The basic design rules for the power distribution network (PDN) tell us that the best performance source is unqualified, and the frequency-independent (or flat) impedance curve. This is a reason which is very important for power supply, because the power of stability can cause impedance peak, and then deteriorate flat impedance curve, and the performance of the power circuit.
Since there is no impedance path is completely flat, we need to do some design adjustments. This article is designed to help you make some compromise on system performance.
Source impedance should match the transmission line impedance.
In general, this is the basic premise of S parameter measurement and all RF equipment. Source impedance (most common is 50 Ω) to connect to impedance and source matching coaxial cable, and the load is also connected to the same impedance. This approach achieves perfect flat impedance, whether it is from the source to see the source from the load, it is consistent.
The output impedance of the regulator can be considered a source, while the PCB layer can be regarded as a transmission line. The rear end decoupling capacitor is load.
Basic principle of transmission line
When the frequency is lower than the transmission line resonant frequency, the transmission line feature impedance can be defined by inductance and capacitance item. The capacitor can be measured when the remote end of the transmission line is not terminated. Inductors can measure when the transmission line is shorted. The feature impedance of the transmission line depends on these two measurement results, namely:
The frequency of inductors and capacitance intersections is characteristic impedance, equal to:
The correct matching transmission line exhibits a complete flat impedance curve, which is equal to the feature impedance. The transmission line that is incorrect is presented as a capacitance or inductance property, and many resonance and anti-resonant frequencies are generated in the multiple of the transmission line resonant frequency. If the transmission line is a capacitive property, the resistance is first happened. If the transmission line is inductive properties, harmonic first occurs. In both cases, the frequency of first resonance or resonance is:
Figure 1 shows these relationships with a 50 Ω coaxial cable simulation. The endless terminal impedance is measured in the case where the cable end is open, short and matching.
Figure 1: Transmission line proximal impedance open circuit (blue), short circuit (red) and correct match (green), another interesting relationship.
In the case where the transmission lines and the source do not match, there are two possible solutions, depending on that the end resistance is greater than the feature impedance. If the end-connected resistance is less than the characteristic impedance of the transmission line, the anti-harmonic peak exceeds the end resistance. These impedance peaks are defined as:
The resonant minimum is equal to the end resistance.
If the end resistance is greater than the feature impedance of the transmission line, the resonance peak is equal to the end resistance. The minimum of the anti-resonant is defined as:
These relationships can be displayed using the front end resistance, respectively, the simulation model of 24.9Ω and 210Ω, respectively, in Figure 2 is matched.
Figure 2: Transmission line is not terminated at terminal impedance 24.9Ω (blue), 210Ω (red) and correct match (green).
These relationships were confirmed in the 50Ω coaxial cable measurement of Figure 3 for 50 Ω coaxial cable measurements.
Figure 3: Measurement results of the 50Ω coaxial cable for end 210 Ω (red) and 24.9Ω (blue).
These concepts are extended to the actual double-sided printed circuit board, and there is a SMA connector in this PCB of 4.5 "x 6.3", there is a SMA connector, as shown in FIG.
Figure 4: Using an area of 4.5 "x6.3", a double-sided copper foil plate with a SMA connector measures the open circuit (green) and short circuit (orange) impedance of the PCB. This impedance also was measured with a 2.7 Ω (blue) and 10Ω (red) end resistance opposite the SMA connector. The resistance is connected to the PCB with a very short brace in order to minimize interconnect inductance.
We can use the oscilloscope measurements in Figure 4 to approximate the characteristic impedance of the PCB. The capacitor is estimated using a tag M3.
The capacitor is estimated to estimate the point of 70 MHz and 10 dBΩ.
The characteristic impedance can be calculated using (1).
In addition, feature impedance can be regarded as an intersection of open-impedance and short-circuit impedance, which occurs at approximately 11.5 dBΩ or 3.76 Ω.
The characteristic impedance of the PCB can also be calculated using an approximate peak impedance (14.5 dBΩ) with 2.7Ω end resistance.
Revert to calculate ZO.
You can use (3) to calculate the first resonant frequency or the resonance frequency, ie:
The measurement is repeated with a 3.6 Ω terminating resistor, as shown in Figure 5.
Figure 5: Substance of the same piece PCB is measured (red) with 3.6 Ω instead of 2.7Ω end resistance. Note that only a small amount of peak indicates a slightly greater than 3.6 Ω after a small amount of peak indicates a slightly greater than 3.6 Ω.
The PCB is simulated and compared to Fig. 5, as shown in Figure 6.
Figure 6: The PCB simulation result is compared to the measurement results shown in Figure 5.
Finally, use the 0.6Ω and 3.6Ω source impedance of the power supply and dynamically instantaneous response in the PCB resonant frequency point simulation. The simulation model is shown in Figure 7, and the simulation results are shown in Figure 8.
Figure 7: Dynamic load instantaneous ADS simulation by 0.6 Ω and 3.6Ω source impedance represents the voltage regulator output impedance.
Figure 8: Instantaneous response simulation results show that the transient response of 0.6Ω lower source resistance (red) is much larger voltage offset than the matching 3.6Ω source resistance (blue).
Power supply
PDN
Our other product:
