Fast pulsed resistance

[Sam Lloyd]

I'm trying to create a controlled resistance "spike" and have had
little success so far. Basically, I want to create a resistance pulse
with a width of around 10nS that is variable from approximately 1
milliohm to around 10 milliohm. In other words, the resistance of the
device would be a nominal 1 milliohm, but would rise to around 10
milliohms for 10nS, then return to nominal. The resistance needs to be
electrically isolated from whatever is driving it. Obviously, rise and
fall times would have to be fast.

I experimented with a pulse generator and FET, but I really need
something like a fast LDR (light dependent resistor) because of the
isolation requirement. Any ideas?

Thanks,
Sam

 

[Kevin Aylward]

I'm trying to create a controlled resistance "spike" and have had
little success so far.

You dont say.

Basically, I want to create a resistance pulse
with a width of around 10nS that is variable from approximately 1
milliohm to around 10 milliohm. In other words, the resistance of the
device would be a nominal 1 milliohm, but would rise to around 10
milliohms for 10nS, then return to nominal. The resistance needs to be
electrically isolated from whatever is driving it. Obviously, rise and
fall times would have to be fast.

I experimented with a pulse generator and FET, but I really need
something like a fast LDR (light dependent resistor) because of the
isolation requirement. Any ideas?

Yes. Try a different problem, this one is non trivial. For starters,
you'll need amps of drive current to drive those bloody huge fets, so a
typical sig gen wont cut it.

What is the application? How do you know what you want to do is what you
want to do?

Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Phil Hobbs]

I'm trying to create a controlled resistance "spike" and have had
little success so far. Basically, I want to create a resistance pulse
with a width of around 10nS that is variable from approximately 1
milliohm to around 10 milliohm. In other words, the resistance of the
device would be a nominal 1 milliohm, but would rise to around 10
milliohms for 10nS, then return to nominal. The resistance needs to be
electrically isolated from whatever is driving it. Obviously, rise and
fall times would have to be fast.

I experimented with a pulse generator and FET, but I really need
something like a fast LDR (light dependent resistor) because of the
isolation requirement. Any ideas?

Thanks,
Sam

You could do this by switching a larger resistor in and out of the
primary of a transformer, with the secondary in series with your
circuit. A 100-turn primary and 1-turn secondary would transform that
10 milliohms up to 100 ohms. This would give you the voltage isolation
as well.

Cheers,

Phil Hobbs

 

[Jim Meyer]

I'm trying to create a controlled resistance "spike" and have had
little success so far. Basically, I want to create a resistance pulse
with a width of around 10nS that is variable from approximately 1
milliohm to around 10 milliohm. In other words, the resistance of the
device would be a nominal 1 milliohm, but would rise to around 10
milliohms for 10nS, then return to nominal. The resistance needs to be
electrically isolated from whatever is driving it. Obviously, rise and
fall times would have to be fast.

Perhaps you could generate your spike at some higher, more
tractable, impedance level and transform it down to the required
impedance level. Transmission line transformers might be suitable.

Jim

 

[Winfield Hill]

Sam Lloyd wrote...

I'm trying to create a controlled resistance "spike" and have had
little success so far. Basically, I want to create a resistance
pulse with a width of around 10nS that is variable from approximately
1 milliohm to around 10 milliohm. In other words, the resistance of
the device would be a nominal 1 milliohm, but would rise to around
10 milliohms for 10nS, then return to nominal. The resistance needs
to be electrically isolated from whatever is driving it. Obviously,
rise and fall times would have to be fast.

I experimented with a pulse generator and FET, but I really need
something like a fast LDR (light dependent resistor) because of the
isolation requirement. Any ideas?

How much current is going through this "variable resistance."
What are you working on?

Thanks,
- Win

whill_at_picovolt-dot-com

 

[John Fields]

I'm trying to create a controlled resistance "spike" and have had
little success so far. Basically, I want to create a resistance pulse
with a width of around 10nS that is variable from approximately 1
milliohm to around 10 milliohm. In other words, the resistance of the
device would be a nominal 1 milliohm, but would rise to around 10
milliohms for 10nS, then return to nominal. The resistance needs to be
electrically isolated from whatever is driving it. Obviously, rise and
fall times would have to be fast.

I experimented with a pulse generator and FET, but I really need
something like a fast LDR (light dependent resistor) because of the
isolation requirement. Any ideas?

