Author Topic: Scope Probes Are Not Just For Oscilloscopes  (Read 559 times)

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Offline German_EE

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Scope Probes Are Not Just For Oscilloscopes
« on: October 19, 2019, 11:15:09 am »
This is not a question, it's a quick heads-up for the beginners out there who might not realize what they have on the test bench.

We've probably all seen scope probes although most of them seem to spend their lives either plugged into a scope or hanging with the other test leads. Rather than just leave them idle here are some other uses:

1. Scope Probes Are Not Just For Scopes
If your scope probe can be switched from X10 to X1 then it becomes (almost) a straight through connection. I measured one of my probes and got 220 ohms series resistance in the X1 position. This makes them ideal for frequency meters and multimeters when set on the volts range. Things get a little complicated in the X10 position though, if your meter has a 1M ohm impedance then you will divide your input by ten but if it's a 10M ohm impedance meter then things get weird because you have a divide by 1.9

2. Scope Probes Work Both Ways
Want to inject a voltage into a circuit? Plug the X1 scope probe into your power supply via a BNC to banana converter but remember that series resistance! It's also good for injecting signals from your audio or RF signal generator and this is where the X10 setting really comes in useful. At X10 you have a 9M ohm series resistor so when you connect the tip to a 50 ohm circuit that's a voltage attenuation of 166,666 times or 104.4 dB, good for working on that receiver.

3. Try it On A Spectrum Analyzer
OK, I know that a spectrum analyzer has 50 ohm ports on the input but hear me out. Assuming that 220 ohm series resistance on X1 and 50 ohm in and out we have an attenuator of about 20 dB (but a horrible mismatch). That 20 dB attenuator can be zeroed out by the analyzer and it MIGHT save your front end if you touch something with the probe that has a large signal. Working out the attenuation in X10 mode with that 9M ohm series resistor is difficult but the above figure of 104 dB looks reasonable.

So, does anyone else have a use for a scope probe that's a little out of the ordinary? I would also appreciate it if someone could check my math when it comes to the attenuators, before someone lets the magic smoke out.
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Offline capt bullshot

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Re: Scope Probes Are Not Just For Oscilloscopes
« Reply #1 on: October 19, 2019, 11:41:05 am »
I've already used the probe clip to pick up a small screw in a narrow space - mostly because I was too lazy to grep the tweezers ;)

Anyway, the series resistance of a 1:1 probe can vary from 100 Ohm to 1k, depending on the model. One would be suprised if he expects just a piece of low resistance coax cable, the center wire of the probe cable is resistive. Look up some old Tek documents for the reasons.
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Offline TheHolyHorse

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Re: Scope Probes Are Not Just For Oscilloscopes
« Reply #2 on: October 19, 2019, 12:10:26 pm »
I've already used the probe clip to pick up a small screw in a narrow space - mostly because I was too lazy to grep the tweezers ;)

Anyway, the series resistance of a 1:1 probe can vary from 100 Ohm to 1k, depending on the model. One would be suprised if he expects just a piece of low resistance coax cable, the center wire of the probe cable is resistive. Look up some old Tek documents for the reasons.

Yeah but if you plug them into a MM in volts mode with like 10M input impedance it wont really matter if it's 100 or 1k, like the OP said.
 

Offline German_EE

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Re: Scope Probes Are Not Just For Oscilloscopes
« Reply #3 on: October 19, 2019, 12:33:03 pm »
I was curious so I went to the spectrum analyzer and actually tried this, the results are surprising:

Test Frequency 10.00 MHz using my GPSDO

50 ohm cable 20cm                 13.3 dBm

Rigol Scope Probe X1                4.4 dBm

Rigol Scope Probe X10            -13.3 dBm

So at X10 with that 9M ohm series resistor I'm only seeing a 26 dB attenuation of the signal, I expected more.
Should you find yourself in a chronically leaking boat, energy devoted to changing vessels is likely to be more productive than energy devoted to patching leaks.

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Offline magic

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Re: Scope Probes Are Not Just For Oscilloscopes
« Reply #4 on: October 19, 2019, 06:27:31 pm »
1. Scope Probes Are Not Just For Scopes
If your scope probe can be switched from X10 to X1 then it becomes (almost) a straight through connection. I measured one of my probes and got 220 ohms series resistance in the X1 position. This makes them ideal for frequency meters and multimeters when set on the volts range. Things get a little complicated in the X10 position though, if your meter has a 1M ohm impedance then you will divide your input by ten but if it's a 10M ohm impedance meter then things get weird because you have a divide by 1.9
I use this trick to check feedback voltage on power supply regulators. I'm a bit paranoid that connecting normal test cables to that node could drive them bonkers (oscillation or noise pickup) and fry the whole PCB.

Division ratio can be fixed to 10x by paralleling the DMM with 1.111MΩ.
 

Offline fourfathom

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Re: Scope Probes Are Not Just For Oscilloscopes
« Reply #5 on: October 19, 2019, 06:50:37 pm »
I was curious so I went to the spectrum analyzer and actually tried this, the results are surprising:

Test Frequency 10.00 MHz using my GPSDO

50 ohm cable 20cm                 13.3 dBm

Rigol Scope Probe X1                4.4 dBm

Rigol Scope Probe X10            -13.3 dBm

So at X10 with that 9M ohm series resistor I'm only seeing a 26 dB attenuation of the signal, I expected more.

Actually, 26dB attenuation is right on the money.  That's roughly what you get if the X10 probe has a 10pF capacitor paralleling the 9Meg resistor, and a 20 pF trim cap (to ground, at the scope side of the 9M resistor). 

Here are some simulation numbers, 50 Ohm source and load, with the above X10 probe:
1 KHz: -98dB
10 KHz: -84dB
100 KHz: -64dB
1 MHz: -44dB
10 MHz: -24dB
100 MHz: -7.5dB

These capacitor values will vary according to the specific probe and scope (trim cap setting), but it's going to be fairly close.
 

Offline German_EE

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Re: Scope Probes Are Not Just For Oscilloscopes
« Reply #6 on: October 20, 2019, 08:55:36 am »
Duh! I forgot about the capacitors in there. :palm:
Should you find yourself in a chronically leaking boat, energy devoted to changing vessels is likely to be more productive than energy devoted to patching leaks.

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Offline MarkF

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Re: Scope Probes Are Not Just For Oscilloscopes
« Reply #7 on: October 20, 2019, 01:55:13 pm »
Inside a x1/x10 scope probe

 


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