What is network port circuit / bobsmith circuit?

Network port circuit is a 10 m / 100 m / 1000 m WAN port protection circuit that we often see in hardware design. It often includes network port transformer, RJ45 terminal, resistor, high voltage capacitor, common capacitor and inductor

According to the processing form of the center tap, we generally divide it into voltage type and current type.

(1) Current mode

For current mode, please note that on the PHY side, the center tap of the transformer is pulled up to the VCC power supply;

The size of the pull-up power depends on the size of the CPU drive capacity, some are 1.8V, some are 2.5V, and there are other voltages, which will not be repeated here.

(2) Voltage type

The voltage type is connected to the capacitor and not to the power supply. The size of capacitance is generally 100nF, if EMC test, but you can try other values.

The difference between current mode and voltage mode lies in the center tap of network transformer near PHY side. The current mode is connected to power supply VCC, and the voltage mode is connected to capacitor to ground.

What is the function of Bob Smith circuit?

(1) Impedance transformation;

I have seen this in a lot of materials, but actually in my later verification experiment, I found that this is not right, which will be described in detail later.

(2) Surge protection;

This is absolutely accurate. Please see the detailed analysis process.

(3) Reduce EMC test radiation

This is also true.

Impedance transformation verification I

The first concept of impedance: the reason why we hold impedance transformation is that the network port must be matched with the network cable, and the impedance of the network cable is 75 ohm of the twisted pair. At this time, 75 ohm is about the differential impedance of the twisted pair;

The second impedance concept: on the PCB board, two pairs of differential line impedance, each of which is 100 ohm differential impedance, and the single end impedance is not specified;

The third concept of impedance: the RJ45 terminal of the network port, for 100 megabytes, is connected in pairs, and then 75 ohm resistance is connected to the ground. What is the 75 ohm impedance for?

The fourth impedance concept: the center tap of the network transformer near the RJ45 terminal is connected with a 75 ohm resistor, passing the capacitor to the ground. What is the 75 ohm for?

The fifth impedance concept: the Gigabit Ethernet port can’t connect two 75 ohm resistors, but the differential impedance is also 100 ohm?

The first three impedance points mentioned above are all mentioned by Bob Smith circuit, but actually no one has made it clear why it is 75 ohm? I also think that it is wrong to use 75 ohm for impedance transformation, because the twisted pair is 75 ohm, but the board is 100 ohm, and the transformer is 1:1 winding. How to transform is totally unreasonable.

Besides, if the resistance here is not 75 ohm, but it is directly removed, changed to 50 ohm or directly 100 ohm, will the impedance transformation not be satisfied at this time? Therefore, I carry out the following verification:

Network port consistency test and verification

Network interface conformance test, which is used to determine the comprehensive physical layer connection index of network interface circuit, including eye diagram, rising edge, falling edge, up rush, down rush, balance degree and transmission jitter. (please refer to the introduction of Tektronix official website in the link)

In high-speed signal transmission, we know that if there is overcharge and undershoot, it must be the signal transmission impedance changes, impedance mismatch, and then lead to signal reflection and other problems.

So I test the network port consistency index from the following three groups of processing:

(1) Four resistors and one capacitor between RJ45 and network transformer are reserved;

(2) Remove four unused pins and two connected resistors from RJ45 terminal of network port;

(3) Remove all four resistors and one capacitor.

At this time, the network interface consistency data is as follows:

The data in the above table is the actual data. At this time, it can be seen that in the result comparison, when the above three kinds of data are used, the result basically changes little, and the network port consistency index is tested and passed. Moreover, if the real bobsmith circuit has the function of impedance transformation, my above operation (retaining resistance and capacitance, removing resistance and capacitance) completely breaks the impedance balance, But in fact, it has little influence on the network port consistency index, which is the first reason why I don’t think the circuit has impedance transformation.

Verification 2

In addition, if it is impedance transformation, I will change the 75 ohm resistance to other values. Will the network port consistency test fail. So I did the following experiments

(1) Change 75 ohm resistance to 50 ohm;

(2) The actual test result of replacing 75 ohm resistance with 100 ohm resistance.

