Why is the internal resistance of the signal source and the input impedance of the measuring device 50 ohms?

乱世煮酒论天下

Why is the internal resistance of the signal source and the input impedance of the measuring device 50 ohms? [Copy link]

 

The signal source is equivalent to a Thevenin circuit with an internal resistance of 50 ohms. The channel input impedance of the test equipment used to measure the signal is also 50 ohms, especially some oscilloscopes and voltmeters.

There are two options for the input line impedance of the oscilloscope, 50 ohms and 1 megohm. Check the equivalent input impedance circuit of the oscilloscope channel. When it is set to 50 ohms, the oscilloscope input channel is a 50 ohm resistor; when it is set to 1 megohm internal resistance, it is a 47 picofarad capacitor and a 1 megohm resistor in parallel.

When it is set to 50 ohms, it is to match the characteristic impedance of the transmission line coaxial cable, but what is the effect when it is set to 1 megohm? Why is the capacitor chosen to be 47 pF and why is the resistor chosen to be 1 megohm?

When measuring what signal, do you choose 50 ohm input and 1 megohm input impedance? What is the basis? What is the relationship with the voltage level and bandwidth of the measured signal?

maychang

[Why is the internal resistance of the signal source and the input impedance of the measuring device 50 ohms?]

That's not necessarily the case.

The internal resistance of the signal generator is 50 ohms to match the characteristic impedance of the coaxial cable. Therefore, this kind of signal generator must be a high-frequency signal generator. The output impedance of signal generators used for lower frequencies such as audio is often 600 ohms. Some signal generators with higher output power have very low internal resistance, even less than 1 ohm.

maychang

[Why is the internal resistance of the signal source and the input impedance of the measuring device 50 ohms?]

Similarly, the input impedance of the measuring device is 50 ohms, which is also to match the coaxial cable. If you are not measuring the signal output by the coaxial cable, such as measuring the waveform of the microcontroller clock generator (quartz crystal), the input impedance of the measuring device (oscilloscope) should be 1 megohm.

beyond_笑谈

I guess you saw this somewhere. It may not be a 50 ohm input impedance. I suggest distinguishing different measurement signals.

乱世煮酒论天下

beyond_笑谈Published on 2024-6-11 14:22 I guess you saw the information somewhere. It may not be 50 ohm input impedance. It is recommended to distinguish different measurement signals

The book Introduction to Electromagnetic Compatibility introduces aspects related to the characteristics of signal sources. The internal resistance of signal sources is mostly 50 ohms, and the input impedance of most instruments for measuring signals is also 50 ohms.

gmchen

Let’s talk about 50 Euros first.

This impedance is usually used when measuring high-frequency signals, because most high-frequency instruments and high-frequency cables are designed according to the 50 ohm characteristic impedance.

Why choose 50 ohms instead of other resistance values? There are technical reasons and historical reasons. According to theoretical calculations, the characteristic impedance of coaxial cable is about 30 ohms with the maximum transmission power capacity, and at 77 ohms with the lowest transmission loss, so choosing 50 ohms is a compromise. The historical reason is that some major American manufacturers set the 50 ohm standard at that time, and later everyone acquiesced to this standard.

gmchen

Let’s talk about 1M Euro.

This impedance is usually applied to low-frequency signals. Because low-frequency signals are usually measured by measuring their voltage, as long as the input impedance of the measuring instrument is much higher than the equivalent internal resistance of the measured point, the influence of the input resistance of the measuring instrument can be ignored. The internal resistance of the measured point of a low-frequency circuit is usually not very high, so the input resistance of a general oscilloscope is mostly 1 megohm or 10 megohm. Using a higher input impedance will cause other problems such as inductive input.

As for the 47pF capacitor, it is a compensation capacitor used to compensate for the frequency response problem caused by the lead capacitance of the oscilloscope. Usually there is a fine-tuning capacitor on the probe of the oscilloscope, which can be adjusted to achieve balance in the entire test frequency band. By the way, if an oscilloscope has adjusted the compensation capacitor, the probe should not be replaced casually. Many people do not pay attention to this compensation capacitor and do not know how to adjust it, which actually affects the measurement results.

