RF Choke
Understanding RF Chokes
RF chokes are a critical but often overlooked component in RF circuit design. Every amplifier, oscillator, and mixer requires DC bias, and the RF choke is the element that separates the DC path from the RF path.
RFC Design Considerations
- Impedance: Must be high enough at the RF frequency (> 5-10x Z0 = 250-500 ohms minimum).
- Self-resonant frequency (SRF): RFC impedance peaks at SRF and drops above it. Operating frequency must be below SRF.
- DC current rating: Must handle the bias current without core saturation.
- Parasitic capacitance: Winding-to-winding capacitance limits high-frequency performance.
RFC Types
- Wire-wound: Traditional coil on a core. Good for low frequencies (< 1 GHz).
- Chip inductor: Surface-mount. Standard for PCB designs. SRFs to 10+ GHz.
- Conical: Wideband RFC with graduated winding pitch for broadband impedance.
- Quarter-wave line: Microstrip RFC using a quarter-wave short-circuit stub. Very high Q at one frequency.
Frequently Asked Questions
What is an RF choke?
An RFC is an inductor that passes DC while blocking RF. It delivers bias voltage to active devices without disturbing the RF signal path. Must have > 500 ohms impedance at the RF frequency while carrying the required DC current.
How do you choose an RFC value?
The impedance at the operating frequency must be much higher than Z0 (> 5x). Z_L = 2*pi*f*L. For 50-ohm system at 1 GHz: L > 50/(2*pi*1e9) x 5 = 40 nH minimum. Also check SRF > operating frequency, and DC current rating.
What is SRF and why does it matter?
SRF (Self-Resonant Frequency) is where the inductor's parasitic capacitance resonates with its inductance. Above SRF, the inductor acts as a capacitor (decreasing impedance). The RFC must operate below its SRF for proper blocking.