Fifty Ohm Probes

[German_EE]

For normal use the scope probes that we buy work just fine, they are manufactured for 1M ohm plus x pF scope input impedance and we all know the advantages and pitfalls of their use. For RF use, especially when measuring low levels of RF power, it is possible to switch a scope to 50 ohm input impedance but here's where the problem starts.

There are no 50 ohm probes.

Do a Google search and ONE applicable link comes up where a brave fellow makes a 50 ohm probe from some hardline. After spending a few hours looking I can't find anybody who makes 50 ohm probes or, even, a standard probe that can be disassembled and rewired with thin 50 ohm coax. Note that a 50 ohm or 'straight through' probe does have other uses. It could be used on a frequency counter where scope probes work but only just and it could be used on an audio or RF voltmeter.

There must be a market for such a device so where are they? Do these probes exist or do I need to get some brass tube and coax so I can make some?

[tautech]

Seem to remember a recent post where a member modded a BNC 50 Ohm socket with tips that could be connected to a std BNC cable.
Lets hope he spots this and links his imagery.

[electr_peter]

Seem to remember a recent post where a member modded a BNC 50 Ohm socket with tips that could be connected to a std BNC cable.
Lets hope he spots this and links his imagery.

Do you mean this post? https://www.eevblog.com/forum/testgear/scope-probes-as-supplied-by-rigol-for-the-low-end-scopes-a-discussion/msg594599/#msg594599

Regarding probes, it is a question of compromise between bandwidth, circuit loading, price, etc.
Passive spectrum covers mostly:

• 1x/10x 9 MOhm probes with 1 MOhm scope - good performance with low BW, high circuit loading in higher frequencies
• 10x 450 Ohm probes with 50 Ohm scope - good performance with med-high BW, low circuit loading in higher frequencies
• 50 Ohm coax with 50 Ohm scope - high circuit loading, wide BW

The reason why 50 Ohm scope probe is not popular because it offers no real benefits when compared to other options. If you need 50 Ohm probe, just connect a coax. And typical coax is not that good compared to high BW scope coax.

[tautech]

Seem to remember a recent post where a member modded a BNC 50 Ohm socket with tips that could be connected to a std BNC cable.
Lets hope he spots this and links his imagery.

Do you mean this post? https://www.eevblog.com/forum/testgear/scope-probes-as-supplied-by-rigol-for-the-low-end-scopes-a-discussion/msg594599/#msg594599

No Peter, not that one.
Member IIRC used SMD resistors soldered in the socket for impedence matching.
There was close-ups of his probe.

[Lightages]

I don't understand where and when you would want a 50 ohm probe. Perhaps this is my ignorance speaking but if the source is a 50 ohm source then just stick a coax with proper termination and probe it with a normal probe. If the source is 50 ohm and it already is in circuit with a 50 ohm load, then adding another 50 ohm load in parallel will change the loading and change what you are trying to see. If the output is a transmitter output then it needs a proper load at all times when outputting and can be damaged with the incorrect load so it needs to be loaded correctly before you probe it. If the transmitter is 1000W, for example, then your 50 ohm probe would need to dissipate 1000W!

[skipjackrc4]

Just solder a thin piece of coax with an SMA connector on it.  That's effective a 50 ohm probe. 

Something to consider, though, is that when you attach your 50 Ohm probe to a 50 Ohm trace, that trace is no longer 50 Ohm...

[nctnico]

For normal use the scope probes that we buy work just fine, they are manufactured for 1M ohm plus x pF scope input impedance and we all know the advantages and pitfalls of their use. For RF use, especially when measuring low levels of RF power, it is possible to switch a scope to 50 ohm input impedance but here's where the problem starts.

There are no 50 ohm probes.

Yes there are. They are called 'FET probes' or 'low-z passive divider probes'. The latter are slightly less expensive. Both offer superior performance compared to standard probes.

[donmr]

Did you recall one like this?

     http://www.emcesd.com/1ghzprob.htm

Its passive, not an FET probe.

I'd like to find an affordable FET probe, or an amplifier to use a 1M probe with a 50 ohm input.

[VK5RC]

I am a bit puzzled,  don't you normally use a high impedance probe on a circuit that is supposed to be working normally,  the probe should have minimal impact on the circuit's behaviour so it should be as high impedance (and low capacitance) as possible (almost impossible at higher frequencies).  50 ohm impedance is a common inter-connection impedance between sections  so if that is required a 50ohm connector,  either a terminator/attenuator/dummy load and good coax is all that is required? That is usually a lot easier to achieve. Eg I have many cables capable of 10GHz 50ohm but no 10GHz probes (nor matching oscilloscope!)

