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Probe into probes. What's up?
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BillyO:

--- Quote from: bdunham7 on February 01, 2023, 03:42:13 am ---That certainly made a lot of difference, and if you were looking to measure the rise time (characterizing re a spec) it should improve.  But is it really better or more accurate or do you just dislike the aberrations less?

--- End quote ---
I think both to some extent.  That document on probe circuits mentioned above states several times that Tek probes are spec'd and calibrated using a 25 ohm source and they make a point of mentioning finding low impedance probe points for HF signals.

It's not really fair to compare a P6139 to the P6156 for rise time and susceptibility to source impedance, however I am scratching my head as to why your 6139 shows practically no degradation in rise time with the 50 ohm source.  What source are you using (not that that would matter)?  Also, what other modifications were done to you 6139?

Since the scope itself would not be much effected by the source impedance through a 9M ohm probe, and it is responding properly to the pulsers when directly connected, then any aberrations must be in the probes.  But they do behave fairly well when applied as suggested by Tek.

In any case, the document discusses the circuit used in the 6139 on page 23 and gives an outline of the adjustment procedure so I now have a fairly good place to start with and see if I can improve things.  However, I feel fairly certain I'm not going to get what you are seeing with you 6139 using a 50 ohm source.
bdunham7:

--- Quote from: BillyO on February 01, 2023, 04:20:51 am ---It's not really fair to compare a P6139 to the P6156 for rise time and susceptibility to source impedance, however I am scratching my head as to why your 6139 shows practically no degradation in rise time with the 50 ohm source.  What source are you using (not that that would matter)?  Also, what other modifications were done to you 6139?

In any case, the document discusses the circuit used in the 6139 on page 23 and gives an outline of the adjustment procedure so I now have a fairly good place to start with and see if I can improve things.  However, I feel fairly certain I'm not going to get what you are seeing with you 6139 using a 50 ohm source.

--- End quote ---

I'm using the same Bodnar pulser.  My 6139A is older than yours, has the threaded attenuator and is not made in China.  It has not been modified that I know of.  My guess is that the degradation doesn't get much worse because there isn't as much in the first place.

I don't know if you can get the same results I do.  David Hess suggests that Tek is juicing the response of these to improve HF response of particular oscilloscopes they were used with, which in your case would be the TDS3000 series if I have that right.  By 'juicing the response' I mean deliberately loading the circuit more at HF in order to make up for losses elsewhere.   So in that case, the source impedance would have an even larger effect than might normally be expected. 
jonpaul:
Rebonjour a tous...

very Fine thread, probes are one of my favorite tools..

At >100 MHz the probe C dominates the impedance and thus the 10X high Z  probes are not so good. 

1/ For 95% of our   P6137 TEK are best. We found some NIB in orig case with all tips/clips/adapters.

2/ At >100 MHz the Zo probes like P6136 are excellent, despite the 500 Ohm..5K load, only  use on 50 Ohm input scopes.

3/ My Easy and cheap wideband DIY:

Thin 50 Ohm  coax Rg175/U or similar , .5..1.5 M
Term to BNC one side
Use  with short shield direct 
OR insert series 450 or 4950 R at cable shield/tip to make a DIY Zo probe.

4/ The TEK calibrate with the comp trim with a 1 kHz cal signal. Beware some scopes change CAL freq with time base (485?)

5/ Active probes (diff/FET) are very rarely needed and can be easy to damage physically or electrically

6/ In SMPS/EBU work,  current is often more useful than voltage info.
We provide small bus loops with Teflon tube insulation on a proto or SMPS PCB.

7/Using  P6021, P6022, and the DC Hall effect probes/amps P6302, 6303, AM502B.
A fine passive current transformer  is the very wideband Pearson CT, 410, 411, The  very best and widest freq resp and current capability.
Use a wire passed thru the toroid...

Hope this is interesting!

Happy probing!

 
Jon

BillyO:
Could it be that my Bodnar is faulty, or at least borderline?

Even if I feed it directly into the scope, the signal into 1M is very much different than into 50.  See the shots below - 1M first, then 50.  You can see the 1M rise time is many times longer, and there is that shift 66% of the way up the rising edge.

@bdunham7, what does yours do in the same situations?

jonpaul:
used Leo Bodnar 40ps pulser for years.

Rated only with 50 ohm load

No spec for open circuit load

Ask Leo,

Jon
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