 

[R.Legg]

I'm trying to create a controlled resistance "spike" and have had
little success so far. Basically, I want to create a resistance pulse
with a width of around 10nS that is variable from approximately 1
milliohm to around 10 milliohm. In other words, the resistance of the
device would be a nominal 1 milliohm, but would rise to around 10
milliohms for 10nS, then return to nominal. The resistance needs to be
electrically isolated from whatever is driving it. Obviously, rise and
fall times would have to be fast.

I experimented with a pulse generator and FET, but I really need
something like a fast LDR (light dependent resistor) because of the
isolation requirement. Any ideas?

Thanks,
Sam

For events occuring in a 10nSec interval, resistances in the milliohm
region will not dominate. So why vary it at all?

RL

 

[Active8]

You could do this by switching a larger resistor in and out of the
primary of a transformer, with the secondary in series with your
circuit. A 100-turn primary and 1-turn secondary would transform that
10 milliohms up to 100 ohms. This would give you the voltage isolation
as well.

Cheers,

Phil Hobbs

he didn't say if he needs it in series with an AC or DC voltage, but
that's a nice idea.

mike

 

[Sam Lloyd]

Thanks for your replies. I'll try to elaborate a little:

I'm trying to validate an automated tester for use as a dynamic
resistance
(noise) tester. We build devices that utilize sliding contact systems,
and one of the parameters we often test is dynamic resistance, or the
variation in resistance as the unit rotates. We currently perform this
test by running a DC voltage through the unit and monitoring the
voltage across the 2 contacts. Any change in resistance results in a
proportional change in voltage, and can easily be seen on the scope.
The problem is that this involves a human constantly watching the
scope and manually changing circuits on a switch box, which is both
expensive and somewhat subjective.

The automated tester purportedly performs 25,000 measurements/S,
so we can assume it's good for any circuit spec'd with a 12.5 KHz or
lower BW, but I would like to be able to use it on higher BW circuits.
Before I can do that I need to determine the minimum "pulse width"
it's capable of detecting. If the tester can repeatedly and reliably
detect a resistance "pulse" as described, I then have some level of
confidence that it can be used for acceptance testing on higher BW
circuits.

So, basically, I want to connect my automated tester to some device
that can vary its resistance as previously described, and either prove
or disprove the viability of the automated tester for detecting such
miniscule and short duration variations in resistance.

Thanks,
Sam

P.S. Winfield- Tried to reply to your email, but it got bounced back.

 

[John Fields]

Thanks for your replies. I'll try to elaborate a little:

I'm trying to validate an automated tester for use as a dynamic
resistance
(noise) tester. We build devices that utilize sliding contact systems,
and one of the parameters we often test is dynamic resistance, or the
variation in resistance as the unit rotates. We currently perform this
test by running a DC voltage through the unit and monitoring the
voltage across the 2 contacts. Any change in resistance results in a
proportional change in voltage, and can easily be seen on the scope.
The problem is that this involves a human constantly watching the
scope and manually changing circuits on a switch box, which is both
expensive and somewhat subjective.

The automated tester purportedly performs 25,000 measurements/S,
so we can assume it's good for any circuit spec'd with a 12.5 KHz or
lower BW, but I would like to be able to use it on higher BW circuits.
Before I can do that I need to determine the minimum "pulse width"
it's capable of detecting. If the tester can repeatedly and reliably
detect a resistance "pulse" as described, I then have some level of
confidence that it can be used for acceptance testing on higher BW
circuits.

So, basically, I want to connect my automated tester to some device
that can vary its resistance as previously described, and either prove
or disprove the viability of the automated tester for detecting such
miniscule and short duration variations in resistance.

---
Since what your tester will be looking at during testing is a changing
voltage, it seems to me that it would make much more sense to use a
pulse generator to determine the bandwidth capabitities of the tester
while it's being evaluated.

During live testing a _much_ better way to determine the resistance of
the contacts would be to force a constant current through them and then
measure the voltage drop across them using a fast ADC (which,
ostensibly, is part of the tester).

 

[legg]

So, basically, I want to connect my automated tester to some device
that can vary its resistance as previously described, and either prove
or disprove the viability of the automated tester for detecting such
miniscule and short duration variations in resistance.

As this is a mechanical contact, it cannot be tested completely any
faster than the mechanism can complete an operation; in this case at
least a full rotation, in both directions.

This is unlikely to happen in a million nanoseconds.

