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probe.


James Theonas
 

A quick question for the group. I have a 2465b and have found a p6139a locally in good condition. Is this probe a good match for my scope?
Dimitris Theonas


Colin Herbert
 

This is intended for use with some TDS scopes and has a 500MHz bandwidth. So probably ok with a 400MHz scope. They also have a problem which can render them scrap. See:
http://w140.com/tekwiki/wiki/P6139
I don't have either a 2465B or a P6139A, so you can take this with a pinch of salt, except for the mentioned problem.
Colin.

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of James Theonas via groups.io
Sent: 28 May 2020 06:40
To: TekScopes Mail List (New: groups.io)
Subject: [TekScopes] probe.

A quick question for the group. I have a 2465b and have found a p6139a locally in good condition. Is this probe a good match for my scope?
Dimitris Theonas


 

On Thu, May 28, 2020 at 07:40 AM, James Theonas wrote:


Is this probe a good match for my scope?
It would be insofar as bandwidth is concerned. The transient response however is not matched to the input characteristics of the 2465B. It'll work but signal fidelity will not be optimal, resulting in visible step response artifacts. For the 2465B, Tek recommended the P6137. With the right tools and equipment, you could optimize the behavior of the probe with its high-frequency adjustments.

Raymond


Chuck Harris
 

Tektronix tweaked the probes meant for the 2465B to match
the response of the inputs on the 2465B, so for specified
performance, only a p6137 will do.

But, likely you will never notice the difference.

The 10M, probe tip, frequency response of these scopes was
simply bragging rights for tektronix.

The parallel capacitive reactance at the probe tip swamps
the probe's 10M resistance long before you get to 400MHz.

It even swamps most circuits where you might think you would
like to make a measurement using a 10M probe...

Amuse your date, calculate Xc for the probe at 400MHz.

P6135,6,7 probes are pretty easily damaged. Often it is
better to get a much slower P6105,6,7 100MHz probe, and
enjoy its near indestructibility.

It is unlikely you will ever be bothered by its frequency
response.

-Chuck Harris

James Theonas via groups.io wrote:

A quick question for the group. I have a 2465b and have found a p6139a locally in good condition. Is this probe a good match for my scope?
Dimitris Theonas


 

James,
Maybe my earlier response was a bit terse and therefore limited.

The general answer to your question "Is this probe a good match for my scope?" should be "no". Chuck's answer and mine show you why:
You will not in practice achieve the "promised" 400 MHz bandwidth with that combination. But, neither will you with the Tek-specified P6137. That does not mean there's no observable difference between the two under any circumstances. Under specific test circumstances the difference is clear.

If you purchase a P6139A with the intention to observe and measure over the full bandwidth (bw) of the 2465B 'scope, you'll be disappointed. In fact, *no* high-impedance passive probe will do, as Chuck explained. One might add that the source impedance used for specifying scope bw is 25 (!) Ohm. Not many real-life signal sources fit that requirement!

At frequencies below 100 MHz or edge speeds slower than ca. 15ns the P6139A will provide acceptable fidelity. Most people use their 'scopes in that range and it makes no real difference if you use a P6139A, P6139B or a P6137.
Remember that for looking at edges, it's not the frequency but the edge speed that determines the speed requirements. For faster speeds either a fast FET probe or a Lo-Z probe with 50, 500 or 500 Ohm is a better choice. Get a copy of Tek's "ABC of Probes" or Doug Ford's "The Secret World of Oscilloscope Probes" and read all about it.

As said, for things like audio, Arduino projects, shortwave Ham and the like, a P6139A or almost any other "HF" quality passive probe will make you happy. That's where Chuck's P6105, 6, 7 choice comes in. They can often be had at low prices. If you get one of those, try and get a low-inductance ground clip/wire, unless you only do audio. The standard ground wire (4 - 6 inches) produces huge ringing, even at frequencies below 100 MHz. And choose an attenuation of 1:10, *not* 1:1. If you want to know why, read the publications mentioned above.

For anything faster, go Lo-Z (cheap/very cheap (DIY possible)) or (Hi-Z) FET.