[tautech]

Did you recall one like this?

     http://www.emcesd.com/1ghzprob.htm

Its passive, not an FET probe.

I'd like to find an affordable FET probe, or an amplifier to use a 1M probe with a 50 ohm input.

Yep that's it.
It was linked to by a member.

There's also some stuff on 50 Ohm setups here:
https://www.eevblog.com/forum/testgear/importance-of-good-terminators-for-rf-pulse-applications/

[mazurov]

I have several passive 50 ohm probes made commercially. One is Tektronix P6158, another is P6058. 50 ohm FET probes that I own are P6201 (one of them proudly wearing Agilent asset tag :-)) and P6202. You will need old Tek scope or 1010 power supply for those.

Don't know if they are still made though; ppl increasingly prefer makeshift low-Z probes these days. P6158 is called "low capacitance probe", you may want to google around to see if anything with this description is offered for sale.

[mazurov]

I am a bit puzzled,  don't you normally use a high impedance probe

That's the probe tip. OP was asking (I hope) about 50 ohm output probe to match 50 ohm input of the instrument.

[GK]

Tektronix, at least, used to make 500 ohm probes. These had a 10:1 attenuation ratio and were designed to be used with their wide-bandwidth plug-in vertical amplifiers which at the time typically had 50 ohm inputs only.

I haven't look too hard yet, but so far I've only found second hand examples of 500 ohm probes on Ebay. I don't know of any contemporary manufacturer of 500 ohm, high frequency probes, but would like to find one.
 

[Hugoneus]

Tektronix, at least, used to make 500 ohm probes. These had a 10:1 attenuation ratio and were designed to be used with their wide-bandwidth plug-in vertical amplifiers which at the time typically had 50 ohm inputs only.

I haven't look too hard yet, but so far I've only found second hand examples of 500 ohm probes on Ebay. I don't know of any contemporary manufacturer of 500 ohm, high frequency probes, but would like to find one.
 

Rigol makes them. 500Ohm, 1GHz bandwidth. The model number is RP6150, it has 1.5GHz of claimed bandwidth. It came with my DS6000 series scope.

[blacknoise]

There is / was even a flexible passive low impedance probe kit with pluggable probe tips to get input impedances beetween 50 ohm (1:1) and 5 kOhm (100:1) - look for HP 10020A in the attached document.

And you may easily build your own passive low impedance probe. Assuming your instrument is already equipped with a 50-Ohm terminated input, you only need to place the correct resistor in front of the inner conductor of a 50-Ohm coaxial probe cable.

The rule for this resistor is ((attentuation-1) * 50 Ohm) - e.g. 0 Ohm for a (1:1) probe, 450 Ohm for a (1:10) probe or 4950 Ohm for a (1:100) probe. For best high frequency performance you may add a compensation capacitor accross the probe tips input resistor. It's theoretical optimal value is:

((capacitance of the scope input) + (capacitance of the coax cable) pF) / (attentuation-1)

To reduce the "ringing" of a probe with a capacity-compensated probe tips input resistor, you may add a fractional of the probe tips input resistor without the parallel capacitor. Then this fractional resistor acts as a damping resistor in the LC resonant circuit beetween the probe tip, across the signal source, and the probe's ground connector.

Commercial probes i have seen used values beetween about 20 and 100 Ohm for this damping resistor. For the accurate attentuation you have to reduce the main tip input resistor by the value of this damping resistor!

For example,

a (1:10) probe:
tip--50Ohm--(400Ohm || compcap)--coax cable--(50Ohm scope-in)

or a (1:100) probe:
tip--50Ohm--(4900Ohm || compcap)--coax cable--(50Ohm scope-in)

compcap = compensation capacitor (see above)

A final note: for best performane (above a few 100 MHz) you should use SMD resistors and capacitors and keep all "wild" wires short.

Kind regards

[tggzzz]

Tektronix, at least, used to make 500 ohm probes. These had a 10:1 attenuation ratio and were designed to be used with their wide-bandwidth plug-in vertical amplifiers which at the time typically had 50 ohm inputs only.

I haven't look too hard yet, but so far I've only found second hand examples of 500 ohm probes on Ebay. I don't know of any contemporary manufacturer of 500 ohm, high frequency probes, but would like to find one.
 

Rigol makes them. 500Ohm, 1GHz bandwidth. The model number is RP6150, it has 1.5GHz of claimed bandwidth. It came with my DS6000 series scope.

Hmmm.
Supplied with a 6" (=140nH) ground lead.
No specification for tip capacitance.
I'd want to measure that before I believed it.

OTOH 25 years ago I was using HP10020 probes (1.5GHz, 0.7pF) and getting extremely good performance on a 1GHz scope observing 800ps risetimes.

