Tek 475 trace/probe problem

[amdx]

Hi Guys,
I recently acquired a Tektronix 475 scope.
While testing it with the internal squarewave calibrator
I noted the trace was wide, I tried the second probe, same thing.
Then I happened set the probe cable across the top of the scope,
when I did that the trace got thin or more normal.
I tried a third probe it is better but the trace width varies somewhat
depending where the probe is positioned, any ideas?

Mikek

 

[mike]

Hi Guys,
I recently acquired a Tektronix 475 scope.
While testing it with the internal squarewave calibrator
I noted the trace was wide, I tried the second probe, same thing.
Then I happened set the probe cable across the top of the scope,
when I did that the trace got thin or more normal.
I tried a third probe it is better but the trace width varies somewhat
depending where the probe is positioned, any ideas?

Mikek

Wide???
As in horizontal width on screen?
As in thickness of the green line?
As on RF pickup?
It's a sensitive scope with wide bandwidth.

 

[amdx]

Mikek

Wide???
As in horizontal width on screen?
No.

As in thickness of the green line?
Yes.

As on RF pickup?

Don't know the cause, trace width Ok with no probe attached.

It's a sensitive scope with wide bandwidth.

Yes, pulling bandwidth limiter to 20Mhz solves the problem.

Here's a photo of calibrator output.

http://s395.photobucket.com/user/Qmavam/media/P1010017_zps53b1b972.jpg.html

Here's a photo of calibrator output with probe cable laid across top of
scope case.

http://s395.photobucket.com/user/Qmavam/media/P1010018_zps1eb5f91b.jpg.html

Note difference in trace width.



Thanks, Mikek

 

[John S]

Hi Guys,
I recently acquired a Tektronix 475 scope.
While testing it with the internal squarewave calibrator
I noted the trace was wide, I tried the second probe, same thing.
Then I happened set the probe cable across the top of the scope,
when I did that the trace got thin or more normal.
I tried a third probe it is better but the trace width varies somewhat
depending where the probe is positioned, any ideas?

Mikek

What is your vertical sensitivity setting?

Try connecting your ground lead to the probe tip and move the probe
around. You may have some pickup from a florescent lamp or something.

 

[amdx]

What is your vertical sensitivity setting?

Calibrator output is 300mv P to P.
Scope is set to 10mv scale with a X10 probe.

Try connecting your ground lead to the probe tip and move the probe
around. You may have some pickup from a florescent lamp or something.

Connecting ground lead to tip, the trace looks OK.
Moving it around doesn't change trace.

My probes are B&K PR-45. I thought these were 35Mhz bandwidth, but a
quick search yielded no reference, so now I'm not sure.
I'll search more later, gotta leave for work.

Thanks, Mikek

 

[John S]

Calibrator output is 300mv P to P.
Scope is set to 10mv scale with a X10 probe.




Connecting ground lead to tip, the trace looks OK.
Moving it around doesn't change trace.

My probes are B&K PR-45. I thought these were 35Mhz bandwidth, but a
quick search yielded no reference, so now I'm not sure.
I'll search more later, gotta leave for work.

Thanks, Mikek

Do you see this on both input channels?

The two pics you posted show that the voltage amplitude is not the same
with probe position (assuming you did not change vertical sensitivity).
You might have an open input on channel 1. Otherwise, you may have some
open probes.

 

[amdx]

Do you see this on both input channels?

The two pics you posted show that the voltage amplitude is not the same
with probe position (assuming you did not change vertical sensitivity).
You might have an open input on channel 1. Otherwise, you may have some
open probes.

I'll check the other channel this evening.
I suspect I did change amplitude between pictures.
BTW, the scope has a 20Mhz bandwidth limiter and that does get rid of
the wide trace.
Mikek

 

[Artemus]

Hi Guys,
I recently acquired a Tektronix 475 scope.
While testing it with the internal squarewave calibrator
I noted the trace was wide, I tried the second probe, same thing.
Then I happened set the probe cable across the top of the scope,
when I did that the trace got thin or more normal.
I tried a third probe it is better but the trace width varies somewhat depending where the probe
is positioned, any ideas?

Mikek

Perhaps an internal broken ground to the input connector?
Laying the probe on top would capacitivly couple the probe
shield to that ground.
Art

 

[amdx]

Perhaps an internal broken ground to the input connector?
Laying the probe on top would capacitivly couple the probe
shield to that ground.
Art

It has the same problem on both channels.
It's late so I'll do some more checking in the morning.
Mikek

 

[[email protected]]

could be RF pickup.

are you located near any AM FM TV or other kind for RF high power transmitter?


Mark

 

[John S]

It has the same problem on both channels.
It's late so I'll do some more checking in the morning.
Mikek

Okay. It sure looks like a signal getting in somewhere. Probably via the
probe based on your reports.

What is confusing is that the probe calibration source must be a very
high impedance (if your probe is good) to have this kind of
interference. You still had the probe attached to the calibrator when
you laid the probe across the top of the instrument, yes?

The 20MHz filter is telling you that you have an HF signal getting in
somewhere, I think. Try moving the scope to some other place and see if
the problem follows. Turn off your lights and anything else while
experimenting with tracing the source.

With the probe not shorted and not connected to your calibrator, move it
all around the area and watch the amplitude. Maybe it will lead you to a
source.

I have a florescent desk lamp that must be turned off for sensitive
measurements.

 

[[email protected]]

I think it's probably just a cheap probe picking something up. For anything to be wrong with the scope I think it would have to be some wierd ass grounding problem.

 

[mike]

Perhaps an internal broken ground to the input connector?
Laying the probe on top would capacitivly couple the probe
shield to that ground.
Art

Assume you've done obvious stuff, like verify continuity between the
connector and the ground wire at the probe end?