Your circuit measures and detects a delta of voltage with a fixed
induced current. You could test the sensor's ability to do this by
applying a simple signal. Anything measurable visually on a scope is
also measurable electronically using the correct configuration of
metering.

You shouldn't have to recalibrate the test system after each test
cycle, so calibration speed cannot be as critical as you suggest.

Any mechanical contact functioning as you describe is going to get a
lot worse with age, so I'm doubtful that this test has real validity,
except as a control of process quality. This can and should be
measured in other ways, that are probably more meaningful, before
final assy.


RL

 

[Tony Williams]

........ We build devices that utilize sliding contact systems,
and one of the parameters we often test is dynamic resistance, or
the variation in resistance as the unit rotates. We currently
perform this test by running a DC voltage through the unit and
monitoring the voltage across the 2 contacts. Any change in
resistance results in a proportional change in voltage, and can
easily be seen on the scope. The problem is that this involves a
human constantly watching the scope and manually changing
circuits on a switch box, which is both expensive and somewhat
subjective.

Have you considered staying with a scope, but changing to one
controlled over the IEEE-488 bus, by a computer? This keeps the
front-end bandwidth and fast sampling rate required to take the
snapshot of the ramp (which you already know works ok).

The inbuilt measurement algorithms that scopes can do on their
snapshot might be enough to do a straight pass/fail on noise
pulses. If not the whole snapshot can be downloaded to the
computer over the IEEE-488, to be examined there.

 

[Sam Lloyd]

The current is adjustable, but the mfr (Omni) suggests 500mA to 1A.

Sam

 

[Sam Lloyd]

The requirement comes from a customer spec. Dynamic resistance on
certain lines results in video noise. I am trying to prove out (or
disprove) the worthiness of the ATE for detecting events of this
magnitude.

Sam

 

[Sam Lloyd]

Tony,

The reason for wanting to use the ATE is that it automatically
switches among hundreds of circuits that must be tested. We would like
to take advantage of the ATE in reducing the manual effort. This
wouldn't be an issue if it wasn't for the fact that I need to test
hundreds of circuits per unit.

Sam

 

[Sam Lloyd]

Just to further clarify:

We are already using the ATE to perform the described "noise" testing-
it simply reports the delta of the 25000 measurements/S. The sliding
contacts move in a >360 degree continuous motion. Given a single
rotation of 360 @ 30 RPM, the ATE would obviously miss certain angular
positions between measurements. My supposition is that by rotating the
unit multiple revolutions I could essentially increase my BW
confidence.

Because of the variation in rotational speed, it is guaranteed that
subsequent rotations (past the initial 360 degrees) would ensure that
different contact surface angular positions would be measured during
each revolution. Basically, I'm suggesting that the BW capability of
the ATE can be artificially increased by testing for longer periods.
Twice the test time should yield twice the BW (in reality the
relationship is probably less than 2/1, but for the sake of
explanation let's assume a direct relationship between time and BW).

What I'm trying to do is prove (or disprove) this hypothesis.

Thanks,
Sam

 

[R.Legg]

The requirement comes from a customer spec. Dynamic resistance on

certain lines results in video noise. I am trying to prove out (or

Reduce the noise where it's generated - snub it.

RL

 

[Marc H.Popek]

perhaps you can use the known and quiet rapid switching associated with
these FUSES:

FA +SACK of FUSES 25 for 99 cents wow!
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=2436022282&cat
egory=39405&rd=1

Thanks For looking

Marco P

8888888888888888
_________________


FA:Support MP quest to pay for college and keep the ACVW fleet running,
Whilst
improving your ride!

FA Mirror
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=2436123985

FA rear deck latch, with FREE CHOICE OPTION

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=2436113078

FA use your single line phone gear (phone-fax-answering machine-modem) on
two line. automatically, plus surge protector and security features, WOW
FREE ship options

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&rd=1&category=1503&item=305132
5831

FA WESTY-VANAGON_ LIKE NEW_ 80-84_ REAR HATCH SCREEN BROWN
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=2436022282&cat
egory=39405&rd=1

FA +SACK of FUSES 25 for 99 cents wow!
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=2436022282&cat
egory=39405&rd=1

BOXES and BOXES of Vintage Pic a parts and specializing in distributors &
carbs

http://cgi6.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewSellersOtherItems&useri
d=lvmarco&include=0&since=-1&sort=3&rows=50