Raymond


Jim Ford
 

I agree. I recently grabbed a P6139A out of Raytheon Technologies (RTX, my day job) e-waste bin and found that it works well with my 400 MHz HP 54504A scope (also an RTX rescue). I was able to adjust the probe to get a nice flat response when connected to the calibrator on the 54504A. Fortunately it also looked flat with the calibrator on my Tek 7904.

So, yes, for most purposes a properly adjusted 500 MHz probe will work with a 400 MHz scope. If you're testing pulses up near the limit (handy rule of thumb is pulse rise/fall time is 0.35/3 dB bandwidth), say 0.9 ns (about 400 MHz 3 dB BW), then no, you won't get an accurate picture unless you use the probe specifically designed for the scope. Also handy to remember 1 ns rise or fall time is equivalent to 350 MHz 3 dB BW, and 1 GHz BW is equivalent to 0.35 ns rise or fall time.

And I'd check the input capacitance of the scope (7 pF in the case of the 54504A) versus the output capacitance of the probe (8 pF in the case of the P6139A), and if they are close, you will probably get good results when you connect the probe to the calibrator and adjust it for flatness. Sometimes you can even find the range of capacitance of the probe. The HP 10430A probes I have with the 54504A scope say "1 Megohm||6.5 pF for 1 Megohm||6-9 pF inputs" on them. Also check http://w140.com/tek_probe_crossref_1990.pdf to see what probe(s) go with what Tek scopes.

There's more, but it's probably covered in those references Raymond mentioned.

Good luck!

Jim Ford

------ Original Message ------
From: "Raymond Domp Frank" <@Raymond>
To: TekScopes@groups.io
Sent: 5/28/2020 4:49:56 AM
Subject: Re: [TekScopes] probe.

On Thu, May 28, 2020 at 07:40 AM, James Theonas wrote:


Is this probe a good match for my scope?
It would be insofar as bandwidth is concerned. The transient response however is not matched to the input characteristics of the 2465B. It'll work but signal fidelity will not be optimal, resulting in visible step response artifacts. For the 2465B, Tek recommended the P6137. With the right tools and equipment, you could optimize the behavior of the probe with its high-frequency adjustments.

Raymond



 

On Thu, May 28, 2020 at 08:36 PM, Jim Ford wrote:


I was able to adjust the probe to get a nice flat response when connected to
the calibrator on the 54504A. Fortunately it also looked flat with the
calibrator on my Tek 7904.
Hi Jim,
The artifacts that I was referring to don't show up on standard 'scope calibrators. Those only allow you to adjust "LF" behavior with a 1 kHz square wave (flat top).
I meant fast-edge behavior, like that visible with <=1 ns steps in the first few ns or in the edge itself. That's where the differences are. You won't just see a flat or single-curved top but all kinds of ripples, resonance-like behavior etc. I know about only one "classic" Tek 'scope that offers that kind of fast step: the 485 in addition to a nice 1 kHz square wave. It allows LF adjustment *and* HF adjustment. The latter is done with separate adjustment regulators in the compensation box of the probe, underneath the plastic cover. *And* you need a well-matched input to the probe, like a probe-shaft-to-BNC adapter
The calibrator's edges in the 7904 are no faster than 250ns so they are useless for showing the differences between e.g. the P6139A and P6137, much less perform the HF adjustment I was referring to.


And I'd check the input capacitance of the scope (7 pF in the case of the 54504A) versus the output capacitance of the probe (8 pF in the case of the P6139A), and
if they are close, you will probably get good results when you connect the probe to the calibrator and adjust it for flatness.
That's not correct: 8 pF is the nominal *input* capacitance of the P6139A and they don't have to be nor will be equal. Otherwise, you wouldn't be able to adjust an 8pF or 1.5 pF FET input capacitance probe to several different 'scopes, like the 2465B with its 15pF nominal input capacitance or a 7A26 with 22pF nominal. It's all about RC times, probe input, cable, probe output, 'scope input. The publications I referred to show it all.

Nothing to worry about for general 'scoping in the sub-100MHz domain though. Who said that 100 MHz actually is DC?