Some probes and some scope inputs have resistance in the ground
lead. If you put current into the ground, it blows the resistor
and the ground is no more.

Have you tried to trigger the scope on the noise?

I noticed ambient noise went way up when I switched from
incandescent to CFL lighting.

 

[amdx]

Okay. It sure looks like a signal getting in somewhere. Probably via the
probe based on your reports.

What is confusing is that the probe calibration source must be a very
high impedance (if your probe is good) to have this kind of
interference. You still had the probe attached to the calibrator when
you laid the probe across the top of the instrument, yes?

Yes, the probe was attached to the calibrator.
The calibrator is low impedance, 300mv at 30ma.

The 20MHz filter is telling you that you have an HF signal getting in
somewhere, I think. Try moving the scope to some other place and see if
the problem follows. Turn off your lights and anything else while
experimenting with tracing the source.

There is not much turned on around my bench, but I understand your
point. I have two CFLs that both create noise at about 45khz, but they
are of.

With the probe not shorted and not connected to your calibrator, move it
all around the area and watch the amplitude. Maybe it will lead you to a
source.

I used delayed sweep to zero in on the noise, it is about
approximately 1Mhz.
I used a 4ft piece of coax with a BNC connector on one end, the other
end I frayed about two inches of the shield out as a ground plane with
the center condutor as the vertical. It picked up the CFLs fine but I
don't see the 1Mhz.
I'll take the scope on a road trip and see what happens.

BTW, I received a P6136 350Mhz probe yesterday, It has the same noise
problem. Also, My Tektronix 2465 scope at the same position does NOT
have this noise problem.

Thanks, Mikek

 

[amdx]

I think it's probably just a cheap probe picking something up. For anything to be wrong with the scope I think it would have to be some wierd ass grounding problem.

Well it is a B&K probe, but I have two of those and a third other
brand, and yesterday I received a P6136 Tektronix probe that has the
shows the same noise when I attached it.

Thanks, Mikek

 

[amdx]

could be RF pickup.

are you located near any AM FM TV or other kind for RF high power transmitter?


Mark

I'm not, But I'm taking the scope on a road trip shortly to see if
I'm getting away from a noise source.

Thanks, Mikek

 

[amdx]

Assume you've done obvious stuff, like verify continuity between the
connector and the ground wire at the probe end?

I've tried four different probes, plus they all work fine on another
scope.

Some probes and some scope inputs have resistance in the ground
lead. If you put current into the ground, it blows the resistor
and the ground is no more.

Have you tried to trigger the scope on the noise?

I haven't, I can try but I think the amplitude is to low.
I did use delayed sweep and it is roughly 1 Mhz.

I noticed ambient noise went way up when I switched from
incandescent to CFL lighting.

Yes, I have two CFLs that sing at about 45khz.

Thanks, Mikek

 

[amdx]

I have a squarewave from the calibrator displayed.
It has noise. I measured the frequency of the noise,
it is about 110Mhz.
I took the scope to another location, it had the same problem.
Here again, is a picture of the noise.

http://s395.photobucket.com/user/Qmavam/media/P1010017_zps53b1b972.jpg.html
This happens on both channels, both probes.

I also notice something else, and this again happens on both channels,
both probes. I have the probe tip connected to the ground of Channel 2
the BNC connector ground. On the screen note the 110Mhz signal.
http://s395.photobucket.com/user/Qmavam/media/P1010034_zps0308c422.jpg.html

Increasing the timebase for a longer time period, you can see the signal
has some modulation. The modulation is unchanging.
http://s395.photobucket.com/user/Qmavam/media/P1010039_zpsbac2e623.jpg.html

Any Ideas?

Mikek

 

[Tim Williams]

I also notice something else, and this again happens on both channels,
both probes. I have the probe tip connected to the ground of Channel 2
the BNC connector ground. On the screen note the 110Mhz signal.
http://s395.photobucket.com/user/Qmavam/media/P1010034_zps0308c422.jpg.html

Increasing the timebase for a longer time period, you can see the signal
has some modulation. The modulation is unchanging.
http://s395.photobucket.com/user/Qmavam/media/P1010039_zpsbac2e623.jpg.html

110MHz is either top of the FM band (modulation would be multipath, but
wouldn't be modulated that fast) or bottom of the aviation band
(specifically, radionav at 108-118; comms are 118-137). Got any stations
nearby?

Try ferrite beads on the probe cable, as near the scope as possible.

If you're concerned that it's from the scope, try taking off the cover and
probing around in air with the probe tip, see if you can find something
squealing. If you're concerned that it's something else, attach a loop
antenna to a long enough BNC cable and probe around your area.

Tim

 

[Artemus]

I have a squarewave from the calibrator displayed.
It has noise. I measured the frequency of the noise,
it is about 110Mhz.
I took the scope to another location, it had the same problem.
Here again, is a picture of the noise.

http://s395.photobucket.com/user/Qmavam/media/P1010017_zps53b1b972.jpg.html
This happens on both channels, both probes.

I also notice something else, and this again happens on both channels, both probes. I have the
probe tip connected to the ground of Channel 2 the BNC connector ground. On the screen note the
110Mhz signal.
http://s395.photobucket.com/user/Qmavam/media/P1010034_zps0308c422.jpg.html

Increasing the timebase for a longer time period, you can see the signal has some modulation. The
modulation is unchanging.
http://s395.photobucket.com/user/Qmavam/media/P1010039_zpsbac2e623.jpg.html

Any Ideas?

Mikek

Since you have 2 (or more) scopes, use the second one to look at the cal out
on the noisy scope. This will rule out, or point to, a noisy cal generator. Be
sure to get your grounds connected. I still think bad connector grounds are
more likely on the noisy scope.
Art