O'scope Noise Measurements

[Rich]

Hello,

Usually, I try to minimize the area between the probe and ground clip by
holding the ground clip against the probe and selecting a close ground with
the probe to minimize noise pick up.

I think the gist is to have a high impedance probe and a low impedance
ground. I've read a bit about using a coax cable on the ground. Would
someone describe this method? I'm trying to measure actual noise on my 5V
rail, my voltage reference and the input to an ADC. I purchased a coax T
to help accommodate this measurement.

Thanks!

 

[martin griffith]

Hello,

Usually, I try to minimize the area between the probe and ground clip by
holding the ground clip against the probe and selecting a close ground with
the probe to minimize noise pick up.

I think the gist is to have a high impedance probe and a low impedance
ground. I've read a bit about using a coax cable on the ground. Would
someone describe this method? I'm trying to measure actual noise on my 5V
rail, my voltage reference and the input to an ADC. I purchased a coax T
to help accommodate this measurement.

Thanks!

read AN70 from here
http://www.linear.com/search/searchResults.do?siteSearch=Search&siteCriteria=an70


martin

 

[amdx]

Hello,

Usually, I try to minimize the area between the probe and ground clip by
holding the ground clip against the probe and selecting a close ground with
the probe to minimize noise pick up.

Hi Rich,
Please see my post in abse (alt.binaries.schematics.electronics),
subject line -- Re: O'scope Noise Measurements
I have a drawing of a method that minimizes noise.

 

[Phil Allison]

I'm trying to measure actual noise on my 5V
rail, my voltage reference and the input to an ADC.

** An un-aided scope is not suitable for measuring low level noise.

For the simple reason they have too much input noise of their own.

First determine the * noise bandwidth * you need to measure.

Then estimate the minimum noise level you need to achieve.

Then use a low noise *pre-amplifier* that has that bandwidth PLUS enough
gain to raise that minimum level to at least 10mV.

The scope then reads the output of the preamp.

Preferably use a wide band, true rms millivolt meter too.

Connection to the DUT is best done with a co-axial cable, either by
soldering the ends or using a BNC plug & socket.




........ Phil

 

[John Larkin]

Hello,

Usually, I try to minimize the area between the probe and ground clip by
holding the ground clip against the probe and selecting a close ground with
the probe to minimize noise pick up.

I think the gist is to have a high impedance probe and a low impedance
ground. I've read a bit about using a coax cable on the ground. Would
someone describe this method? I'm trying to measure actual noise on my 5V
rail, my voltage reference and the input to an ADC. I purchased a coax T
to help accommodate this measurement.

About the best you can easily do is get a piece of coaxial cable, BNC
on one end into the scope, and solder the shield/center to the board
on the other end, using a ground pad close to the signal pad. It helps
to wind it through a ferrite core midway along the cable, to break up
RF ground loops between the scope and the DUT. This technique of
cource assumes a low-impedance signal that isn't changed much by the
coax+scope capacitance.

Designing a few SMA connector footprints into your board layout, to
snoop critical signals, is great, if you think about it beforehand.

Regular passive 10:1 probes are terrible for characterising wideband,
low-level noise on a board. Fet probes are a lot better, but still
cost you 10:1 on amplitude. There are exotic/expensive active
differential probes, too.


John

 

[Harry Dellamano]

About the best you can easily do is get a piece of coaxial cable, BNC
on one end into the scope, and solder the shield/center to the board
on the other end, using a ground pad close to the signal pad. It helps
to wind it through a ferrite core midway along the cable, to break up
RF ground loops between the scope and the DUT. This technique of
cource assumes a low-impedance signal that isn't changed much by the
coax+scope capacitance.

Designing a few SMA connector footprints into your board layout, to
snoop critical signals, is great, if you think about it beforehand.

Regular passive 10:1 probes are terrible for characterising wideband,
low-level noise on a board. Fet probes are a lot better, but still
cost you 10:1 on amplitude. There are exotic/expensive active
differential probes, too.


John

What about using a 50R coax with BNC unterminated at the scope and at DUT
tie shield to ground and tie signal thru 50R to center coax lead. Keep
shield/center and 50R as short as possible. Will that not work for high
gain/high Z scope measurements up to 20MHz?
Regards,
Harry

 

[Phil Hobbs]

About the best you can easily do is get a piece of coaxial cable, BNC
on one end into the scope, and solder the shield/center to the board
on the other end, using a ground pad close to the signal pad. It helps
to wind it through a ferrite core midway along the cable, to break up
RF ground loops between the scope and the DUT. This technique of
cource assumes a low-impedance signal that isn't changed much by the
coax+scope capacitance.

Designing a few SMA connector footprints into your board layout, to
snoop critical signals, is great, if you think about it beforehand.

Regular passive 10:1 probes are terrible for characterising wideband,
low-level noise on a board. Fet probes are a lot better, but still
cost you 10:1 on amplitude. There are exotic/expensive active
differential probes, too.


John

A 450-ohm chip resistor soldered on to the centre conductor of a
flush-cut end of RG58 makes a pretty good X10 probe.

Cheers,

Phil Hobbs

 

[John Larkin]

A 450-ohm chip resistor soldered on to the centre conductor of a
flush-cut end of RG58 makes a pretty good X10 probe.

Cheers,

Phil Hobbs

Caddock makes some axial resistors that plug directly into an SMA
female and make an instant 10:1 or 20:1 probe, 0.25 pF equivalent,
good to 6 GHz or so and fine for hundreds of volts peak. HP used that
idea in their 54006A probe kit.

There's something magical, I guess, about the resistance/distributed
capacitance of those resistors. We've used them as pickoffs in our
high-voltage pulsers.


Hey, take a look at the price!

http://www.home.agilent.com/USeng/nav/-536900342.536883024/pd.html


John

 

[John - KD5YI]

Caddock makes some axial resistors that plug directly into an SMA
female and make an instant 10:1 or 20:1 probe, 0.25 pF equivalent,
good to 6 GHz or so and fine for hundreds of volts peak.

Hi, John -

I could not find such a beast on Caddock's site. Perhaps I'm too blind. Do
you have a link?

Thanks,
John

 

[John Larkin]

Hi, John -

I could not find such a beast on Caddock's site. Perhaps I'm too blind. Do
you have a link?

Thanks,
John

They're the MG series; see pics in a.b.s.e.

John