乱世煮酒论天下

gmchen posted on 2024-6-11 20:25 Let's talk about 1M ohm. This impedance is usually used for low-frequency signals. Because low-frequency signals are usually measured by measuring their voltage, as long as the input impedance of the measuring instrument is...

Is it the knob on the oscilloscope probe? Is it that you clamp the oscilloscope in the calibration position and use a small plastic piece to turn the knob on the probe?

乱世煮酒论天下

gmchen posted on 2024-6-11 20:15 Let's talk about 50 ohms first. This impedance is usually used when measuring high-frequency signals, because most high-frequency instruments and high-frequency cables are based on 50 ohm characteristic impedance...

The so-called high frequency and low frequency, how high is considered high? I checked some information but there is no clear statement, but the frequency is limited to 100M.

gmchen

乱世煮酒论天下 posted on 2024-6-11 20:53 Is it the knob on the oscilloscope probe? Is it to clamp the oscilloscope clip in the calibration position and use a small plastic piece to turn the knob on the probe?

Yes

gmchen

Luan Shi Zhu Jiu Lun Tian Xia published on 2024-6-11 20:57 The so-called high frequency and low frequency, how high is high, I checked some information and there is no clear statement, 100M frequency is the limit.

There are no clear boundaries. The key is the need for measurement.

For example, the intermediate frequency of a radio is only 465kHz, but when it is measured with a high-frequency instrument such as a frequency sweeper, it is measured according to the characteristic impedance of 50 ohms. However, if an op amp is used to form a voltage amplifier, the signal frequency may be as high as tens of megahertz, but the instrument used to measure its output voltage (high-frequency millivoltmeter or oscilloscope) still uses high-impedance input.

If we have to give a limit, then anything below 100k is definitely low frequency, and anything above 100M is definitely high frequency. The part in between, haha.......

乱世煮酒论天下

gmchen posted on 2024-6-11 20:25 Let's talk about 1M ohm. This impedance is usually used for low-frequency signals. Because low-frequency signals are usually measured by measuring their voltage, as long as the input impedance of the measuring instrument is...

You must be talking about the lead inductance of the oscilloscope probe. I did a simulation, but didn't adjust the parameters. I just selected some random resistance, capacitance and inductance parameters. The simulation data is as above. The inductive reactance of the inductor and the impedance of the parallel connection of resistance and capacitance are connected in series, which means using the capacitive reactance of the capacitor to offset the inductive reactance of the probe lead and fine-tune the adjustable capacitor.

maychang

乱世煮酒论天下Published on 2024-6-12 18:37 You are probably talking about the lead inductance of the oscilloscope probe. I did a simulation and didn't adjust the parameters. I just selected some random resistors and capacitors...

[Simulation data as above]

I think you set the signal output by XFG1 to be a square wave. The period can be seen from the waveform you posted, which is 1ms, that is, the frequency is 1kHz.

maychang

乱世煮酒论天下Published on 2024-6-12 18:37 You are probably talking about the lead inductance of the oscilloscope probe. I did a simulation and didn't adjust the parameters. I just selected some random resistors and capacitors...

[You are probably talking about the lead inductance of the oscilloscope probe]

Teacher gmchen is not talking about the lead inductance of the oscilloscope probe. The 7th post clearly states: "The 47pF capacitor is a compensation capacitor used to compensate for the frequency response problem caused by the lead capacitance of the oscilloscope."

乱世煮酒论天下

maychang posted on 2024-6-12 18:43 [You should be talking about the lead inductance of the oscilloscope probe] Teacher gmchen is not talking about the lead inductance of the oscilloscope probe. The 7th floor said it very clearly: & ...

Why is it lead capacitance? It is the equivalent capacitance of the loop formed by the probe tip and the ground?

maychang

Luan Shi Zhu Jiu Lun Tian Xia published on 2024-6-12 20:05 Why is it lead capacitance? It is the equivalent capacitance of the loop formed by the probe tip and the ground of the probe?

The probe tip and ground do not form a loop.

