was heavier near the road where the plough had been, of course, but no worries. Bright sunshine, well above freezing, sun melting the last little bits of snow left behind on the driveway.
A curve tracer is a qualitative measurement device. I use a TEK 7CT1N for simple stuff and only drag out the 576 when I gotta have it. I use it more for blowing shorts out of NiCd's than for testing transistors.
If you try to make one using a series of fixed measurements and graphing them, you get mixed results. I tried that back in the day. The device thermal time constants significantly affect the measurement details.
And the parasitics of the test fixture swamp device parasitics, unless you use network analyzer fixturing.
Picoamps to amps is a wide dynamic range and will require some serious attention to range switching.
Not saying it can't be done, just that it's not nearly as simple as it sounds.
I was expecting a visit from a Pratt&Whitney guy. He lives near Hartford CT. His driveway is 400 feet long and maybe 40 feet wide. I calculated that he had roughly 400 tons of snow in the driveway.
He rescheduled.
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John Larkin Highland Technology, Inc
jlarkin at highlandtechnology dot com
It looks like simple but working curve tracers are going unobtainium these days. It's either modular high-end systems for IC Fabs with the prices skyrocketing up the ISS or the homegrown fleabay stuff designed by people with no grasp of anything beyound current shunt resistors.
There's a lot of hobby project schematics published in a hidden corners of the web that very nearly miss ending up on
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but only because the daily WTF does not know they exist (and does not care much about test equipment either). Also there are some boat anchors in unknown condition for sale, sometimes way overpriced because audiophools use the higher-voltage models in order to match tubes (whoever sells the QT2, listen, there's a new market for you).
It looks like there's just no supply of affordable and usable curve tracers in the western corners of the world. There's a lot of Chinese curve tracers available, mostly analog ones, but the Chinese see no market for them in the west and don't even bother offering export versions with English user manuals or sometimes even front panel texts. Even worse, English-speaking search engines have difficulties finding anything, so most types are only findable with the proper "native" search terms.
The "classic" instruments are really heavy (look up "QT2" for example, that thing is still produced) and haven't heared about a digital interface (or about the last 4 decades for that matter).
There are newer versions like
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(XJ4836). It's mains powered, but it at least has an USB interface. It's a rather sensitive small-signal tracer (by local standards), collector current down to
100nA/div, but no way for it to measure picoamps. Somewhat pricey too, given its specs, but at least it is standalone, PC-less and it is supposed to store settings and plots somewhere nonvolatile.
If you search further, you may find the WQ4828 to WQ4830 type curve tracers too (
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), but they only go down to 200nA/div and their user interface will be a completely non-intuitive guesswork to anyone who does not have some Chinese friends to help translate it. They seem to be able to calculate more parameters however.
It's only qualitative because it only has a CRT readout. A curve tracer is a sort of stripped-down version of a semiconductor parameter analyzer such as an HP4145B, which is very quantitative indeed. (I wouldn't mind one of those--I used to have one at my PPOE, but it takes a lot of rack space.)
Yup. You have to be quick about it if you're dissipating any significant power. Of course with a MCU and DACs, you can do the measurements out of order to keep the average dissipation constant.
Not necessarily. My Boonton 72BD easily measures a few femtofarads. You just null it out before attaching the hot lead to the device.
I don't need amps--I'd be quite happy with a 50-100 mA top end. Most of what I need a curve tracer for is measuring weird RF devices with lousy datasheets.
Sure, I know how to do it fully manually, and could build a proto of the front end in a day or so. What I want to avoid is spending a week getting all the features right. (Time is money and all that.)
The 7CT1 is perhaps mainly a qualitative device. The 576, which I use regularly, is capable of quite accurate measurements, used correctly. (Reminder to self, I must dig out the 2N7000 sub-threshold measurements I did in response to Jim T, and curve fit them)
That's what pulsed measurements are for. I regularly test devices at up to
200 amps, using a 176 pulsed high current fixture, without thermal problems.
That applies to measurements generally.
--
"For a successful technology, reality must take precedence
over public relations, for nature cannot be fooled."
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