[w2aew]

Tek P6150 is a low capacitance Z0 probe - 500ohm input impedance  when terminated at the scope with 50ohms (thus is a 10x probe), and has 9GHz BW.  The same probe has "feedthrough" tips which basically make it simply an extension of the coax.

Tek P6158 is a 20x probe, thus a 1K input impedance.  3GHz BW

Both of these are current products from Tek.

If you look used, you may also find things like the:
P6057, 100x 5Kohm 1.4GHz
P6056, 10x, 500ohm, 3GHz.

[darrell]

Tektronix, at least, used to make 500 ohm probes. These had a 10:1 attenuation ratio and were designed to be used with their wide-bandwidth plug-in vertical amplifiers which at the time typically had 50 ohm inputs only.

I haven't look too hard yet, but so far I've only found second hand examples of 500 ohm probes on Ebay. I don't know of any contemporary manufacturer of 500 ohm, high frequency probes, but would like to find one.
 

I have a pair of these from Agilent. I got then used for a fraction of the $3400 price. The 450 and 950 ohm resistors are Caddock non inductive high voltage resistors. Rise time is limited by my scope to about 140 ps. I also have a pair of LeCroy 1.5 GHz 10:1 transmission line probes (PP061, discontinued).

http://www.keysight.com/en/pd-37010-pn-54006A/6-ghz-passive-divider-probe-kit?nid=-34025.536883024&cc=US&lc=eng

When I'm doing anything more than verifying the presence and shape of a signal, I tend to solder a 950 ohm 0402 resistor to what I want to probe and a stripped end of a small coax to the other end of the resistor and nearest ground. The 0402 resistor parasitics result in about 3 dB of peaking by 6 GHz for the 950 ohm 20:1 probe. Stacking two or three resistors should improve the performance. I suspect that is part of why Agilent used the inch long high voltage resistors.

[w2aew]

Tektronix, at least, used to make 500 ohm probes. These had a 10:1 attenuation ratio and were designed to be used with their wide-bandwidth plug-in vertical amplifiers which at the time typically had 50 ohm inputs only.

Tek still makes them. The P6150 is 500 ohm (thus 10x), 9GHz bandwidth.

[helius]

The Tek P6507 is a 1X 50-ohm probe. It's made for semi-permanent board mounting.

[GK]

lol, like I said, I haven't looked too hard yet

I like the Tek 20:1 1k probe and it would go nicely with my 7904A, but ouch on the price for a brand new one (>600 USD).

[Mark_O]

I made this 20X probe from a section of hardline today.   I removed and cut away part of the Teflon to solder the 0605 950 ohm resistor in-line.    Sewing needle was soldered to the outside for the ground tip.  Shrink tube applied to to outside.   

Very clean construction.  Nicely done.     Thanks for sharing your technique.

[tggzzz]

I made this 20X probe from a section of hardline today.   I removed and cut away part of the Teflon to solder the 0605 950 ohm resistor in-line.    Sewing needle was soldered to the outside for the ground tip.  Shrink tube applied to to outside.   

Blue showing the difference between this cheapo probe against a LeCroy 10Meg 350MHz probe. 

That's the neatest homebrew construction for the resistor I have seen.
It does, however, rely on there being an available circuit ground at precisely that distance from the point being probed.

One alternative to the sewing needle would be "piano wire" or "music wire", say 0.8mm diameter. It has the potential advantage that it can be bent into any desirable shape using strong pliers, but is also sufficiently stiff to act as a ground bayonet/spear. This would enable construction of a probe tip similar to the old HP10020A 1.5GHz probe (see PDF attached to https://www.eevblog.com/forum/testgear/fifty-ohm-probes/msg601417/#msg601417 ), or the high impedance 500MHz probe shown below.

Another alternative to the sewing needle would be the flexible sheet ground used in the Keysight 2870 series.

[joeqsmith]

Thanks. 

Copper foil cut into strip used to form a clamp.  Easy to make but would guess the performance with just a wire wrapped around the end would be better.   

[tggzzz]

Copper foil cut into strip used to form a clamp.  Easy to make but would guess the performance with just a wire wrapped around the end would be better.

That's another neat easy construction. You might like to consider documenting it in some form of blog that is less transitory than this forum, and then submitting it to something like hackaday.com. That would enable people to find it more easily.

I'm unsure of the performance of the spear vs wire, and no longer have access to equipment that could measure it.

I have used the spear on the ancient HP10020 1.5GHz <0.7pF probe to measure digital risetimes of ~800ps with a 1GHz scope.

I presume one advantage of the foil on the modern Keysight 2870 is slightly lower inductance, due to it being wider.