BIAS (DC bias device) is a three-port network device, the three ports are the radio frequency port RF, the DC bias port DC and the radio frequency DC port RF&DC. Because these three ports are often arranged in the shape of a T, they are called Bias.

Bias (DC bias device)
Bias (DC bias device)

Bias's DC port is composed of a feeding inductor, which is used to add a DC bias to prevent the AC signal of the RF port from leaking to the power supply system. Under ideal conditions, the DC terminal will not have any impact on the RF terminal signal; the RF port is composed of a blocking capacitor, which is used to input the RF signal and can block it at the same time.The DC voltage of the bias port; the RF&DC port is connected to the device, which can see the DC bias voltage and the RF signal at the same time. If the internal devices of Bias choose ultra-wideband, near-idealized, high-frequency inductors and capacitors without resonance points, then when BIAS is used to set the DC bias points of certain electronic components, it will not interfere with other components.

Figure 1-1 Schematic diagram of the internal structure of Bias
Figure 1-1 Schematic diagram of the internal structure of Bias

1.2. Bias index

For Bias, the more important indicators are the DC terminal bias voltage and current; the isolation between the RF terminal and the DC terminal; RF bandwidth, group delay, insertion loss and return loss, etc. The meaning of each indicator will be explained separately.

1.2.1 DC terminal bias current

The DC terminal bias current is the current input from the DC terminal and passing through the inductor, which is mainly limited by the maximum current that the DC port inductor can withstand. The larger the value of the DC-terminal bias current, the better the indicator. When the actual current exceeds the upper limit of the bias current, the inductance of the DC port is in a current saturation state. The performance of Bias will be affected in this state, and the device may be damaged due to excessive current, causing Bias to fail to work properly.

1.2.2 Isolation between RF terminal and DC terminal

Isolation refers to the ability of inductance to prevent the flow of radio frequency signals from the RF port to the DC port. The unit is usually expressed in dB. In theory, the greater the isolation, the better. If the isolation index of a BIAS is poor, the radio frequency signal will leak into the power supply system, affecting system performance. As shown in Figure 1-2, a schematic diagram of the isolation between the RF terminal and the DC terminal is shown. The input signal power of the RF port in the figure is 40dBm. When the isolation is 10dB, the signal power received at the DC terminal is 30dBm, which is equivalent to 1W, which may cause irreparable damage to the power supply system.; When the isolation is 40dBm, the signal power received at the DC terminal is 0dBm, which is equivalent to 1mW, and will not cause interference or damage to the power supply system.

Figure 1-2 Schematic diagram of the isolation between the RF terminal and the DC terminal
Figure 1-2 Schematic diagram of the isolation between the RF terminal and the DC terminal

1.2.3 insertion loss

The insertion loss of bias refers to the amount of signal attenuation between the RF input terminal and the RF&DC terminal, and the unit is usually expressed in dB. Expression formula for insertion loss：

Bias's insertion loss calculation formula
Bias's insertion loss calculation formula

​ (Equation 1)

Where Ut is the transmitted signal voltage and Ur is the received signal voltage.

插入损耗可以用网络分析仪的S21参数进行评估，插入损耗反映了系统传输路径的情况，插入损耗的数值越小代表指标越好。如图1-3所示为典型的BiasT的S21参数测试图，从图中可以看出在5GHz的频点，BiasT的插损约为0.5dB;在30GHz的频点，BiasT的插损约为1.3dB。0.5dB<1.3dB，说明5GHz的插损指标更好，插入损耗的绝对值值越小代表指标越好。

Figure 1-3 S21 parameter test diagram
Figure 1-3 S21 parameter test diagram

1.2.4 RF bandwidth

The RF bandwidth is the frequency range between the upper limit 3dB cut-off point and the lower limit 3dB cut-off point, which is the pass band of Bias. The 3dB frequency cut-off point is the point where the average power in the pass band is attenuated by 3dB relative to the pass band. The wider the bandwidth of the RF, the better. If the signal frequency range exceeds the Bias pass band, the signal outside the Bias pass band will be greatly attenuated.

Figure 1-4 Schematic diagram of RF bandwidth
Figure 1-4 Schematic diagram of RF bandwidth

As shown in Figure 1-4, it is a schematic diagram of the RF bandwidth. From the figure, it can be seen that the upper limit of 3dB cut-off point is about 15GHz, and the lower limit of 3dB cut-off point is about 7GHz, so the pass band ranges from 7GHz to 15GHz. If a 5GHz signal is input to the device, the signal will be attenuated by 15dB. If a 10GHz signal is input to the device, the signal will only be attenuated by 1dB, that is, the signal in the RF pass band has less attenuation, and the signal outside the pass band has a lot of attenuation. It can be concluded that the larger the RF bandwidth, the larger the frequency range of the signal that is allowed to pass normally.