The structure of an ordinary oscilloscope probe is a probe (your name), which is often covered with a cap. There is a spring inside the cap, which can retract a hook into the cap). Inside the probe is a resistor, one end of which is connected to the probe, and the other end is connected to the core wire of the lead. The core wire is connected to the input terminal of the amplifier inside the oscilloscope through a plug, and the outer skin (metal mesh) of the lead is connected to the ground wire (shell) of the oscilloscope.

Obviously, there is a considerable distributed capacitance between the core wire and the outer skin of the lead wire. The resistor inside the probe and the input resistance of the oscilloscope amplifier form a voltage divider circuit, and the distributed capacitance between the core wire and the outer skin of the lead wire is connected in parallel to the input resistance of the amplifier, which will affect the voltage divider ratio of the above voltage divider circuit.

With such a structure, it is obvious that the voltage division ratios of signals of different frequencies are different, because the capacitive reactance of the capacitor is inversely proportional to the frequency.

In order to avoid different voltage division ratios of signals with different frequencies, a variable capacitor is connected in parallel across the resistor connected to the probe, which is the compensation capacitor mentioned by teacher gmchen on the 7th floor.

If the ratio of the compensation capacitor to the lead distributed capacitance is equal to the ratio of the oscilloscope amplifier input resistance to the probe internal resistance, then the voltage divider ratio of the probe, the amplifier input resistance and the two capacitors does not change with frequency.

maychang

Luan Shi Zhu Jiu Lun Tian Xia published on 2024-6-12 20:05 Why is it lead capacitance? It is the equivalent capacitance of the loop formed by the probe tip and the ground of the probe?

If you want to understand what I said on the 16th floor, you'd better draw a picture and show the oscilloscope's internal amplifier input resistance, lead distributed capacitance, probe internal resistance, and compensation capacitance.

maychang

Luan Shi Zhu Jiu Lun Tian Xia published on 2024-6-12 20:05 Why is it lead capacitance? It is the equivalent capacitance of the loop formed by the probe tip and the ground of the probe?

As for the two waveforms you posted on the 12th floor, the lower one is actually the waveform of the upper one in the schematic diagram (with inductor L1). Why there is such a waveform is because you use a square wave to drive the upper one in the schematic diagram, which is caused by the series resonant circuit composed of L1 and C1, and has nothing to do with the simulation oscilloscope.

maychang

Luan Shi Zhu Jiu Lun Tian Xia published on 2024-6-12 20:05 Why is it lead capacitance? It is the equivalent capacitance of the loop formed by the probe tip and the ground of the probe?

The upper limit of the working frequency of the ordinary oscilloscope probe mentioned on the 16th floor is generally 10MHz or 20MHz. At higher frequencies, such as 100MHz, the probe lead (coaxial cable) can no longer be regarded as a lumped parameter conductor and distributed capacitance, but must be considered as a transmission line. Because the wavelength of a 100MHz electromagnetic wave is 3 meters, the length of the probe lead is comparable to the wavelength. Therefore, the compensation measures for the probe are more complicated.

乱世煮酒论天下

maychang posted on 2024-6-12 20:25 The probe tip and the ground of the probe do not form a loop. The structure of an ordinary oscilloscope probe is a probe (your name), often with a cap on the outside, ...

According to what you said, there is a considerable distributed capacitance between the core wire and the outer skin of the lead wire. The resistor inside the probe and the input resistance of the oscilloscope amplifier form a voltage divider circuit, and the distributed capacitance between the core wire and the outer skin of the lead wire is connected in parallel to the input resistance of the amplifier, which will affect the voltage divider ratio of the above voltage divider circuit.

I may understand that the external equivalent capacitance of the probe is very large, and the adjustable capacitance inside the oscilloscope is the same as the external equivalent capacitance after adjustment. In this way, the external equivalent capacitance will not affect the capacitive reactance of the capacitor on the equivalent capacitance due to frequency issues, because the inside and outside of the channel are completely consistent! According to the 10:1 ratio of ordinary probes, the ratio of internal and external capacitance of the oscilloscope is equal to the ratio of resistance of 10:1.