Accessory probes are a vital part of measurement systems. Here you will find
- Logic Probes
- Multimeter Probes
- Oscilloscope Probes
- RF Probes
- Temperature Probes
A Logic Probe is a type of test probe used for analyzing and troubleshooting the logical states of a digital circuit. They are powered either by the test circuit or by batteries. These probes can be used on either TTL or CMOS integrated circuit devices by selecting the circuit type on the probe. All logic analyzers feature the ability to operate in high and low states (many have specific audible tones to signify each state) and the ability to register pulses. Some logic probes, such as some from GW Instek and Elenco, also incorporate a logic pulser function. Logic probes are relatively simple in construction and therefore are cheap, however the major drawback of logic probes is that they can only test one signal at a time. If you need to assess many logic signals simultaneously, you should look instead for a logic analyzer.
Features to consider when selecting Logic Probes
- Operating Voltage. Most logic probes operate between about 4-18V. This is a pretty standard range and any difference from this range is usually small
- Frequency Response. This is the range of signal frequencies a logic probe can register. The most common frequency response is 50MHz
- Detectable Pulse Width. Most logic probes can register pulses as small as 10ns however some cannot. Know your application and choose the appropriate probe
Multimeter Probes connect a multimeter to the circuit or device which is in need of testing. These probes typically feature either a pointed metal tip, retractable hook tip, or alligator clip tip; some probes feature only a pointed metal tip but include attachments that feature hook and alligator tips. Also, the majority of multimeter probes connect to multimeters via banana style connectors; some benchtop meters may require BNC connectors. While a probe may seem like a fairly standard component, there can be many differences between seemingly similar probes.
High Voltage Probes
A High Voltage Probe is a type of multimeter probe designed to measure voltages that are much higher than those a regular probe could handle. For example, a regular multimeter probe typically can measure a maximum voltage of about 1,000V. A high voltage probe can measure voltages of several thousand volts, 40 kV for example.
Features to consider when selecting Multimeter Probes
- Safety Rating. Multimeters and their probes are rated based on four categories that concern intended application. Probes often can be rated by multiple categories at different voltages (Ex: CAT IV 600 V, CAT III 1000 V). It is very important that you choose a meter that is rated for your intended application otherwise you can be hurt from unsafe, unintended probe use.
The categories are:
- CAT I: used where equipment is not directly connected to the mains
- CAT II: used on single phase mains final sub-circuits
- CAT III: used on permanently installed loads such as distribution panels, motors, and 3 phase appliance outlets
- CAT IV: used on locations where fault current levels can be very high, such as main panels, supply meters and primary over-voltage protection equipment
- Voltage Rating. This is very simply the maximum voltage (V) a probe is able to measure. A typical probe can measure up to 1000V while a high voltage probe can measure voltage well upwards of 5kV
- Maximum Current. All probes are also rated for the amount of current in amps (A) they can measure
An Oscilloscope Probe is the component used to make the connection between the oscilloscope and the signal source to be tested. These probes can be as simple as a basic length of wire or as complex as an active differential probe. However regardless of their unique characteristics, all oscilloscope probes are at their simplest connections between the test source and the oscilloscope input.
3 Key considerations when choosing Oscilloscope Probes
Additional features to consider when selecting Oscilloscope Probes
- Physical attachment. To physically attach a probe to a test source a probe must have a long enough cable to get from the oscilloscope to the test site, typically 10 ft (3 M) cable. However the trade-off with cable length is that the longer the cable, the more the probe’s bandwidth is reduced (the smaller the range of frequencies a probe can pick up)
- Impact on circuit operation. Probing a circuit also has the potential to impact the circuit’s operation and skew test results. This is called source loading. When a probe is attached to a signal source it draws current from that circuit which in turn can change the circuit operation behind the test point. It is impossible to eliminate source loading completely because to do so a probe would need infinite impedance. To minimize source loading one should look for a probe with high input impedance; this combined with probe attenuation further reduces source loading
- Signal transmission. One should also consider how well a probe transmits a signal. Signal fidelity is an important part of this because it basically is a measure of how accurately a probe represents a test signal. Signal fidelity is best preserved by active probes. Most probes also incorporate shielded cables that prevent most environmental electrical noise from interfering in signal transmission, however with low-level signals noise can often still be an issue
Types of Oscilloscope Probes
- Attenuation. Probes are available with different attenuation factors for different voltage ranges. For example, a passive probe typically includes 10X attenuation while a high-voltage probe can include attenuation up to 1000X. Attenuation minimizes source loading but reduces the amplitude of the signal delivered to the oscilloscope
- Maximum Voltage. This is simply the maximum voltage a probe can measure. It is important to select a probe with a voltage range that meets your application. Active probes typically have small voltage ranges while high-voltage probes offer the highest max voltage possible
- Bandwidth. Bandwidth quantifies the range of frequencies a probe can measure. Active probes typically have the largest bandwidths. You must select a probe with a bandwidth that either meets or exceeds the bandwidth of your oscilloscope
- Input Impedance. This is the resistance within the probe itself. Higher input impedance will reduce source loading
- Probe Interface Type. The probe interface is what connects the probe to an oscilloscope. Most probes connect via basic BNC connectors. Many Tektronix probes now incorporate special Tektronix-specific connectors for their oscilloscopes such as TekProbe, TekVPI, and TekConnect. Each has specific advantages such as communication of signal scale and power supply and management
- Passive Probes. Passive probes are the most common type of oscilloscope probe. These are the standard probes that are included with a new oscilloscope purchase. Because these probes are passive, they do not require power to operate (like some other more advanced probes) and are therefore relatively simple. This makes passive probes the most durable and affordable probes on the market. Passive probes commonly include attenuation abilities to expand measureable voltage range; the most common attenuation is 10X. They also typically have bandwidth ranges from about 100 MHz – 500 MHz. There is also a special type of passive probe often referred to as a Z0 or Lo Z probe. These probes feature 50 Ohm impedance and typically have bandwidth ranges up to several gigahertz
- Low Capacitance Passive Probes. A low capacitance passive probe is a unique Tektronix probe. These probes are specifically designed for certain Tektronix oscilloscopes. They offer industry-leading 3.9pF capacitive loading
- Active Probes. Active probes get their name because they incorporate active components that require power to operate. In most active probes the main active component is a type of transistor called a field-effect transistor (FET). FET active probes feature ultra-low capacitance, high input impedance, and bandwidths ranging from 500 MHz to several GHz. The most important feature of these types of probes is that they offer such low source loading that they can be used on high-impedance circuits that would be seriously loaded by passive probes. However the major drawback to active probes is their limited voltage range, typically anywhere from ±0.6 V to ±10 V. A passive probe can measure from millivolts to tens of volts; the passive probe’s voltage range is substantially larger
- Differential Probes. A differential probe is a type of active probe used to measure the voltage difference between two different signals. A differential probe uses an active component called a differential amplifier to subtract voltages for two signals and find the differential signal. Today’s technology has allowed the differential amplifier to be miniaturized and placed directly in the probe head. This has allowed a bandwidth of 1 GHz for modern differential probes. This process of determining differential signal can be accomplished instead by using two single passive probes however this method is much less accurate and much more susceptible to noise
- High Voltage Probes. High voltage is a relative term. In the case of probes, high voltage is any voltage that cannot be measured with a typical 10X attenuation passive probe. Basic passive probes can usually measure a maximum voltage of 400-500 volts; a high voltage probe can measure voltages as high as 20,000 volts. Because these probes deal with such high voltages, safety becomes increasingly important. To maintain the safety of the operator and the equipment high voltage probes are usually offered with extended cables up to 25 ft (7.6 M) long.
- Current Probes. When current flows through a conductor it causes an electromagnetic flux field to form around the conductor. A current probe can sense the strength of that field and is used to convert that strength value to a voltage value that can be measured by an oscilloscope. There are two types of current probes: AC current probes which are usually passive and AC/DC current probes which are usually active
An RF Probe is designed to measure radio frequency oscillation in an electronic circuit. Because RF can be challenging to measure depending on conditions there are a variety of different types of RF probes. For example, RF energy may be at too high of a frequency to be measured. In this case a special RF probe can be used to convert RF signal to DC. For another example, some circuits are very sensitive to changes in their electrical environments and therefore require probes that draw very little energy from the circuit (very little source loading). In this case a high impedance RF probe should be used to minimize source loading. It is important to know your application well so you can find the RF probe that best meets your needs.
Features to consider when selecting RF Probes
- Bandwidth. What frequency range can a probe measure? Auburn offers a probe with a wide range of 100kHz – 3GHz
- Accuracy. Most probes are accurate to +3db. Pomona offers a probe that is accurate to +2db
- Maximum Input Voltage. How much voltage can a probe handle? This can be anywhere from a few volts to upwards of 200 volts
Temperature Probes come in a variety of sizes, shapes, materials of construction, ranges, connectors, and technologies. They are largely self-explanitory and are used simply for taking temperature measurements.