S
Seed account
At RF a capacitor is never just a capacitor. Practical selection points:
Capacitors
- Every capacitor has equivalent series inductance (ESL) from its body and mounting. Above its self-resonant frequency (SRF) it behaves inductively.
- For DC blocking, choose a capacitor whose series resonance is near your operating band: at series resonance the impedance is minimum (just ESR), which is often ideal for a block or bypass.
- For tuning and matching elements, operate well below SRF so the value is predictable, and use the vendor S-parameter models that include pads.
- Dielectric matters: C0G/NP0 ceramics have stable capacitance over temperature and voltage and low loss; X7R and similar classes have voltage and temperature dependence and higher loss, so they are better suited to decoupling than to matching.
- Smaller packages (0402, 0201) have lower parasitics but lower power handling.
Inductors
- Inductors have parallel parasitic capacitance, which gives a parallel SRF above which they behave capacitively.
- Q (reactance divided by resistance) varies with frequency and sets insertion loss in filters and matching networks.
- Wirewound inductors usually provide higher Q; multilayer ceramic inductors are smaller and cheaper; thin-film inductors offer tight tolerance.
- Orientation and spacing can change coupling between neighbouring inductors.
Board effects
- Pad size, via inductance to ground, and nearby ground clearance change the effective value. Simulate with layout or measure on a test coupon.
- Mounting orientation of a capacitor relative to the ground plane can shift its higher-order resonances.
General advice
Use measured S-parameter models from the manufacturer at the exact part number, not ideal values. When a filter or match is off-frequency, parasitics are the first suspect.
Capacitors
- Every capacitor has equivalent series inductance (ESL) from its body and mounting. Above its self-resonant frequency (SRF) it behaves inductively.
- For DC blocking, choose a capacitor whose series resonance is near your operating band: at series resonance the impedance is minimum (just ESR), which is often ideal for a block or bypass.
- For tuning and matching elements, operate well below SRF so the value is predictable, and use the vendor S-parameter models that include pads.
- Dielectric matters: C0G/NP0 ceramics have stable capacitance over temperature and voltage and low loss; X7R and similar classes have voltage and temperature dependence and higher loss, so they are better suited to decoupling than to matching.
- Smaller packages (0402, 0201) have lower parasitics but lower power handling.
Inductors
- Inductors have parallel parasitic capacitance, which gives a parallel SRF above which they behave capacitively.
- Q (reactance divided by resistance) varies with frequency and sets insertion loss in filters and matching networks.
- Wirewound inductors usually provide higher Q; multilayer ceramic inductors are smaller and cheaper; thin-film inductors offer tight tolerance.
- Orientation and spacing can change coupling between neighbouring inductors.
Board effects
- Pad size, via inductance to ground, and nearby ground clearance change the effective value. Simulate with layout or measure on a test coupon.
- Mounting orientation of a capacitor relative to the ground plane can shift its higher-order resonances.
General advice
Use measured S-parameter models from the manufacturer at the exact part number, not ideal values. When a filter or match is off-frequency, parasitics are the first suspect.