Oscilloscope probes

[starlight_tools]

Thanks gang,

Pintek CP-3308R it is, got a confirmation this morning that they have been shipped from Taiwan, estimated delivery of June 6 to the 13th. 

https://www.ebay.ca/itm/2pc-Oscilloscope-Probe-Kit-CP-3308R-100-1-300Mhz-1500V-100M%CE%A9-Readout-Pin-PINTEK/142102785647


Walter, TTDr

Probes arrived on the 4th, still waiting for the 'silly scope.

Be careful looking at the "see other items" cart filled up fast with all sorts of other stuff.

[starlight_tools]

Oscilloscope arrived in and I calibrated the CP-3308R probes and the stock PP510 probes.

Noticed that with the CP-3308R probes, one did not have the red insulator over the adjusting screw, but had the metal screw showing, and that the adjusting screw driver did not fit as well as it did with the probes with the red plastic "screw caps".  the supplier stated, when asked that this was the new design.

Hope this does not present a problem later on.

[Finderbinder]

May someone explain how to check probe's BW properly don't having expensive gear?

[tggzzz]

May someone explain how to check probe's BW properly don't having expensive gear?

What matters is the scope, the type of probe, the probe's compensation, the way the probe is attached to the circuit, and the circuit. Concentrating on the probe alone is an academic exercise

Of those, the key points people tend not to consider are choosing the appropriate type of probe, and the probing technique.

[exe]

May someone explain how to check probe's BW properly don't having expensive gear?

I connected probes to siggen and measured amplitude of sine wave. To be sure it's not just peaking at a particular frequency I did test over multiple frequencies. Turned out that cheap probes (afaik pp-150) were noticeably better (in terms of amplitude) than the bundled once. I don't remember if siggen output was terminated with 50 or not, but it probably should. Also, ground lead should be eliminated for such measurements.

One may probably use a pulse gen or something. But I'm not sure how to make a correct setup and how to evaluate results.

[nctnico]

May someone explain how to check probe's BW properly don't having expensive gear?

I connected probes to siggen and measured amplitude of sine wave. To be sure it's not just peaking at a particular frequency I did test over multiple frequencies. Turned out that cheap probes (afaik pp-150) were noticeably better (in terms of amplitude) than the bundled once. I don't remember if siggen output was terminated with 50 or not, but it probably should. Also, ground lead should be eliminated for such measurements.

One may probably use a pulse gen or something. But I'm not sure how to make a correct setup and how to evaluate results.

Probes are specified with a signal source impedance of 25 Ohms.

[tggzzz]

One may probably use a pulse gen or something. But I'm not sure how to make a correct setup and how to evaluate results.

In a practical sense that is easy. In a theoretical sense it is more tricky.

A scope shows the time-domain shape of the waveform, so if you know (by independent means) the waveform shape, you can compare it with what is displayed on the screen. The simplest such waveform is a rectangular wave with flat top/bottom and a risetime that is faster than the scope+probe combination.

You are unlikely to get that from a general purpose sine/square/triangle waveform generator, since the risetime is unlikely to be fast enough and there may be ringing.

Your best bet is either Leo Bodnar's generator, or you can make one as shown here: https://www.eevblog.com/forum/testgear/show-us-your-square-wave/msg1902941/#msg1902941

The above practical test is especially useful in that you will be able to see the effect of your probing technique, which will dominate that a properly adjusted probe.

Probe manufacturers are interested in avoiding the problems with probing technique, so they have adaptors that connect a signal generator directly to a probe tip, with other specified conditions. Such theoretical tests are of little value when making real-world measurements.

One example of poor and improved probing technique (plus theory) is found here:
https://entertaininghacks.wordpress.com/2015/04/23/scope-probe-accessory-improves-signal-fidelity/
https://entertaininghacks.wordpress.com/2016/09/17/scope-probe-accessory-higher-frequency-results/

[jonpaul]

Rebonjour a tous: Fine to see so much activity on this old topic.

Glad my notes were useful to some.  A few comments:

1/ Testing and CAL of probes:  Highly recommend  Leo Bodnar 40 pS pulser for risetime test and transient adjustments.
http://www.leobodnar.com/shop/index.php?main_page=product_info&products_id=295
UK L 50. (pre Brexit!)


For fast transients, The test setup is critical. Minimum ground return path on the probe is critical, best is a bayonet or direct BNC adapter NOT  a wire ground  lead.

Any constant amplitude RF gen can be used for BW, we use TEK SG503, 191, SG504, etc. The huge classic HP 8640B  RF gen is very useful with regulated output 500 kHz - 1250 MHz.

