Good uses for 1:1 probe

Question

We all know why using a properly compensated 10:1 probe is a must when viewing MHz-speed signals on a scope with a 1 MOhm input impedance. Now who can supply a good use for a 1:1 probe? These probes have not found much use in my lab.

The only thing I can think of is that the 1:1 probes might be useful for making measurements of power supply ripple, switching artifacts, etc. I, however, question whether the 1:1 probe is readily capable of a connection with low-enough ground transfer impedance to really see what's going on in, for example, a switching power supply rail. Howard Johnson ("Healthy Power") and Jim Williams ("Minimizing Switching Regulator Residue in Linear Regulator Outputs", page 11) both discuss a similar technique but use plain coax instead of a 1:1 probe. In Howard Johnson's example, the coax shield is then soldered to the board with bus wire to achieve the lowest possible ground transfer impedance. Eliminating inductance in the ground wire is key to probing the fast switching artifacts. I'm not sure how well a 1:1 probe would do in this case, but it can probably be made to work okay.

Can anyone recommend any other uses for the 1:1 probe??

Answer 1

Noise in oscilloscope front ends is quite high, maybe 1mVp-p.

Using the 1:1 probe lowers the input-referred noise floor by an order of magnitude. Still pretty crappy, but opens a few doors.

Answer 2

Convenience. A 1:1 probe (or the x1 setting on a switchable x10 probe) will probably have slightly lower capacitance than a 50ohm coax of the same length, and also handy clips on signal and ground. It's therefore a convenient tool for small signals where noise makes a 10:1 probe unusable, and for low frequencies where the relatively long ground lead doesn't cause a problem.

For more critical monitoring situations, you might use the scope's 50 ohm input directly, or an active probe, or a DIY probe, or a plain piece of coax.

I use fixed x10 probes. No switch means one less thing to go wrong, and I find switchable probes' switches are often in the wrong position, and it's difficult to spot when they are. When I need x1, I use a short bit of coax.

Answer 3

Coax vs 1:1 probe. I've used both. It depends on the source impedance to a large degree. The probe does a better job matching to the 'scope input impedance (R//C) over the entire frequency range and this can matter with higher source impedances. (Where the capacitive loading of a long piece of coax may degrade the HF response.)

Answer 4

Can anyone recommend any other uses for the 1:1 probe??

With a 5MHz analog scope you got for free out of a dumpster dive, probe frequency response becomes a wee bit less important ;)

For a beginner, it is a lot better than no scope!

Answer 5

Unlike a random piece of 50/75/93 Ohm coax cable - which at first sight seems to be a perfect replacement for a 1:1 probe - a 1:1 or switchable probe still gets the benefit from using an intentionally lossy coax (which 1:10, 1:100 probes use as well), so reflections are dampened more even if the system is sorely mismatched.

So in the end, the 1:1 probe serves well as a connection cable to any source that is relatively low impedance and low level - like audio signals, output from passive (eg inductive or strain gauge) sensors. Mind that not every scope (or scope plugin) goes down to 1mV/div - and that 1mV/div with a 1:10 probe already means you need 80mVpp to fill the screen, 400mVpp at 5mV/div (minimum of eg the Tek 7A18/7A26), 2-3Vpp(!!) at 50mV/div (minimum of many really old scopes resp their general-purpose plugins - think 545B/CA. Not typically 4Vpp since that kind of scope is usually 4 or 6 div high, not 8).

Also, DC accuracy will likely be better (unless the lossy cable is really in the tens of kiloohms), which can matter if the readout function of the scope is pressed into service as a DVM.

Answer 6

It had very limited use for signals < 20 MHz where 1M load with ~50pF or more with signals below 1 to 50mV.

if larger a 10:1. Probe is better and if smaller then a FET buffered diff probe is best or 50 Ohm terminated if possible.

You can alway get more bandwidth by removing the clips and ground leads with twin prongs.

You can use them as EMI sniffer proves to a spectrum analyzer using a short open wire or better a ground loop for RF

Many scopes have a 20MHz or similar BW filter. This makes the 1:1 probe more useful because it is incapable or accurately capturing risetimes extending past this band without ringing. The probe is simply not balanced for impedance due to the input RC impedance and probe inductance.

Answer 7

A 1:1 probe minimizes oscilloscope noise, but comes at a cost of lower bandwidth.

1:1 probes are very popular for ripple measurements and power measurements. Basically, a 10:1 probe means you get less probe loading (capacitance) but get 10X the scope front-end noise.

I go into some more detail on that here:

http://www.electronicdesign.com/test-measurement/how-pick-right-oscilloscope-probe