Ceramic capacitors are the major contributors to circuit performance that affect the stability of the voltage regulator control loop and the impedance of the power supply network (PDN). When designing with this type of circuit, you have to operate over a wide frequency range Provide good impedance data.The right measurement technology is the key to achieve accurate measurement.This article discusses how to effectively use two simple techniques to achieve accurate and broadband frequency measurement by expanding the range of Vector Network Analyzer (VNA).
Ceramic capacitors are difficult to measure because of their high impedance dynamic range, while larger tantalum and aluminum electrolytic capacitors have a smaller dynamic range and are easier to measure.
To measure a 100nF ceramic capacitor, the capacitive reactance (capacitive reactance) is about 1.6MΩ at 1Hz. The equivalent series resistance (ESR) is typically about 10mΩ at series resonance. Precisely over a wide frequency range When measuring this capacitance, the dynamic range (the ratio of the capacitive impedance at the lowest frequency to the ESR at low impedance) must be at least 164dB.
Some engineers may think that measuring capacitance as low as 1Hz is too small because even the Keysight Technologies E5061B network analyzer can only measure capacitance impedances down to 5Hz.The techniques discussed in this article can be used to measure Scaling up to 1Hz and dynamic range up to 164dB. Starting from 10Hz reduces the dynamic range to 144dB; however, adding 10dB of noise allowance to each impedance limit will get you back to the required 164dB range right away.
Choose the correct instrument
Few instruments, except for specialized impedance analyzers, support such a large dynamic range. If measuring these components is your primary job and you have no budget constraints, using a dedicated high-performance impedance analyzer may be a A good choice; otherwise this may not be the case, so a vector network analyzer is the next best option. Vector network analyzers measure impedance and display capacitance, inductance, real, imaginary, and magnitude.
Vector Network Analyzer is inherently able to measure impedance using three different techniques, all based on scattering parameters (S-parameters) .Table 1 lists the three methods and the transformation from S-parameters to impedances Fortunately, these impedance transformations are typically built into vector network analyzers and do not require any calculations.
The three measurement techniques are accurate over the specified impedance range, and the recommended ranges for each measurement technique are shown in Table 2.
These are the recommended ranges and most vectorial network analyzers can do better as long as careful instrument calibration is performed prior to measurement.
One of the most common decoupling values is 0.1μF, which is why I chose a 0.1μF low-ESR ceramic capacitor for the purpose of this article This article uses the OMICRON Lab Bode 100 as an example for the capacitive samples of the above three technologies The measurement results are shown in Figure 2. The measurement results are exported directly to the Touchstone format from the Bode 100 Vector Network Analyzer and then simultaneously displayed on the Keysight ADS Data Display. As you can see, the measurement techniques shown here can be extended Bode 100 dynamic range.
All three methods perform equally well over the frequency range from approximately 100 Hz to 300 kHz, and their measurement performance is much better than the recommended range. Deviations also occur at frequencies less than 100 Hz and greater than 1 MHz. (Shunt Thru) is very accurate at low impedance levels, so it measures 10mΩ impedance at resonance. The Series Thru is very accurate in measuring impedance and can therefore be operated at 1Hz The correct impedance is provided, which is 1.6 MΩ in this example. The single-port reflection measurement is less accurate in both low and high impedance ranges.
Two available measurement techniques
You can use two techniques to extend the dynamic range to accurately measure capacitance from 1Hz to 50MHz One way is to use an impedance adapter that is commonly used as a component of a vector network analyzer Impedance adapter uses a resistor bridge Extended Dynamic Range The impedance adapter connected to the Vector Network Analyzer in Figure 2 supports the recommended range of 20mΩ at 1Hz and the recommended minimum impedance measurement is about 6Ω at the 20MHz resonant frequency.The maximum recommended at low frequencies The impedance is 600kΩ.
The second method is to insert a resistor between each port of the vector network analyzer Figure 3 shows the components measured in the 'TEE' configuration Select the series resistance to modify the impedance range of this method.
The connection of series resistor and shunt capacitor is shown in Figure 4.
Capacitance measurements are made using both impedance adapters and two-port shunt and series capacitors. The measurements are exported directly to the Touchstone format from the Bode 100 Vector Network Analyzer and then displayed on the Keysight ADS Data Display as shown in Figure 5 Show.
Both of these methods are capable of accommodating the entire 164dB dynamic range from 1.6MΩ at 1Hz to 10mΩ at the resonance point.The two-port with series resistor allows for a more accurate evaluation at 10mΩ resonance, due in part to the direct soldering of capacitors to the printing Circuit board, eliminating the effects of changes in the contact resistance between the impedance adapter and the measurement adapter. Either way, the performance of the impedance adapter is well beyond the specified 6Ω performance at 20MHz, and both methods provide very good High dynamic range.
Comparison of two methods
Both of these methods have their advantages and disadvantages. Impedance adapters are easy to use and do not have to be soldered onto a printed circuit board. In addition, they have a high minimum impedance, require a short open circuit and load calibration, and can not be adjusted By choosing a series resistor, you can optimize for a specific range of two-port parallel methods with series resistance, a method that typically requires the component to be mounted on a circuit board for measurement.
to sum up
This article describes three kinds of native vector network analyzer impedance measurement methods: single-port emission method, dual-port series and straight-through method and two-port parallel straight-through method, and three methods were supported dynamic range and impedance range. There are two A simple vector network analyzer method can be used to accurately measure the 164dB dynamic range of a 100nF ceramic capacitor.