The oscilloscope probe has two important performance problems to consider, that is, how much influence and interference the probe has on the measured signal, and how well the probe can reflect the actual signal. In order to get the best performance, it is usually necessary to keep the probe connection as short as possible, but sometimes this is not possible, so it is necessary to know the performance of various probes and their accessories configuration, and understand the balance between the indicators.
To understand how the probe interferes with the detected signal, the influence of the probe input impedance needs to be measured.To understand how well the probe reflects the real signal under test, you need to measure the probe response or VOUT/VIN.
These measurements can be made in the frequency domain using a three-port network analyzer or in the time domain using a high-speed step pulse source and a two-channel oscilloscope. No matter use which kinds of instrument, is driven by signal source by 50 Ω transmission network analyzer or an input of the oscilloscope, received the second input output probe VOUT, probe inputs received a 50 Ω pass-through fixture, and then measured with a probe attachment VIN, it represents the probe input.The use of oscilloscope with probe can be used for qualitative analysis, but high bandwidth sampling oscilloscope should also be used to accurately evaluate the performance of the probe.
Take probe to 50 Ω pass-through fixture, by observing the probe on the VIN, can determine the probe the influence of input impedance of the signal.For this measurement, the time domain effect may be of more interest.
VSRC is the signal that does not pass the probe, VIN is the signal that USES the probe connection, obviously at this time the probe has serious interference to the signal, such probe configuration can be considered unacceptable.As a comparison, figure 2b shows the results of the same measurement with a better probe, where the probe has much less interference with the signal and thus can meet the measurement requirements.
The probe response can be evaluated by calculating the ratio of VOUT to VIN. In order to accurately represent the input signal, the probe should have a flat response within the specified bandwidth. At this time, the frequency domain method should be used for measurement.
Poor probe response: when observing signals with fast jump edge, the peak value of +3dB at 1.8ghz will lead to considerable overshoot and ringing. Some manufacturers specially designed the probe frequency response to have a spike, to compensate for the input impedance of the probe signal loss, this approach may make you think that the signal looks good, but in fact the probe has greatly attenuated the measured signal, so as to cover up the error caused by the probe spike response. For comparison, the probe response is flat up to 2GHz and then attenuates -3db at the expected 3.5ghz bandwidth point.
In order to accurately measure the high bandwidth signal with the oscilloscope, the probe used must not have obvious influence and interference to the measured signal, serious interference may lead to the original normal circuit work abnormal, or make the original good quality signal looks very bad on the oscilloscope.The probe must not only accurately reproduce the signal under test, but also minimize the effect on the signal under test.The probe configuration can be accurately evaluated with the measuring device provided in this article, so that you can measure the high bandwidth signal with more confidence.