2/ For wideband applications > 300MHz the Zo probes such as TEK P6156  are best and go to 3 GHz. These MUST be used with a 50 Ohm scope input and impose a résistive load 500 Ohm...several K Ohm.

3/ FET probes are large size and easy to blow the FETs and impossible to fix.  Many on epay are damaged or missing tips etc.

4/  for 300..500 MHz scopes and plugins, suggest  TEK 6137, best compromise in size, BW and ruggedness. Many tip adapters available. Other similar TEK 10X are too delicate or worse response.

5/ 10X: CHeck scope input capacitance, vs probe spec, 10x probes have a range of allowed Cin that the compensation will accommodate.
 eg P6137 is for 246x TEK and 15 pF.

6/ Avoid switched probes with 1X/10X, never reliable switches and often poor quality or poor 10X response.

7/ We never  needed 1X probes: Use a BNC with coax, connect direct. RG178 is 50 Ohm and ~ 0.125" ~ 2 mm dia. Works on either 1M or 50 Ohm input.

8/ High voltage probes are a different animal with many uses and precautions. Beware of Freon requirement in TEK P6015 1000x.

9/ For professional or serious application, especially if high power or regulatory compliance is involved, strongly suggest to avoid the cheap Chinese knockoff junk and stick to Tektronix probes. Yokogawa are a very good alternative.

10/

Just the ramblings of an old retired EE!

HAPPY PROBING AND FINE HOLIDAYS TO ALL

Jon

[nctnico]

One may probably use a pulse gen or something. But I'm not sure how to make a correct setup and how to evaluate results.

In a practical sense that is easy. In a theoretical sense it is more tricky.

A scope shows the time-domain shape of the waveform, so if you know (by independent means) the waveform shape, you can compare it with what is displayed on the screen. The simplest such waveform is a rectangular wave with flat top/bottom and a risetime that is faster than the scope+probe combination.

You are unlikely to get that from a general purpose sine/square/triangle waveform generator, since the risetime is unlikely to be fast enough and there may be ringing.

Your best bet is either Leo Bodnar's generator, or you can make one as shown here: https://www.eevblog.com/forum/testgear/show-us-your-square-wave/msg1902941/#msg1902941

No. I don't get why people keep bringing up fast edges to check oscilloscope bandwidth. It is not a 100% correct method because not all oscilloscopes adhere to the factor 0.35 for the steepness of the edge. The only way is to use a levelled signal generator.

[tggzzz]

One may probably use a pulse gen or something. But I'm not sure how to make a correct setup and how to evaluate results.

In a practical sense that is easy. In a theoretical sense it is more tricky.

A scope shows the time-domain shape of the waveform, so if you know (by independent means) the waveform shape, you can compare it with what is displayed on the screen. The simplest such waveform is a rectangular wave with flat top/bottom and a risetime that is faster than the scope+probe combination.

You are unlikely to get that from a general purpose sine/square/triangle waveform generator, since the risetime is unlikely to be fast enough and there may be ringing.

Your best bet is either Leo Bodnar's generator, or you can make one as shown here: https://www.eevblog.com/forum/testgear/show-us-your-square-wave/msg1902941/#msg1902941

No. I don't get why people keep bringing up fast edges to check oscilloscope bandwidth. It is not a 100% correct method because not all oscilloscopes adhere to the factor 0.35 for the steepness of the edge. The only way is to use a levelled signal generator.

That's true, but who really cares what a scope's frequency response is?!

A scope makes time-domain measurements, and needs to do that accurately. If you want to make frequency-domain measurements, use a spectrum analyser or VNA etc.

Where scopes post-process their time-domain information into frequency-domain information, they ought to know and take account of their internal characteristics. For modern professional scopes (i.e. not typical amateur scopes) an "intelligent" probe can also aid removal of distortions in the probe.

But in most cases of relevance to people on this forum, the probing technique will dominate all that - and cannot be removed in the scope.

[tggzzz]

2/ For wideband applications > 300MHz the Zo probes such as TEK P6156  are best and go to 3 GHz. These MUST be used with a 50 Ohm scope input and impose a résistive load 500 Ohm...several K Ohm.

... and at even 100MHz those have an input impedance higher than, say, Tektronix P6137 and all the similar ones

Plus you can make them yourself and even solder them into the circuit. Just solder a 500ohm resistor with short leads directly to the testpoint, and solder a minature coax to that resistor and the ground plane.

