Recommendations for hobbyist quality 500 MHz scope probes

[geostep]

Hi All,

I scored a Tektronix TDS 540C 4 channel 500 MHz oscilloscope on eBay.  It's on route to me now.  However I'm fairly certain there are no probes included with it.
A quick look around shows that high quality passive probes will set you back $400 - $500 apiece.  That's way out of my hobbyist budget.

Do you guys have recommendations for 500 MHz passive probes that are not obvious junk but are not budget busters either?
It's a 4 channel scope, but at this stage I'll probably just get a pair. The scope and probes will be put to general use, nothing exotic.

Thanks for your input!

- George

[KaneTW]

https://www.picotech.com/accessories/passive-oscilloscope-probes 180 each. I'm sure there's something a bit cheaper if you find the OEM.

https://www.staubli.com/en/connectors/test-and-measurement/high-frequency-measurement/touch-protected-passive-probes/ seem to go for 80-100 depending on accessories.

[AJ3G]

Hi George:

Let me Caveat by saying I have not used their Oscilloscope probes, but the Multimeter Probes are excellent. It would stand to reason that their scope probes are probably decent too. 

https://probemaster.com/5900-series-oscilloscope-probes/

These too are in the $180.00 range brand new. Might be worth a look. I am looking for 1GHz probes myself at the moment, and even on the used market probes are very pricey. If I were interested in 500MHz probes I would give these a try based on my experience with their other probe products.

Rich

[dzseki]

If you really need 500MHz probe, forget passive probes... To see how useful are these just calculate their impedance when the tip capacitance is 10pF. Measuring at such bandwidth isn't going by just tapping here or there with the probe anyway.
For such bandwidth you either need a Low-Z probe (reasonably easy to DIY) or a real active probe.
Usefulness of passive probes tops around 200-300MHz.

[james_s]

Beware that active probes are easily damaged and virtually impossible to repair so it's easy to end up buying junk if you're not careful. The P6139A is the passive probe that would have come with that scope, while trying to actually measure up to 500MHz with a passive probe has the caveats mentioned above they are nice probes. Just don't buy probes from China ebay sellers, I did that recently, received hacked together garbage and had to file a claim through ebay to get a refund.

[joeqsmith]

I bought some new old stock resistive probes and have made my own.  These are fairly inexpensive.  You may want to have a look at used active probes.   I have a very old Tektronix active probe and a couple of differential probes that I picked up for a decent price.   

[tggzzz]

Hi All,

I scored a Tektronix TDS 540C 4 channel 500 MHz oscilloscope on eBay.  It's on route to me now.  However I'm fairly certain there are no probes included with it.
A quick look around shows that high quality passive probes will set you back $400 - $500 apiece.  That's way out of my hobbyist budget.

Do you guys have recommendations for 500 MHz passive probes that are not obvious junk but are not budget busters either?
It's a 4 channel scope, but at this stage I'll probably just get a pair. The scope and probes will be put to general use, nothing exotic.

Thanks for your input!

- George

What are you going to be probing?

If ordinary digital logic then that can probably tolerate a loading of 500ohms//0.7pF (*10), or 5000ohms//0.7pF (*100). Strongly consider a "low" impedance Z0 resistive divider probe. They are easy to make.

However, that requires a scope with a 50ohm input. It isn't clear to me whether the tds540 has a proper 50ohm input (like the tek485) or merely has 50ohm resistor in parallel with 15pF//1Mohm (like the tek24x5 and many others).

[geostep]

Thanks for all the insightful replies so far!  I'll be mainly using it for digital logic.  I don't *really* need 500 MHz of bandwidth, just like how many times have you gone 180 MPH in your car just because the speedometer tops out at 180.     
But the scope was priced right and has four channels which made it desirable. And I do want to see how well it will perform.  Can I really only get 3 db of drop at 500 MHz?  No, I haven't looked at the specs for the scope or probes to see what the losses are as one approaches 500 MHz.  I want to do that myself and see what results I can get.
And yes, I have been resisting the temptation to get a P6139A or P6139B probe from eBay.  Who knows what you will actually get. And I don't have the proper gear to test out a probe, I have to take it on faith it will perform as advertised.
Also, I like the low Z idea that has been mentioned. I'l be looking into it.

- George

[Gyro]

This is probably the thread you want for constructing Z0 probes...

https://www.eevblog.com/forum/projects/lo-z-probe/

I humbly suggest that the ones in reply #23 look particularly nice.

Don't forget that, when constructing such probes, you need a 50 ohm BNC through-terminator at the scope end (I don't remember the 540C having a built-in 50R termination option - I may be wrong though).

[KaneTW]

Can you use Z0 probes with a 50 Ohm terminator into a 1Meg scope input?

[nctnico]

Can you use Z0 probes with a 50 Ohm terminator into a 1Meg scope input?

Ofcourse. With a 50Ohm terminator (either as a feed-through or terminator on a tee).

[edavid]

And yes, I have been resisting the temptation to get a P6139A or P6139B probe from eBay.  Who knows what you will actually get. And I don't have the proper gear to test out a probe, I have to take it on faith it will perform as advertised.

I don't see why you are worrying so much.  As long as they are not mechanically damaged, they should be fine.  Also, you can test them with the scope's calibrator.

[Gyro]

Wasn't there an issue with very early P6139s developing cable shorts? I don't know if that was a P6139 / P6139A change?

http://w140.com/tekwiki/wiki/P6139

I never experienced any issues myself - other than the rubber on one going sticky... https://www.eevblog.com/forum/repair/tek-p3169a-_probe-rot_-repair/

[tggzzz]

Can you use Z0 probes with a 50 Ohm terminator into a 1Meg scope input?

Ofcourse. With a 50Ohm terminator (either as a feed-through or terminator on a tee).

Except that it will actually be 50ohm//20pF or whatever the scope's input capacticance is. 20pF@500MHz is 15ohms.

And since the transmission line isn't properly terminated, there will be reflections and the response will be frequency dependent.

[edpalmer42]

Thanks for all the insightful replies so far!  I'll be mainly using it for digital logic.  I don't *really* need 500 MHz of bandwidth, just like how many times have you gone 180 MPH in your car just because the speedometer tops out at 180.     

Exactly!  So for 'daily driver' use, I'd recommend a cheap set of 100 MHz probes.  Whatever you get for higher frequencies, keep them safely stashed away until you really need them.

Also, I like the low Z idea that has been mentioned. I'l be looking into it.

Rather than go the DIY route, I ended up with a pair of HP 10020A low impedance resistive probes.  They have <1 pf capacitance, variable divider ratios from 1:1 to 100:1 and a frequency response of 700 MHz.  They sometimes show up on ebay and aren't usually too expensive - until now!   

[tggzzz]

Thanks for all the insightful replies so far!  I'll be mainly using it for digital logic.  I don't *really* need 500 MHz of bandwidth, just like how many times have you gone 180 MPH in your car just because the speedometer tops out at 180.     

Exactly!  So for 'daily driver' use, I'd recommend a cheap set of 100 MHz probes.  Whatever you get for higher frequencies, keep them safely stashed away until you really need them.

But throw away the 6 inch ground lead, since that will introduce a resonance at @100MHz (150nH//20pF). Use the curly spring ground, and a groundplane.

Also, I like the low Z idea that has been mentioned. I'l be looking into it.

Rather than go the DIY route, I ended up with a pair of HP 10020A low impedance resistive probes.  They have <1 pf capacitance, variable divider ratios from 1:1 to 100:1 and a frequency response of 700 MHz.  They sometimes show up on ebay and aren't usually too expensive - until now!   

I have three But two of then are 10:1 only

I even used them as inspiration for a 3D printed "bayonet" for a "high" impedance *10 probe, to learn about 3D modelling and materials - and reduce the inductance. The results were good. (N.B. the Tek485 has a real 50ohm input, not 50ohm//15pF!)

[nctnico]

Can you use Z0 probes with a 50 Ohm terminator into a 1Meg scope input?

Ofcourse. With a 50Ohm terminator (either as a feed-through or terminator on a tee).

Except that it will actually be 50ohm//20pF or whatever the scope's input capacticance is. 20pF@500MHz is 15ohms.

But there will be some series inductance too. A properly designed 1M Ohm input isn't purely capacitive at higher frequencies by design. Otherwise it wouldn't make sense to have >100MHz bandwidth on a 1M Ohm oscilloscope input.

