Multimeter Oscilloscope Scope
How do I determine what type of probe I will need for my oscilloscope?
I have found a scope I am interested in on Ebay, but it has no probe. Would someone please walk me through the various types (in not an engineer) and what need each type serves? I understand they can be as simple as a set of multimeter test leads to very elaborate. For example I see some for sale over 10,000 dollars (gasp)
The oscilloscope input is usually the equivalent of a 1 megohm resistor to ground with some tens of pF capacitance in parallel. It measures only voltage level, so any other parameter needs some sort of conversion.
Unshielded test leads can cause instability - outputs of the circuit being measured get coupled back to their inputs by stray impedance. These tend to only be suitable for DC or very low frequencies.
If you use just a shielded cable there is series inductance and shunt capacitance which becomes more significant as the frequency rises. The capacitance loads the circuit being measured, and modifies the resulting waveform, or even affects the operation of the circuit. The impedance of the source is also important here. In many electronics circuits the impedance of the source is significant when considering the capacitive loading of shielded cable. This method is suitable for DC or low frequencies like power line frequencies. In some cases it may suit for audio frequencies, but remember there can be 100s of pF of cable capacitance shunted across the circuit. Just 100pF has about 1 Megohm impedance at 1590 Hz. At 1.59Mhz the impedance of 100pF is only 1000 ohms.
Coaxial cable (usually 50 ohms) can be used if it is terminated in its characteristic impedance at the oscilloscope input (a 'T' connector with a 50 ohm resistor for example. This is called a transmission line. The inductive and capacitive loading effects in the cable are minimal. This means the resistor absorbs power though, and needs to be rated accordingly, especially if there is any DC present. A capacitor or transformer might be used to isolate the DC at the measured circuit. The source being measured should also equal the impedance. This method is useful in radio devices and modules where 50 ohm or 75 ohm circuits are used. This method has the best bandwidth where it can be used, as the circuit is loaded only by the 50 ohm resistor, but that means the circuit has to be designed for a 50 ohm load. The input capacitance of the oscilloscope might be 30pF which represents 333 ohms at 15.9 MHz, so can be ignored in a 50 ohm circuit up to something like this frequency. This approach (often with 75 ohm cable and circuits) is also suitable for analogue television video signals.
Using a x1 or 1 megohm probe, the shunt capacitance and impedance are similar to the input of the oscilloscope itself. This is more sensitive than divider probes, but the bandwidth suffers. It is most likely useful in the audio range only, depending on the impedance of the measured circuit. The capacitance is too high for good indication of logic circuits.
Use a x10 probe or 10 megohm probe (it is a divider, but means multiply the reading by 10). This suits a lot of applications. The shunt resistance is increased by the division ratio to 10 megohms, and the shunt capacitance is reduced by 10 too, so 30pF oscilloscope becomes 3pF on the probe. At 159KHz this is 333K ohms, so generally this probe is great for audio frequencies, and usable in RF circuits, for those with lower internal impedance up to 20MHz or 30MHz. It can also give good results with logic circuitry. It is fine for most day to day purposes. This is the minimal requirement.
Power mains.
This needs special probes for safety. Normal probes and oscilloscope inputs are not rated for 240V to ground (340V peak). Usually an isolated or differential probe is used too, in case the power circuit is not of the polarity expected. You wouldn't want to connect the active to the oscilloscope ground. Expensive! Thus a specialised probe is required for mains use. It may even be a specialised oscilloscope with differential inputs (floating or isolated). Other higher voltage also needs specialised probes. For example a divide by 1000 probe has only fractions of a pF of capacitance, but sensitivity is well down so it is only useful for higher voltages, and it can be rated to thousands of volts if properly designed.
Active probes.
For reasonable or increased sensitivity, and very low capacitance, special probes using active internal amplifiers exist. These can be expensive, but have a fraction of a pF capacitance, and are designed for better high frequency operation. The capacitance of the oscilloscope input is more or less isolated. Others include differential inputs, preamplifiers etc.
There are also current probes. These convert current to voltage. There are AC only types that use a current transformer, and AC/DC types that use hall effect devices.
A look at what is currently available on eBay
 SCOPE PROBE for UT81B Digital Multimeter Oscilloscope US $4.99 
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