This post is about experimentally using the SynthUSBII SG and the CPDETLS-4000 RF power detector to check the high frequency response characteristics of scope probes. These are not the optimal instruments for this task, but in my opinion, they can produce moderately accurate results on a low budget. The test subjects are a number of inexpensive oscilloscope probes with alleged bandwidths of 200MHz or more that I have acquired during the last 2 years.
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I've known for some time that the SDS7102 DSOs had some surplus bandwidth. So during the last couple of years I've been shopping for inexpensive higher bandwidth probes in an attempt to better exploit this extra bandwidth. However, until now, all I was able to do to evaluate the new probes is compare them to the stock probes that came with the DSO using an uncalibrated 310kHz - 110MHz RF generator as the signal source. The results were at times frustrating and bewildering, since based on my comparisons the stock probes seemed to outperform the allegedly higher bandwidth probes. Unfortunately, due to my test equipment limitations, I couldn't even get an approximation of the actual bandwidth and response curve of both the new and the stock probes.
Now, with the help of the SynthUSBII SG and the CPDETLS-4000 RF power detector, I can get a better idea of the bandwidth and response curve of my new, allegedly higher bandwidth, probes. To test these probes I'll be using the same procedure I used for the stock Owon T5100 as discussed on the previous 2 posts. The probes will be very carefully compensated before performing each test. Compensation will be done with the probe connected to the 1KHz signal source via a probe tip adapter instead of using the probe clip and ground lead.
Li Hua P6200 200MHz passive probes bought on eBay
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I bought these probes at eBay in June 2014 for $33.90 (free shipping). As soon as I tried them it became evident that these probes' bandwidth was nowhere near 200MHz. After contacting the seller I was issued an immediate refund and I was also allowed to keep the probes. The following is a link to the post where I describe the tests I performed on these probes at the time:
https://www.eevblog.com/forum/testgear/review-of-owon-sds7102/msg460398/?topicseen#msg460398Now I have dug up these probes again to test them with my new gear.
Attachment #1 - Probe picture from the web.
Attachment #2 - Probe specs as shown in the manual. The specs given at the eBay page were identical.
Attachments #3 & 4 - Li Hua P6200 + DSO Response at the probe tip & Li Hua P6200 + DSO Response at the probe tip with dBm difference expanded Y scale.
Based on the response curve and dBm difference I think it's evident that these probes' bandwidth is way below 200MHz. I've seen a post by another member that tested the Li Hua 500MHz version (P6500) with equipment far superior than what I'm using here and reported results just as dismal.
https://www.eevblog.com/forum/testgear/oscilloscope-probes-41026/msg579768/#msg579768The eBay seller from whom I bought these probes no longer sells this brand, however, if you search on Amazon for P6100, P6200, P6300, or P6500, you'll find plenty of marketplace sellers that offer them. Note that so far I haven't seen the brand Li Hua mentioned in the description, but look at the pictures closely and you'll see that they are the same as the ones pictured on #1.
Attachment #5 - This is the spreadsheets used to plot the line chart. In addition to the line chart it contains all the tabulated readings obtained during the experiment.
250MHz passive probe bought from Saelig
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I bought this probe from Saelig on December 2014 for $35. When I first tried it and compared it to my stock Owon T5100 probes I noticed that peak to peak readings were quite a bit higher in the 30MHz to 100MHz range with the largest difference at around 100MHz. However, due to test equipment limitations I couldn't confirm that these higher readings were less accurate than the ones obtained with the T5100 probes.
Now that I'm ready to test this probe with my new gear, I decided to investigate it further. First I found out that the probe pictured at the Saelig's website looks similar but is definitely not the same as the one I received. In addition, the specs given in the manual for the probe I received are slightly different than the specs given at the Saelig's website. For example, there is a 3pF difference on the input capacitance spec, also, the bandwidth spec is stated a little different. The manual identifies this probe as model 05SPGL250, however, the probe is labeled GLF-250. A web search revealed that this is a discontinued GW Instek probe intended for use with the GW Instek GOS-6200 scope (a discontinued 200MHz analog scope). In addition, the specs given on a Japanese GW Instek dealer's website once again are somewhat inconsistent with both the manual and the Saelig website.
The probe I received is nice looking and well built. However, in view of the above findings, I'm now inclined to believe that the exaggerated peak to peak readings are due to the fact that this probe was meant to be used with a different scope, namely, a GOS-6200. So I suspect that when used in conjunction with my SDS7102 DSO this probe's performance is subpar.
Attachment #6 - This shows a picture of the probe and its specs as it appeared at the Saelig website when I purchased it. Note that the input capacitance is specified as 11pF and the Bandwidth as 250MHz +/- 3dBs.
Attachment #7 - This shows a picture of the probe I received and its specs as it appeared at a Japanese GW Instek dealer's website. The Google translation is a little goofy (e.g. "start-up-time" instead of "rise time") but clear enough. Note that the GOS-6200 is the only applicable oscilloscope model mentioned. In addition, the specified input capacitance is different, 17pF instead of 11pF (my LC meter reads 16.2pF), and the bandwidth specification is stated differently, "DC - 250MHz" instead of "250MHz +/- 3dBs".