1.2.5 Return loss

Return loss (return loss), also known as reflection loss, is the ratio of the reflected wave power to the incident wave power of the transmission line port. It is expressed in logarithmic form. The unit is dB, which is generally a negative value. It can be evaluated with the S11 and S22 parameters of the network analyzer. Return loss is an indicator used to measure the mismatch of link impedance. unqualified return loss means that the data transmission system is in a state of impedance mismatch. The lower the value of the return loss, the smaller the return loss, and the less signal reflection and signal distortion. Generally, the return loss is maintained between -20dB and -40dB, and the formula is as follows：

Return loss calculation formula
Return loss calculation formula

(Equation 2)

Among them, RL is the abbreviation of return loss, P reflection refers to the power of the reflected signal, and P incident refers to the power of the incident signal.

As shown in Figure 1-5, a typical Bias return loss parameter test diagram is shown. As can be seen from the figure, the return loss at the 25GHz frequency is =-10dB, indicating that 1/10 of the power is reflected; at the 5GHz frequency, the return loss is -20dB, which means that 1% of the power is reflected, that is, the smaller the return loss value, the better.

Figure 1-5 S11 parameter test diagram
Figure 1-5 S11 parameter test diagram

1.2.6 Group delay

The group delay is the rate of change of the phase (phase shift) of the system at a certain frequency to the frequency. When a wideband signal passes through a linear element in a media transmission path or equipment, the phase velocity of each spectral component is different, and the response of the component to each spectral component is also different. This will cause the signal reaching the receiving end to be disturbed by the phase shift or delay of each frequency component and the phase relationship, that is, phase distortion.. The ideal group delay curve should be a straight line without fluctuations. The smaller the vibration amplitude of the group delay, the better.

For Bias, the smaller the group delay change, the smaller the impact of the signal. If the delay of the Bias group of an access system changes greatly, it will cause the phase distortion of the signal, and a significant phase shift can be observed on the oscilloscope.

Figure 1-6 is a delay test diagram of a bias group.

Figure 1-6 Group delay test diagram
Figure 1-6 Group delay test diagram

Bias can be used in all fields from chips to test and measurement equipment and systems. Bias is mainly used to power transistor or amplifier circuits and drive electro-optical modulators and lasers.

1.3.1 Broadband amplifier

Bias can be used for the feeding of wideband amps, which is a very mature application. The amplifier that needs power supply at the output requires the addition of Bias to provide stable operating conditions for the amplifier.

Figure 1-7 Schematic diagram of amplifier and Bias
Figure 1-7 Schematic diagram of amplifier and Bias

As shown in Figure 1-7, a schematic diagram of the amplifier circuit that requires a biasing device. In the figure, Bias is added to the output of the amplifier for feeding power to the output.

When the amplifier needs to add BIAS for feeding, focus on certain indicators of Bias: BIAS pass band, maximum input power at the RF terminal, S21 parameter, S22 parameter, S11 parameter, rise and fall time, jitter, etc.

When purchasing Bias, the first step should be to determine the frequency range of the amplifier and the frequency range of Bias. As shown in Figure 1-8, a schematic diagram of the frequency band connected between the amplifier and Bias is shown. The frequency range of the amplifier in the figure is from 40kHz to 40GHz, and the frequency range of Bias is from 50kHz to 18GHz. After the two are connected, the passband is the overlapping part of the frequency range of the two. In BIAS's out-of-band, such as 18GHz to 40GHz, the signal will have a lot of loss and cannot achieve the expected effect of the amplifier. Therefore, when choosing Bias, make sure that the pass band range of Bias is greater than or equal to the frequency range of the wideband amplifier.

Figure 1-8 Schematic diagram of amplifier and BIAS frequency range
Figure 1-8 Schematic diagram of amplifier and BIAS frequency range

The second step of purchasing Bias is to check the maximum output power of the amplifier and compare the maximum input power of the RF terminal of BIAS. Figure 1-9 is a schematic diagram of the maximum input power of the amplifier and Bias. The maximum output power value of the amplifier in the figure is 60dBm, while the maximum input power of Bias is only 30dBm. The final output terminal cannot get the expected 60dBm output, and the signal power output by the amplifier may also damage the BIAS device.Therefore, when purchasing Bias, make sure that the maximum input power value of Bias is greater than the maximum output power value of the amplifier.

Figure 1-9 1dB compression point and Bias diagram
Figure 1-9 1dB compression point and Bias diagram

The third step of purchasing BIAS is to check the GAIN of the amplifier and compare the S21 parameter indicators of BIAS.

