The difference between X1 and X10 gears of oscilloscope probes

Engineers who are familiar with oscilloscope probes all know that oscilloscope probes have X1 and X10 gears. How should you choose when measuring a signal? Today PRBTEK will share with you.

1. Let’s first look at their differences?

X1 level means that the signal enters the oscilloscope without attenuation.

X10 gear means that the signal is attenuated 10 times before entering the oscilloscope (when the oscilloscope is also set to X10 gear, just read directly. When the oscilloscope is set to X1 gear, the reading on the oscilloscope should be expanded 10 times to be the true value)

Let’s look at the equivalent circuit of an oscilloscope probe?

Simplify the probe model into an RLC circuit, as shown in the figure below:

As shown in the figure above, Rprobe is the input resistance of the probe. The larger the input resistance Rprobe, the better. However, Rprobe cannot be infinite. It creates a voltage divider with the circuit under test, making the measured voltage smaller than the actual voltage. In order to avoid the impact caused by the resistive load of the probe, it is generally required that Rprobe is more than 10 times greater than Rsource and Rload. The input impedance of most probes ranges from tens of kilohms to tens of megohms.

Cprobe is the input capacitance of the probe itself. This capacitance is not deliberately built in, but is the parasitic capacitance of the probe. This parasitic capacitance is also the most important factor affecting the probe bandwidth, because this capacitance will attenuate high-frequency components and slow down the rising edge of the signal. Generally, the parasitic capacitance of high-bandwidth probes is relatively small. Ideally Cprobe should be 0, but this is not possible. Generally, the input capacitance of passive probes is between 10pf and several hundred pf, and the input capacitance of active probes with higher bandwidth is generally between 0.2pf and several pf.

Lprobe is the parasitic inductance of the probe wire. Usually the ground wire of a 1mm probe has an inductance of about 1nH. The longer the signal and ground wires, the greater the inductance value. The probe's parasitic inductance and parasitic capacitance form a resonant circuit. When the inductance value is too large, high-frequency resonance may occur under the excitation of the input signal, causing signal distortion. Therefore, the length of the signal and ground wires needs to be strictly controlled during high-frequency testing, otherwise ringing will easily occur.

When using an oscilloscope, you need to set the coupling method and input impedance of the oscilloscope measurement channel. There are two coupling methods: AC and DC, and two types of input impedance: 1MΩ and 50Ω. There are many types of oscilloscope probes, but the matching of oscilloscopes always only has two choices: 1M ohm or 50 ohm. Different types of probes require different resistors to match them. The circuit diagram of the oscilloscope input interface is shown below:

When measuring ordinary signals, the DC coupling method is generally used. When testing the ripple/noise of the power supply, the AC coupling method is required. When the oscilloscope is connected to an active probe, the input impedance will automatically switch to the 50Ω level. When a passive probe is connected, the input impedance will be manually switched to 1MΩ range. From the perspective of voltage measurement, in order to reduce the impact on the circuit under test, the oscilloscope should use a high input impedance of 1MΩ. However, the bandwidth of the high-impedance circuit is easily affected by parasitic capacitance. Therefore, the input impedance of 1MΩ is widely used for measurements below 500M bandwidth. For higher frequency measurements, a 50Ω transmission line is usually used, so the oscilloscope’s 50Ω matching is mainly used for high frequency measurements.

In order to better illustrate the impact of the oscilloscope's input impedance and parasitic capacitance on the measurement channel bandwidth, the equivalent impedance of the oscilloscope's parasitic capacitance is 1/2pifc. At low frequencies, the equivalent impedance of C is very large, and most of the current flows through R. When As the signal frequency increases, the impedance becomes smaller and smaller, and the input impedance decreases. In order to reduce the impact of parasitic capacitance on the oscilloscope's input impedance, when testing high-frequency signals, the oscilloscope's input impedance is set to 50 ohms.

The attenuated signal is to use a probe. After connecting this probe and using the x10 level, the amplitude of the signal that reaches the input end of the oscilloscope after passing through the probe is attenuated to 1/10, and from the perspective of the probe input end, the input impedance becomes 10 times. In fact, from the oscilloscope Looking at the input end, the input impedance is still the same, but for the system (oscilloscope + probe), the impedance increases by 10 times. A high input impedance will cause less loss of the input signal. The input impedance is equivalent to the load of the signal. The higher the input impedance, the lighter the load of the signal.

2. When to use X1 and X10?

(1) For unknown signals, it is best to use X10 gear to avoid burning out the instrument

(2) For example, the VTUNE pin of a VCO, it is best to use the X10 level because the signal is relatively sensitive. Less interference to the signal after attenuation

(3) When measuring lines with large internal resistance and large output impedance such as crystal oscillators, use the X10 gear to obtain a more accurate amplitude value. Play the role of improving input impedance.

Summary: t probe attenuation and input impedance matching are two concepts and should be treated differently.