I now realize that when I purchased this probe I overlooked the +/- 3dB qualifier. Probably because I was expecting the bandwidth spec to adhere to the 3dB down industry standard. Inadvertently ignoring the added +/- 3dB amplitude distortion qualifier was a mistake I hope I won't forget. In the future I will be paying more attention to this red flag. I suspect that the +/- 3dB qualifier was added so that the probe could be marketed for use with scopes other than the GOS-6200. With this change the probe's frequency response is within specs even if the peaks and dips throughout the entire frequency range are as large as +/- 3dBs.
An industry standard bandwidth specification identifies the point where the signal amplitude versus frequency drops 3dBs below its low frequency value. A good quality probe is expected to exhibit a fairly flat response to at least 1/3 of its bandwidth followed by a slow roll-off to the 3dB down point. Another measure of quality is how well peaks and dips along the way are minimized. I don't think that +/- 3dBs peaks and dips would qualify as even mediocre quality.
Attachment #8 - Probe specs as shown in the manual. Note that the model number given is different from the label on the probe (GLF-250). Also note that the input capacitance is specified as 14pF (different from #6 & #7 above) and the bandwidth is stated slightly different, "DC to 250MHz +/-3dB".
Attachments #9 & #10 - GW Instek GLF-250 + DSO Response at the probe tip & GW Instek GLF-250 + DSO Response at the probe tip with dBm difference expanded Y scale.
Based on the response curve and dBm difference I think it's evident that this probe when used in conjunction with my SDS7102 overstates the amplitude of signals in the 50MHz to 120MHz frequency range. It also exhibits a sharper than expected roll-off starting at about 180MHz. Since the bandwidth spec on the probe's manual is given as "DC to 250MHz +/- 3dB" instead of the industry standard, the output signal amplitude may be up to 3dBs above or below the amplitude of the actual signal throughout the entire frequency range. Therefore, the frequency response curve is within specs since in this case, at its worst, it only overstates the 100MHz signal amplitude by a little less than 2dBm (about 120mVpp). I don't know what to say about this "within specs" performance, but the word crappy comes to mind! This makes this probe nearly useless when used in conjunction with my SDS7102. I suspect that when this probe is used in conjunction with other scopes, except the GOS-6200, the results will be similar.
Attachment #11 - This is the spreadsheets used to plot the line chart. In addition to the line chart it contains all the tabulated readings obtained during the experiment.
Hantek PP-200 200MHz passive probes bought from Amazon
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I bought these probes from Amazon.com in June 2014 for $31.99 (free shipping). When I first tried these probes they seemed to perform about on par with the Owon T5100 probes that came with my DSO. However, due to my test equipment limitations at the time, I couldn't evaluate their bandwidth beyond 110MHz. Now, with the help of my new gear, I hope to find out if their 200MHz rating is justified.
Attachment #12 - This shows a picture of the probes as they appeared at the Amazon.com page when I purchased them.
Attachment #13 - Probe specs as shown in the manual. The specs given on the Amazon.com page were identical.
Attachments #14 & #15 - Hantek PP-200 + DSO Response at the probe tip & Hantek PP-200 + DSO Response at the probe tip with dBm difference expanded Y scale.
Based on the response curve and dBm difference I think it's evident that, when used in conjunction with my SDS7102, the bandwidth of these probes is better than 200MHz. The response curve is similar to what I got with the Owon T5100 probes, but in some ways, in my opinion, is slightly better. For example, the dips and peaks along the 35MHz to 150MHz range are less pronounced. On the downside, the input capacitance spec for these probes is 18.5pF to 22.5pF compared to 14.5pF to 17.5pF for the T5100 probes. However, my LC meter reads 15.2pF for the PP-200 and 14.4pF for the T5100. So it appears that the actual input capacitance of the probes that I have is nearly the same.
Attachment #16 - This is the spreadsheets used to plot the line chart. In addition to the line chart it contains all the tabulated readings obtained during the experiment.
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Some things I learned along the way that in my opinion should be considered when trying to match new probes to an existing scope:
Matching a particular scope to probes from a different manufacturer can be a tricky endeavor. Although important, making sure that the compensation range matches the scope's input capacitance does not guarantee an adequate response curve. A probe by itself is not associated to a particular bandwidth, the specified bandwidth applies to the combination of the probe and a compatible scope. Claims that a probe is compatible with most scopes are probably exaggerated, specially if only LF compensation is available. Watch out for red flags, like an amplitude distortion qualifier added to the bandwidth spec (e.g. DC to 250MHz +/- 3dB). Search the web to see if someone has tested the response of the particular probe you are considering, other reviews based on non-technical observations can be deceiving.
Some things I'd like to try in the future:
The probes I tested so far only offer LF compensation. However, even if the probe's compensation range matches the scope's input, different scopes with the same input specs will still differ in their parasitic capacitance and inductance characteristics. As a result, unless the probe is specifically designed for a particular scope, the response curve obtained via LF compensation is a compromise and not the most optimal response curve possible. To get around this pitfall some probes offer HF compensation in addition to the standard LF compensation. The Pico Technology TA131 is an inexpensive example of a probe with this extra feature, so I'm currently considering purchasing one. This 250MHz probe with an 11pF input capacitance sells for about $42:
https://www.picotech.com/accessories/passive-oscilloscope-probes/250-mhz-scope-probeAnd here is a link to an article that explains how to perform the HF compensation on a virtually identical probe:
https://www.picotech.com/library/application-note/how-to-tune-x10-oscilloscope-probes---------------------------------------------------------------------
Next I'll try to plot the SynthUSBII response curve from 40MHz to 4GHz