After the first two steps of screening, Bias can be guaranteed to be applied to the amplifier. Bias feeds the amplifier, the two are in the same transmission link, and the final gain is the superposition of the S21 parameters of the two. Figure 1-10 shows a schematic diagram of the amplifier and Bias in the transmission link. When the amplifier gain is 10dB and the Bias insertion loss is -1dB, the final output gain is 9dB. When choosing Bias, pay attention to the maximum insertion loss of the S21 parameter, and superimpose the amplifier gain and Bias insertion loss to obtain the final output gain value to determine whether it meets the requirements of use.

Figure 1-10 Schematic diagram of S21 parameter overlay
Figure 1-10 Schematic diagram of S21 parameter overlay

The fourth step of purchasing Bias is to compare the S22 and S11 parameters of Bias of each manufacturer. Figure 1-11 shows the return loss when the signal enters the Bias and the signal is output from the BIAS. When the output of the amplifier enters the Bias, the actual input power P transmission = Pin-P reflection, the greater the P reflection, the greater the S11 parameter, and the signal strength of the actual transmission into the biasThe smaller it is. Similarly, P transmission= Pout+P reflection, the greater the P reflection, the greater the S22 parameter, and the smaller the actual signal strength output from Bias. Therefore, the smaller the S11 and S22 parameters, the better.

Figure 1-11 Schematic diagram of S22 and S11
Figure 1-11 Schematic diagram of S22 and S11

The fifth step of purchasing Bias is to check the rise time/fall time and jitter of Bias from each manufacturer. Because the amplifier and Bias are in the same signal link, the rise and fall time and jitter will accumulate and superimpose. The formula is：

Bias's rise time/fall time and jitter formula
Bias's rise time/fall time and jitter formula

(Equation 3)

Tr1 is the rise time of Bias, Tr2 is the rise time of the amplifier, and the fall time and jitter are similar.

Therefore, we only need to pay attention to the rise and fall time and jitter values of BIas-tee. The smaller the value, the better the indicator.

1.3.2 Electro-optical modulator drive

Bias can be used for the DC bias port of the electro-optical modulator, which is a very mature application. Bias has become a necessary component of the electro-optical modulator. Bias plays a vital role in the electro-optical modulation process. As shown in Figure 1-13, a schematic diagram of BIAS driving an electro-optical modulator is shown. The RF terminal of BIAS is used as the data input source of the modulator, the DC terminal is used as the power supply input for MZM bias adjustment, and the RF&DC terminal inputs the signal to the modulator to complete the electro-optical modulation. It can be seen that the modulation of the electro-optical modulator cannot be completed without Bias.

Figure 1-13 Schematic diagram of Bias and electro-optical modulator
Figure 1-13 Schematic diagram of Bias and electro-optical modulator

The DC bias port voltage and electro-optical bandwidth of the electro-optical modulator are the key indicators of the electro-optical modulator, and the index that should be compared with it is the frequency range of Bias. For example, the electro-optical bandwidth of a certain electro-optical modulator is 0.15GHz to 10GHz. If the frequency range of BIAS is from 10MHz to 4.2GHz, then the frequency range between 4.2GHz and 10GHz will not be modulated normally.

1.3.3 Laser drive

Bias can be used to drive lasers and is a mature application. Lasers are extremely sensitive to overcurrent, overvoltage, and electrostatic interference. Therefore, stable working conditions and reliable data input are required. The current mainstream solution is to use Bias for laser driving. Figure 1-14 is a schematic diagram of BIAS and laser drive. The drive current required for the laser is injected through the DC terminal of the biaser, and the required communication data is injected through the RF port.

Figure 1-14 Schematic diagram of Bias and laser drive
Figure 1-14 Schematic diagram of Bias and laser drive

Lasers have the following key indicators: threshold current, operating current, operating voltage, and modulation frequency. When considering the modulation frequency of the laser, what needs to be paid attention to is the frequency range of Bias. Suppose the modulation frequency range of the laser is >100MHz, and the frequency range of BIAS is from 500MHz to 40GHz. Then the signal from the frequency of 100MHz to 500MHz will be sharply attenuated, resulting in this segment of the signal can not be input to the laser normally, can not achieve the purpose of modulation. It can be concluded that a bias with a frequency range greater than the modulation frequency of the laser should be selected.

When considering the threshold current and operating current of the laser diode, the Bias indicator that needs to be paid attention to is the maximum current value of the DC terminal. The threshold current of a laser diode is the minimum current value that a laser diode can generate a laser. When the current is lower than the threshold current, the laser cannot emit a laser. When the maximum current value of Bias is greater than the operating current of the laser diode, the laser diode can work normally.

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Zhongxing Lianhua Technology

  • Creation date: February 20, 2020
  • Revision date: July 14, 2024