Raymond


Roy Thistle
 

From what I've read...Tek took a systems approach with this series of 2400 scopes... so it's fair to consider a 2465B scope plus the Tek recommended P6137 as what Tek intended... when interpreting the Tek specs about them.
Taking that way of looking at it... the DUT is on one end of the system, and you are on the other... and so what you see is what you get.

I've a feeling that Tek enthusiasts might buy a Tek scope just based on the nominal bandwidth, and then look for probes with a matching nominal bandwidth.
Given just the above, and assuming too, that the front end, filter response, of a 2465B is Gaussian, then:

(1) BW(system) = 1 / sqrt[ 1/(probe bandwith ^2) + 1/(scope bandwidth ^2) ]

So from (1) it is easy to see... if the bandwidth of the scope and the probe match... the nominal bandwidth of the system is scaled by 1/sqrt(2)
Thus, for a 400 MHz nominal bandwidth... (1) gives a system bandwidth of approximately 282 MHz. (Check the calculation!)
For a 2465B... that's a "sweet spot" in the sense that reducing the nominal bandwidth of the probe is only going to reduce the system bandwidth... but given the high non-linearity of (1), its not a proportional reduction.

So consider using a probe with a nominal bandwidth of 100 MHz. Then (1) gives a system bandwidth of approximately 97 MHz. That is pretty close to the nominal bandwidth of the probe.

The takeaway from (1) is that just considering bandwidth specs... if you are working on 10 MHz timebase, with a 2465,B... say just for the features that Tek offers.. then using a P6105 probe is probably okay. (I say probably...because Tek specs a P6105 for 15 pf, on the lower end of the compensation... and Tek specs the input of some? 2465Bs at 15 pF + - 2 pF.)

Best regards and wishes.
Roy


Jim Ford
 

I stand corrected. Sometimes you can teach an old dog new tricks! ;)

Thanks, Raymond!

Jim

------ Original Message ------
From: "Raymond Domp Frank" <@Raymond>
To: TekScopes@groups.io
Sent: 5/28/2020 1:31:23 PM
Subject: Re: [TekScopes] probe.

On Thu, May 28, 2020 at 08:36 PM, Jim Ford wrote:


I was able to adjust the probe to get a nice flat response when connected to
the calibrator on the 54504A. Fortunately it also looked flat with the
calibrator on my Tek 7904.
Hi Jim,
The artifacts that I was referring to don't show up on standard 'scope calibrators. Those only allow you to adjust "LF" behavior with a 1 kHz square wave (flat top).
I meant fast-edge behavior, like that visible with <=1 ns steps in the first few ns or in the edge itself. That's where the differences are. You won't just see a flat or single-curved top but all kinds of ripples, resonance-like behavior etc. I know about only one "classic" Tek 'scope that offers that kind of fast step: the 485 in addition to a nice 1 kHz square wave. It allows LF adjustment *and* HF adjustment. The latter is done with separate adjustment regulators in the compensation box of the probe, underneath the plastic cover. *And* you need a well-matched input to the probe, like a probe-shaft-to-BNC adapter
The calibrator's edges in the 7904 are no faster than 250ns so they are useless for showing the differences between e.g. the P6139A and P6137, much less perform the HF adjustment I was referring to.


And I'd check the input capacitance of the scope (7 pF in the case of the 54504A) versus the output capacitance of the probe (8 pF in the case of the P6139A), and
if they are close, you will probably get good results when you connect the probe to the calibrator and adjust it for flatness.
That's not correct: 8 pF is the nominal *input* capacitance of the P6139A and they don't have to be nor will be equal. Otherwise, you wouldn't be able to adjust an 8pF or 1.5 pF FET input capacitance probe to several different 'scopes, like the 2465B with its 15pF nominal input capacitance or a 7A26 with 22pF nominal. It's all about RC times, probe input, cable, probe output, 'scope input. The publications I referred to show it all.

Nothing to worry about for general 'scoping in the sub-100MHz domain though. Who said that 100 MHz actually is DC?

Raymond