Coupling circuits that are 50 ohm to 50 ohm with Low Q, high ESR MLCCs’ will have high coupling losses. Even with the best SMD MLCC coupling cap you can lose 0.5 dB per stage especially at frequencies above 1 GHz. With typical off-the-shelf MLCC this can be several dB’s per stage many dB’s per device. In a portable device this could mean higher battery consumption during transmit and poor receiver sensitivity causing dropped calls. 

The capacitance, “C” value of a coupling capacitor, is often a trade-off of its SRF/PRF. A large value “C” would be best with an impedance approaching 1 ohm for minimal insertion loss (S21) for a 50 ohm circuit. The SRF/PRF resonance often limits the “C” to a much lower value. The best value is typically closer to a few ohms impedance at UHF/Microwave frequencies. This can be realized with an MLCC designed especially for very Low ESR, High Q and High SRF/PRF as the AEC UHF Series MLCCs’.

Band Pass Filters with Low Q, high ESR MLCCs’ causes them to exceed their bandwidth, have high insertion loss and unacceptable stop-band performance. Temperature compensation is also important requiring an NPO level of TC in most UHF/Microwave circuits. 

Another consideration is the SRF and PRF of the MLCC if the resonance points are at or near the filter’s operation frequency. SRF/PRF in band resonance will cause wide swings in amplitude and phase. This would distort the complex QPSK/QAM modulation commonly used in RF digital circuits. The distortion effect maybe only at some channel frequencies but not on all channels making the failure mode difficult to test and isolate. Only MLCCs’ designed for very high SRF/PRF should be used.

In a VCO circuit Low Q, high ESR MLCCs’ can cause the varicap controlled tuning to not have the correct total tuning range and frequency versus voltage curve. Low performance MLCCs’ often will cause severe output level roll-off over the frequency range. Some product may fail to lock over their full tuning range due to the MLCC’s in band resonance (SRF/PRF). All parts used in the RF circuitry of a VCO must be temperature stable usually with an NPO TC.

Digital Receiver demodulators often use a quad detector circuit. A high Q Tank will yield a high level output, low Q, low level output, possibly below the digital detector level. A high Q, low ESR MLCC with TCC = NPO is always specified for this circuit to operate correctly.

Digital circuits need effective bypass capacitors. The key for an effective bypass capacitor is its impedance (Z) ratio to the source impedance of the circuit to be bypassed. If the capacitor is attempting to bypass digital noise on a DC power rail, with an impedance of 1 ohm, the capacitor best have an impedance of a small fraction of 1 ohm. 

Ideally this ratio should be at least 20:1 or more (>26 dBi) at the lowest clock frequency of the circuitry bypassed. Therefore the bypass impedance should be 50 milliohms (or lower). Common off the shelf MLCCs’ often are several Ohms ESR making them ineffective for decoupling. 

The UHF Series AEC MLCC are below 50 Milliohms at 100 MHz and below 100 Milliohms at 1 GHz!

At Self Resonant Frequency on Bypass Application the capacitor's net reactance is zero (X_C-X_L = 0) and the net impedance is equal to ESR. At this point, the lowest impedance path is required for optimal bypassing. Therefore, using a low ESR capacitor could be very critical. aec's Ultra High Q, Ultra Low ESR capacitors would be perfect for such application.