Table Of Content
Table Of Content
Characteristic impedance (Impedance) What is the characteristic impedance of the cable? The transmission line is affected by the structure of the conductor, and there is a resistance value of the high-frequency signal, the unit is Ohm. the
1. When any load impedance is connected to the output end of the transmission line loop at low frequencies, the ratio of the voltage at the input end to the current is called the loop input impedance (like Ohm’s law). When the reactance is very small (f is very small) at low frequencies, the main thing to look at is impedance, so the impedance measured at low frequencies is resistance.
2.The impedance is encountered when the signal (electromagnetic) wave follows a uniform transmission line without reflection. For a uniform transmission line, when the terminals of the line are matched, the voltage wave (V) and current wave at any point in the line The ratio of (I) is called characteristic impedance. When the frequency is high (when f is large), the impedance is very small, mainly because of the reactance (capacitive reactance and inductive reactance effect), so the reactance is the impedance at a high frequency.
This is like a network, and the network has a characteristic impedance, the formula is:
R: Resistance C: Capacitance L: Inductance G: Conductance (reciprocal of insulation resistance) ω=2πf (f=frequency) j: Imaginary number Zo: Characteristic impedance When the frequency is high, ω is very large (when approaching ∞) We can ignore R and G without timing and get:
Simply put, the characteristic impedance is the ratio of inductance to capacitance. the
3. The transmission line (coaxial line) is affected by the conductor structure, and there is a high-frequency signal resistance value, which can be regarded as a characteristic impedance. When transmitting signals, the characteristic impedance affects two important factors. z high power signal processing capability; z low signal transmission loss capability. According to the theory (experiment), it shows that the maximum power processing capability occurs at a characteristic impedance of about 30Ω, and the minimum signal attenuation capability occurs at a characteristic impedance of 77Ω, so the 50Ω coaxial cable is the best choice for both, so large Some high-frequency microwave systems choose a characteristic impedance of 50Ω. On the other hand, a characteristic impedance of 75Ω is used in a cable TV system because it needs to transmit long-distance analog (simulation) video signals.
Attenuation (Attenuation) Attenuation is the loss value of the signal strength after passing through a section of wire or cable (or connector). It is affected by the material and structure of the insulator (medium), and it also refers to how much the output power (Pout) or voltage (Vout) is lower than the incident power (Pin) or voltage (Vin ), and it is expressed in dB.
The main factors affecting attenuation are: z wire structure z conductor size z dielectric constant of insulator z other factors
Return Loss When the impedance of a certain point on the transmission line changes, this point is regarded as a discontinuous point, which will cause the reflection of the incident voltage or current wave, and the incident energy will be lost. expressed in dB. It refers to the ratio of the incident power to the reflected power and reflects the matching between the characteristic impedance of the coaxial line and the theoretical impedance.
For transmission lines, the larger the absolute value of the reflection loss, the better. In an ideal transmission line, the reflection loss is theoretically infinite. However, due to the influence of the wire structure and the acceptance sensitivity of the network analyzer, its value cannot reach infinity. Generally speaking, when the absolute value of RL is greater than 60dB, it can be considered an ideal impedance-matching situation
Three extreme cases of impedance matching: z Z0=ZL, input impedance and system impedance are perfectly matched, RL=infinity z Z0=0, open circuit, RL=0 z Z0=infinity, short circuit, RL=0
Standing Wave Ratio (SWR) The standing wave ratio refers to the ratio of the highest voltage to the minimum voltage of the synthetic wave of the reflected waveform and the incident waveform. It is also another way of expressing the reflective quality of coax transmission. As mentioned in the previous paragraph, the reflection is caused by the mismatch of the impedance of the coaxial line, and the reflected voltage will generate an electrical standing wave along the coaxial line, and a maximum voltage and a minimum voltage will be generated on the transmission line. The ratio of the two voltages is the standing wave ratio
Transmission delay (Delay) Transmission delay refers to the time required for a signal to pass through a fixed-length transmission line at a fixed frequency. The unit is usually ns/m. It is related to the insulating material. The formula is as follows
For example, the dielectric constant of FM-PE is about 1.7, so its delay Delay is about:
Time Delay Skew Time Delay Skew refers to the difference between the maximum value or the minimum value of the propagation delay of two or more transmission lines.
The velocity of Propagation is also called VP value or wavelength shortening rate. It refers to the ratio of the propagation speed of the signal in the medium to the propagation speed in a vacuum. It is determined by the dielectric coefficient Ke of the conductor and the insulating material, and the percentage (%) of the propagation speed and the speed of light in the transmission line is determined by the following formula:
For example, the dielectric constant of PE is 2.32, which means it has about 66% of the propagation speed; the dielectric constant of Teflon is 2.1, which means it has 69% of the propagation speed. That is, the propagation speed of PE is: speed of light╳66%=3╳108 m/s╳2/3=20cm/ns The smaller the dielectric constant, the faster the propagation speed and the larger the VP value. For example, RG6U uses physical foamed PE, and the VP value can reach about 82%, while the VP value of conventional chemical foamed PE can only reach about 77%.
Eight, crosstalk (Crosstalk) In an integrated communication cable, there are many telephone calls back. When talking on one loop, it can be heard on other adjacent loops. This phenomenon is called crosstalk. Crosstalk plays a role in interference, which reduces the clarity of the call. In severe cases, the call cannot be carried out. As the transmission frequency of communication cables increases, the problem of cable crosstalk becomes more and more serious. Therefore, the quality of communication in symmetrical cable communications depends not only on Loop attenuation but often depends more on the mutual influence of the loop. How to reduce or eliminate crosstalk is a key issue on high-frequency lines. Loop crosstalk is divided into z near-end crosstalk and z long-range crosstalk. The crosstalk received at the same end as the signal source of the main loop in the cross-connected loop is near-end crosstalk, and the crosstalk received at the other end is remote crosstalk.