As shown in the figure above, the actual result is not as we imagined. After changing to 50 Ω and 100 Ω, the network port conformance test fails. Instead, when the network port conformance test is 75 Ω, 50 Ω and 100 Ω, all the network port conformance tests pass, which indicates that the circuit has the function of impedance transformation, but it is not feasible. In fact, the above results in some test items, 50 or 100 ohm is better than 75 ohm, so it is incorrect to verify that the circuit has the function of impedance transformation again.

Another point is that eight signal lines are used in the Gigabit network. At this time, there is no place to connect the 75 ohm resistance. But this is a common practice. Why don’t we consider the impedance characteristics.

In addition, I also test the IP packet pinging of the network port, and there is no delay, and the link is normal.

Surge protection

Iec61000-4-5 is the lightning surge test standard for electronic products. Please refer to the corresponding description of the link for details.

When testing the surge protection capability of communication port, two groups of common mode test and differential mode test are needed. The test times are based on each company and professional certification test company, generally 5 times for positive and 5 times for negative. Generally speaking, we need to test common mode 4KV (voltage level) and differential mode 2KV (voltage level). The common mode test and differential mode test will be introduced below.

Common mode protection

Common mode test is to connect all the 8 lines together with the network port test tooling, and add 4KV standard waveform voltage between the 8 line nodes and the motherboard ground. The test voltage waveform is shown in the relevant standards of communication port surge test. What is the surge discharge path at this time?

As shown in the figure below:

Because eight wires are connected together, the direction of surge is consistent. Therefore, one signal wire (blue) and one unconnected wire (green) are used as the description of surge relief.

Blue signal line surge relief: the surge is first through one end of the transformer, tapped from the center, and then through the 75 ohm resistance and high-voltage capacitor to the ground; therefore, the transformer, resistance and capacitor are required to withstand the 4KV surge impact.

Green line surge discharge: the surge discharge is directly to the ground through the resistor and then the capacitor, so the resistor and capacitor are required to resist the 4KV surge impact.

Differential mode protection

The differential mode test is conducted on the differential line of data transmission, one end of which is connected with 2KV positive and the other end is connected with 2KV negative; there are 2 groups of 100m network port test and 4 groups of gigabit network port test. The 100 MB network port test is as follows:

The following is a set of difference lines.

Blue is the surge path on the right side of the network transformer. It can be seen that the network transformer must be able to resist the 2KV surge because there is no other discharge path.

Green is the surge path on the left side of the network transformer, that is, close to the CPU port. Because the differential mode is directly coupled to the coil on the side of the CPU through the transformer, the two pins of the CPU in the figure above must resist 2KV impact. However, in order to protect the CPU, we usually place small resistors on the data line close to the CPU and two-way TVs devices between the differential lines connected to the CPU to protect the CPU Surge damage.

Reduce EMC radiation inductance

As shown in the figure above, in the current mode Bob circuit, pull up the power supply and connect the inductor to reduce the high-frequency interference. What is the harm of high-frequency interference? High frequency signals contain abundant harmonics in frequency domain, or lead to strong electromagnetic field changes, resulting in excessive radiation.


What is the function of the center tap capacitance? The function of the center tap is also to reduce the AC impedance and short circuit the high-frequency AC signal to the ground. Therefore, l and C form LC filter to reduce the high-frequency interference.

Common mode inductor

The common mode inductance of network port transformer is T10, T14, T17 and T18 in current mode. We know that the radiation exceeding the standard is generally caused by common mode current, and the common mode inductance is to consume the common mode current in differential line, but has no effect on the differential mode current.


Bob Smith circuit we began to talk about his basic circuit form, components, voltage type and current type differences, and then we talked about his role?

The articles I read all say that it has the function of impedance transformation and surge protection. Therefore, I prove that impedance change is not very correct from three points. It may be wrong;

In addition, the role of bobsmith circuit in surge protection is elaborated from the perspective of surge protection. In order to achieve better protection effect, 3216 package is generally selected for resistance, and 3216 package is also selected for capacitor, because large package has better high voltage impact resistance.

Finally, the function of this circuit in preventing EMC test radiation is discussed.

I hope you can have a basic understanding and grasp of Bob Smith circuit.

Note: the above test data can be guaranteed to be true, and the above mentioned experiment is also verified by actual test. However, the above 75 Ω is the definite value of impedance matching, which I think is inappropriate. This is my personal view. If you have new views, you can exchange them more.

Editor: hfy

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