Besides that your circuit is also wrong. At the input the impedance from the divider towards the cable should be 50 OHms as well; that is likely the cause for the reflections in your simulation.

[edpalmer42]

Thanks for all the insightful replies so far!  I'll be mainly using it for digital logic.  I don't *really* need 500 MHz of bandwidth, just like how many times have you gone 180 MPH in your car just because the speedometer tops out at 180.     

Exactly!  So for 'daily driver' use, I'd recommend a cheap set of 100 MHz probes.  Whatever you get for higher frequencies, keep them safely stashed away until you really need them.

But throw away the 6 inch ground lead, since that will introduce a resonance at @100MHz (150nH//20pF). Use the curly spring ground, and a groundplane.

Totally agree!  Some cheap probes *don't* include the springs so check carefully!

[james_s]

But throw away the 6 inch ground lead, since that will introduce a resonance at @100MHz (150nH//20pF). Use the curly spring ground, and a groundplane.

Don't throw it away, it is useful. Just know not to use it when you are trying to measure high frequency or fast rise time signals. Proper probing technique is a whole subject in itself.

[tggzzz]

Can you use Z0 probes with a 50 Ohm terminator into a 1Meg scope input?

Ofcourse. With a 50Ohm terminator (either as a feed-through or terminator on a tee).

Except that it will actually be 50ohm//20pF or whatever the scope's input capacticance is. 20pF@500MHz is 15ohms.

But there will be some series inductance too. A properly designed 1M Ohm input isn't purely capacitive at higher frequencies by design. Otherwise it wouldn't make sense to have >100MHz bandwidth on a 1M Ohm oscilloscope input.

You'll have to explain that, preferably with a schematic to show the location and magnitude of that inductance.

Besides that your circuit is also wrong. At the input the impedance from the divider towards the cable should be 50 OHms as well; that is likely the cause for the reflections in your simulation.

The circuit is correct. See, for example, fig 3.2 in http://w140.com/tekwiki/images/6/62/062-1146-00.pdf

If the capacitor is removed, the ripple disappears.
If the transmission line delay/length is changed, the frequency/spacing of the peaks changes as you would expect with a (very imperfect) comb filter.
Try the simulation yourself, and see.

[nctnico]

Here is the difference between using a 50 Ohm input or 1M Ohm input (at 500MHz bandwidth) on a 66MHz square wave from a 50 Ohm source.
A cheap 50 Ohm feed-through:


A 50 Ohm terminator on an SMA tee:


Direct 50 Ohm:


As can be seen the signals are not dramatically different.

[eevcandies]

Keep your eyes on HP 10441A or especially 10441B probes...very nice...I have a bunch, sometimes you get the entire "kit" of clips, tips, etc for $25 or so, I even got one for $10 once (new, in bag, with accessories).  I only buy when it includes the tophat.   Another time, I got 5 of this series probe for about total of $87 and they also came with a big bagful of  extra gnd clips, top hats, etc,either a few or all 5 were new in sealed bags.

sometimes you see this exact same thing for $10 to $30 (5 for $87 was a once-in-a-decade deal)
https://www.ebay.com/itm/Agilent-HP-10441A-Oscilloscope-Probe/123724232534?epid=1900713148&hash=item1cce89f756:g:HUAAAOSwBt5ZD3Mr

[edpalmer42]

I've seen probes similar to the HP 10441 and I've always been puzzled by one thing.  The specs state that the input impedance of the scope must be 1 Mohm in parallel with 6 - 9 pf.  How many scopes have capacitance that low?  I have a LeCroy 9384 1 GHz scope and even it has an input capacitance of 15 pf.  If I was using a 10441 probe, I couldn't compensate it.   

[dzseki]

I've seen probes similar to the HP 10441 and I've always been puzzled by one thing.  The specs state that the input impedance of the scope must be 1 Mohm in parallel with 6 - 9 pf.  How many scopes have capacitance that low?  I have a LeCroy 9384 1 GHz scope and even it has an input capacitance of 15 pf.  If I was using a 10441 probe, I couldn't compensate it.   

You wouldn't guess, HP scopes usualy made such way to save on two way attenuator.

[KaneTW]

R&S RTB2004 has 1MΩ||9pf input impedance.