Note that a typical scope's 20pF input capacitance in parallel with a 50ohm termination is an issue. That's one reason I like my Tek 485: it has a proper 50ohm attenuator on the inputs

[jonpaul]

Hello again:

We make both time and frequency tests with our analog and digital scopes. Reliable specs are important in both domains. We rely on Tektronix and Yokogawa for specified performance in both BW and transient response. We have see many Chinese knockoff scopes and probes that have poor results in either or both areas.

1/ Depending on the application, BW or transient resp may be important.
Examples: Transient: Testing of high speed digital signal transmission eg transformers and cables.
Bandwidth: Checking generator outputs, broadcast exciters and transmitters, etc.
Notice that a good BW scope and probe could have a poor transient response and a perfect transient response will not have the widest BWs due to phase response.

In CAL of wideband scopes eg TEK 2465/7/B or 7104 + 7A29, transient response is the most difficult and critical adjustment.
BW is easier and if transient is good, probably BW is OK after CAL.

2/ We also use direct connect with coax. Many wideband and RF PCBs in fine instruments have builtin tiny coax connectors or probe tip receptacles for testing, esp TEK and HP of 1970s...1990s vintage.

3/ A well calibrated probe and/or wideband scope should pass BOTH BW and transient tests:
0.5 db, 3 db and BW = 6db with 50 Ohm term
Transient rise/fall time, abberation, preshoot, overshoot, etc.
The pulse source must be verified for fidelity.
VSWR of both scope and pulse source will cause reflections and give a worse transient result.

Standard practice is to insert a 6 - 20 db wideband attenuator at the scope 50 Ohm input, to attenuate reflections and improve VSWR.

We highly recommend the Mini_Circuits BNC><BNC inline attenuators,  HAT-xx+  series eg HAT-10+ 10 dB, HAT-20+ 20 db.

Specified to 2 GHz. https://www.minicircuits.com/pdfs/HAT-20+.pdf $10 ea  VSWR 1.05..1.10.

4/ tests that use cables are naturally dependent on the cable transient and BW performance. ALL CABLES ARE NOT CREATED EQUAL!
Ordinary coax ><bnc or F or RCA CANNOT be used for calibration or fine measurements! Even USA made Pomona are NOT precision Zo or BW. Let alone the Chinese clone  junk calbes.
Special calibration coax are available from TEK and GenRad, eg the 0.5m precision 50 Ohm 012-0482-00 BNC><BNC or the Rg8U GenRad 874 hermaphrodit connector 1,2,5,10 nS cables

Best connection is direct eg a BNC F><BNCF adapter and NOT a cable!

5/ BNC terms eg 50 Ohm can have issues eg poor quality connectors and inaccurate resistors. The internal construction and type of R determines the BW. Best inline BNC><BNC feedthru are the TEK and Mini-Circuits. The cheapo terms cant be relied on.

Enjoy!

Jon

[Finderbinder]

So many usefull information! Just need endless time to study it all 

[Elasia]

7/ We never  needed 1X probes: Use a BNC with coax, connect direct. RG178 is 50 Ohm and ~ 0.125" ~ 2 mm dia. Works on either 1M or 50 Ohm input.

Just curious, what's your opinion on RG178 vs RG316?

[jonpaul]

>>Just curious, what's your opinion on RG178 vs RG316?
  Finderbinder
 

Have used only RG178/u, normally in 30.....75 cm lengths, so losses were not significant.

Searching the comparison, RG316 is much more costly and less flexible and thicker.

As its PTFE, harder to terminate.

RG316U: Advantage: Mil Spec, 200 dec C temp rations, slightly lower losses.

REFS: http://rfcablescoax.blogspot.com/2012/02/rg-316-rg-174-rg-188-coaxial-cable.html

Belden specs fpr their SKU equiv:

https://catalog.belden.com/techdata/EN/8216_techdata.pdf
https://catalog.belden.com/techdata/EN/83284_techdata.pdf

Enjoy!

Jon

PS: Belden has the best info and range. Their app engineer Steve Lampen is the worlds expert on cables.

[tggzzz]

3/ A well calibrated probe and/or wideband scope should pass BOTH BW and transient tests:
0.5 db, 3 db and BW = 6db with 50 Ohm term
Transient rise/fall time, abberation, preshoot, overshoot, etc.
The pulse source must be verified for fidelity.
VSWR of both scope and pulse source will cause reflections and give a worse transient result.

Probing technique, especially ground lead inductance, is a major issue at these speeds. I'm sure you know that, but the OP might not.

If you have a 6inch/15cm ground lead on a standard passive *10 10Mohm probe, then you will get ringing at ~100MHz. I would be somewhat surprised if a scope could be calibrated to remove that!

[Elasia]

PS: Belden has the best info and range. Their app engineer Steve Lampen is the worlds expert on cables.

