RF/Microwave MLCC Application in Coupling
The role of capacitors in coupling and DC blocking circuits is to transfer the RF energy from one part of the circuit to another, and the capacitors are connected in series. In theory, all capacitors can block DC. However, in order to achieve the most efficient RF energy transfer, appropriate coupling capacitors must be selected.
It is connected between the signal source and the signal processor circuit or the two-stage amplifier to cut off the DC power and allow the AC or pulsating signal to pass through, so that the DC operating points of the adjacent amplifiers do not affect each other.
The coupling capacitor is a capacitor used for coupling. The function of the coupling capacitor is to add the front-stage signal to the post-stage circuit as much as possible without loss, and at the same time remove the needless DC signal.
RF microwave capacitors are widely used between two-stage amplifiers, so they are also called interstage coupling capacitors.
Coupling locations in RF microwave circuits require capacitors to have minimal impedance in the carrier frequency range. The capacitor has the smallest impedance at the self-series resonant frequency, Z=ESR.
RF Interstage Coupling Circuit
RF interstage coupling capacitors need to focus on the following parameters:
- Select the capacitor according to the transmission signal frequency, so that the output signal frequency is within the the capacitor's FSR range ± 100MHz, the closer to the FSR is the better, and please refer to the corresponding FSR value from the FSR curve in the manual.
- According to the voltage requirements where the coupling capacitor is used in the circuit, select capacitors with suitable size and corresponding series.
For instance, DLC70A470GW151XT and DLC70B200GW501XT, the FSR is about 2GHz and the rated voltages are 150V and 500V.
- Within the transmission signal frequency range, the smaller the capacitor’s ESR is the better, and generally is less than 0.1 ohm.
- The frequency of the transmission signal should be smaller than capacitor’s FPR.
Coupling Application Analysis
With 1.03GHz to 1.15GHz using frequency, customer uses DLC70B100pF capacitors for coupling and the output power drops by 30%.
Use E4981A impedance analyzer to make impedance analysis for DLC70B100pF capacitors and the impedance characteristics are as below:
The ESR in Fab frequency band is smaller than 0.1 ohm and can be used for coupling.
The ESR rises sharply in Fcd frequency band and is not suitable for coupling.
From DLC70B100pF capacitor S21 curve, we can see the following:
1. When FPR (M1) equals to 1.15GHz, the insertion loss becomes significantly larger.
2. When the capacitor’s using frequency M2 is below 1.10GHz, S21 is lower than 0.1dB.
3. The frequency used by customer is from 1.03GHz to 1.15GHz, which is just between M1 and M2, and the insertion loss gradually increases, which causes the power output to drop rapidly as the frequency increases.
In conclusion, the FPR of the capacitor must be firstly considered in selecting the interstage coupling capacitor.