Let us read the underlying topics related to Oscilloscope probes in this series of articles. Please, suggest any other information that could be added to these tutorials.
When hardware designers starts testing his design, they start using different tools and techniques. The tools could be Multimeter, Oscilloscope, TDR meter, LCR Meter, Network Analyzer, Spectrum analyzer, Debugger, etc. For all these tools and equipments, probe is the primary interface. Probe helps tap the electrical signal from the board under testing and provide input to measurement equipment. The probe should neither disturb the functionality of the board nor generate an erronous signal which could mislead the test engineer.so, let us list some important probing techniques which could help the test engineer to do the job as expected.
1. Selecting a compatible probe is the first step. Always try to use a probe supplied by the oscilloscope vendor for that specific model. For tektronix Oscilloscope models, we have a probe selector, which helps select a probe for an application and oscilloscope model.
https://www.tek.com/probe-selector
Similarly, another major test equipment vendor Keysight Technologies has a probe selector tool.
https://www.keysight.com/en/pc-1000000101%3Aepsg%3Apgr/oscilloscope-probes?cc=IN&lc=eng
One has to remember that, the above tools might suggest a suitable probe but it is the responsibility of design/test engineer to study the circuit and probing needs in detail to arrive at a decision. The critical factors which affect the measurement are:
a. Probe Type - Active, Passive, Signle-Ended, Differential
b. Probe Electrical Characteristics - Voltage, Current, Capacitance, Resistance
c. Bandwidth
d. Probe Tip type & extensions possible
e. Attenuation options - x1, x10, x100
f. Rise Time
2. While selecting a probe, one of the major decision is to verify the type of probe required. Probes can be classifed as Active or Passive probes and the usage depends on the signal frequency for which the probing is required. High frequency signal probing, like the DDR, Ethernet, USB requires active probes for good signal reproduction on the measurement equipment. For low frequency signals, passive probe must be enough.
3. The impedance of the probe is another critical factor. As mentioned previously, probe should not load the design under test (DUT) or circuit under test. The requirement for this is that probe should have a high impedance. This means that probe should tap only a fraction of voltage and rest of the processing should be done by the measuring equipment. when we talk about high impedance here, the input resistance of the probe shall be in the range of 8-10Mohms. This 8-10ohms resistance is for passive probe. For an active probe, there shall be an amplifier, which amplifies and also isolates (buffer) the circuit from the equipment.
4. Probe Capacitance is another major parameter. The loading effect of capacitance on the circuit is high in the passive probes and this distorts the signal. But if the signal is low frequency, the capacitance might not matter as the rise time of the probed signal would be lesser. One has to rememeber that probe has a capacitance and oscilloscope has it's own input capacitance. Also, the circuit under test has it's own capacitance. Choosing a probe which satisfies these requirements and which passes the signal without distortion is important. So, when we talk about Probe capacitance here, we are talking about signal frequency and hence the measurement bandwidth.
5. While measuring the signal, the ground tip must be as close as possible to probe tip. This helps eliminate the inductance and hence probe input impedance is not varied. If the grounding of the probe is done away from live terminal, this creates ground loop and hence the signal integrity is lost causing ringing, overshoot and undershoot.
6. Ensure that you probe the signal with the probe tip directly conencted to the circuit. Sometimes designers tend to solder wires to the board and then probe away from the circuit. The wires soldered to the board adds inductance and signal integrity is lost causing ringing, overshoot and undershoot.
7. The best way to ensure proper probing is to verify the probe functionality by giving a stable waveform input to oscilloscope. If the set up looks fine, this can be used for measurements.
8. Perform clock probing at the output of clock buffers rather than on the oscillator output directly. This helps isolate the signal source and probe input.
When hardware designers starts testing his design, they start using different tools and techniques. The tools could be Multimeter, Oscilloscope, TDR meter, LCR Meter, Network Analyzer, Spectrum analyzer, Debugger, etc. For all these tools and equipments, probe is the primary interface. Probe helps tap the electrical signal from the board under testing and provide input to measurement equipment. The probe should neither disturb the functionality of the board nor generate an erronous signal which could mislead the test engineer.so, let us list some important probing techniques which could help the test engineer to do the job as expected.
1. Selecting a compatible probe is the first step. Always try to use a probe supplied by the oscilloscope vendor for that specific model. For tektronix Oscilloscope models, we have a probe selector, which helps select a probe for an application and oscilloscope model.
https://www.tek.com/probe-selector
Similarly, another major test equipment vendor Keysight Technologies has a probe selector tool.
https://www.keysight.com/en/pc-1000000101%3Aepsg%3Apgr/oscilloscope-probes?cc=IN&lc=eng
One has to remember that, the above tools might suggest a suitable probe but it is the responsibility of design/test engineer to study the circuit and probing needs in detail to arrive at a decision. The critical factors which affect the measurement are:
a. Probe Type - Active, Passive, Signle-Ended, Differential
b. Probe Electrical Characteristics - Voltage, Current, Capacitance, Resistance
c. Bandwidth
d. Probe Tip type & extensions possible
e. Attenuation options - x1, x10, x100
f. Rise Time
2. While selecting a probe, one of the major decision is to verify the type of probe required. Probes can be classifed as Active or Passive probes and the usage depends on the signal frequency for which the probing is required. High frequency signal probing, like the DDR, Ethernet, USB requires active probes for good signal reproduction on the measurement equipment. For low frequency signals, passive probe must be enough.
3. The impedance of the probe is another critical factor. As mentioned previously, probe should not load the design under test (DUT) or circuit under test. The requirement for this is that probe should have a high impedance. This means that probe should tap only a fraction of voltage and rest of the processing should be done by the measuring equipment. when we talk about high impedance here, the input resistance of the probe shall be in the range of 8-10Mohms. This 8-10ohms resistance is for passive probe. For an active probe, there shall be an amplifier, which amplifies and also isolates (buffer) the circuit from the equipment.
4. Probe Capacitance is another major parameter. The loading effect of capacitance on the circuit is high in the passive probes and this distorts the signal. But if the signal is low frequency, the capacitance might not matter as the rise time of the probed signal would be lesser. One has to rememeber that probe has a capacitance and oscilloscope has it's own input capacitance. Also, the circuit under test has it's own capacitance. Choosing a probe which satisfies these requirements and which passes the signal without distortion is important. So, when we talk about Probe capacitance here, we are talking about signal frequency and hence the measurement bandwidth.
5. While measuring the signal, the ground tip must be as close as possible to probe tip. This helps eliminate the inductance and hence probe input impedance is not varied. If the grounding of the probe is done away from live terminal, this creates ground loop and hence the signal integrity is lost causing ringing, overshoot and undershoot.
6. Ensure that you probe the signal with the probe tip directly conencted to the circuit. Sometimes designers tend to solder wires to the board and then probe away from the circuit. The wires soldered to the board adds inductance and signal integrity is lost causing ringing, overshoot and undershoot.
7. The best way to ensure proper probing is to verify the probe functionality by giving a stable waveform input to oscilloscope. If the set up looks fine, this can be used for measurements.
8. Perform clock probing at the output of clock buffers rather than on the oscillator output directly. This helps isolate the signal source and probe input.
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