+1 for Belden, we use their brands for a good bit of stuff, their extreme cables for off highway are fantastic

[jonpaul]

>>If you have a 6inch/15cm ground lead on a standard passive *10 10Mohm probe, then you will get ringing at ~100MHz. I would be somewhat surprised if a scope could be calibrated to remove that!>>

tggzzz: Cal of a SCOPE is different from CAL of the probe. We do scope CAL as per TEK serv manuals. The pulse source is a TD pulser (or we use Leo B 40 ps) DIRECT to scope BNC.

Probe CAL is just the LF comp for scope input C. Have never needed to cal HF of a probe.

The ground lead length indeed inserts an inductance, which can be calculated or measured.

That is why the best probing technique at HF uses a matched coax onboard and Zo matched cable OR if a probe is used, the bayonet adapters that clip at the probe tip and surrounding ground ring. They have NO ground wire.

A TEK Zo probe like P 6156 can give fine fidelity above 200 MHz with the proper tip and ground bayonet or adapters.

There is NO "calibration" to remove ringing of poor ground return of a probe!

Cheers,

Jon

[jonpaul]

We tested 18 types of cables 110 Ohm TP and 75 Ohm coax for broadcast use  at 100M legnth for atten and digital sig distortion.

Belden 1694A wideband 75 Ohm coax was the best, requires special they F connectors and term tooling.

https://catalog.belden.com/techdata/EN/1694A_techdata.pdf

Jon

[tggzzz]

>>If you have a 6inch/15cm ground lead on a standard passive *10 10Mohm probe, then you will get ringing at ~100MHz. I would be somewhat surprised if a scope could be calibrated to remove that!>>

tggzzz: Cal of a SCOPE is different from CAL of the probe. We do scope CAL as per TEK serv manuals. The pulse source is a TD pulser (or we use Leo B 40 ps) DIRECT to scope BNC.

Probe CAL is just the LF comp for scope input C. Have never needed to cal HF of a probe.

The ground lead length indeed inserts an inductance, which can be calculated or measured.

That is why the best probing technique at HF uses a matched coax onboard and Zo matched cable OR if a probe is used, the bayonet adapters that clip at the probe tip and surrounding ground ring. They have NO ground wire.

A TEK Zo probe like P 6156 can give fine fidelity above 200 MHz with the proper tip and ground bayonet or adapters.

There is NO "calibration" to remove ringing of poor ground return of a probe!

We basically agree; the differences are probably due to (my) poor use of language.

I will, however, note that some *10 passive probes do have HF tweaks inside the case. One example is the Tek 6137 - see p6 of the manual on TekWiki.

[exe]

How practical is it to poke around with a 10x probe at freqs higher than, say, 100MHz? They load circuit quite a lot.

If I needed to see, say, a 500MHz signal, I'd try to gen an active probe, too bad they are freakin' expensive (except may be DIY).

[0culus]

Yes, but you're looking at Low-Z probes past 500-700 MHz or so. Keysight offers 10:1 passive probes up to 1.5 GHz. I, personally, have a Tek P6055 low-Z probe and accessories.

[tggzzz]

Yes, but you're looking at Low-Z probes past 500-700 MHz or so. Keysight offers 10:1 passive probes up to 1.5 GHz. I, personally, have a Tek P6055 low-Z probe and accessories.

I'd suggest a lower frequency than that.

I have three HP10020 probes (1.5GHz 0.7pF)

[0culus]

Yes, but you're looking at Low-Z probes past 500-700 MHz or so. Keysight offers 10:1 passive probes up to 1.5 GHz. I, personally, have a Tek P6055 low-Z probe and accessories.

I'd suggest a lower frequency than that.

I have three HP10020 probes (1.5GHz 0.7pF)

https://www.keysight.com/en/pd-1661717-pn-N2873A/passive-probe-101-500-mhz-13-m?nid=-32557.898995&cc=US&lc=eng

[tggzzz]

Yes, but you're looking at Low-Z probes past 500-700 MHz or so. Keysight offers 10:1 passive probes up to 1.5 GHz. I, personally, have a Tek P6055 low-Z probe and accessories.

I'd suggest a lower frequency than that.

I have three HP10020 probes (1.5GHz 0.7pF)

https://www.keysight.com/en/pd-1661717-pn-N2873A/passive-probe-101-500-mhz-13-m?nid=-32557.898995&cc=US&lc=eng

Input impedance ~j33ohms @500MHz, compared with ~500+j450 ohms for the hp10020.
Input impedance ~j67ohms @250MHz, compared with ~500+j900 ohms for the hp10020.

I know which I prefer