An RL filter high pass is a passive two-component circuit that allows high-frequency AC signals to pass through to the output while attenuating lower frequencies, achieved by taking the output voltage across the inductor in a series resistor-inductor network. When you drop this network into a real circuit or PCB trace, it fundamentally changes the impedance profile and introduces a frequency-dependent phase shift, forcing low-frequency noise to dissipate across the resistor while high-frequency signals develop across the inductor. Builders commonly confuse this topology with the RC high pass filter (which uses a capacitor and blocks DC entirely) or accidentally wire an RL low pass filter by mistakenly taking the output across the resistor instead of the inductor.
The Core Mechanism: Inductive Reactance and the Flywheel Effect
To understand why this filter passes high frequencies, you have to look at inductive reactance ($X_L$). The opposition an inductor presents to alternating current is calculated as:
$X_L = 2 \pi f L$
Where f is frequency in Hertz and L is inductance in Henrys. As frequency increases, the inductor's reactance increases. In a series RL circuit acting as a voltage divider, the output is taken across the inductor. At low frequencies, $X_L$ is tiny compared to the resistor ($R$), so almost all the input voltage drops across the resistor, leaving near-zero voltage at the output. At high frequencies, $X_L$ dominates, and the inductor hogs the voltage, passing the signal to the output.
Worked Numeric Example: Designing a 1.59 kHz Crossover
Let’s design a filter on the bench using standard, easily sourced components. We will use a 100 Ω carbon film resistor and a 10 mH radial drum-core inductor.
Step 1: Calculate the Cutoff Frequency ($f_c$)
The -3dB cutoff frequency for an RL filter is defined by the point where the resistance equals the inductive reactance ($R = X_L$). The formula is:
$f_c = \frac{R}{2 \pi L}$
Plugging in our values:
$f_c = \frac{100}{2 \times 3.14159 \times 0.010}$
$f_c = \frac{100}{0.06283}$
$f_c \approx 1591.5 \text{ Hz}$
Step 2: Verify Attenuation at a Lower Frequency
What happens if we feed a 500 Hz signal into this network? First, find $X_L$ at 500 Hz:
$X_L = 2 \times 3.14159 \times 500 \times 0.010 = 31.42 \text{ Ω}$
Now, use the AC voltage divider formula to find the output magnitude:
$V_{out} = V_{in} \times \frac{X_L}{\sqrt{R^2 + X_L^2}}$
$V_{out} = V_{in} \times \frac{31.42}{\sqrt{100^2 + 31.42^2}}$
$V_{out} = V_{in} \times \frac{31.42}{104.8} \approx 0.299 \times V_{in}$
At 500 Hz, the output is roughly 30% of the input voltage, which equates to an attenuation of about -10.5 dB. The low-frequency signal is successfully being choked off.
Where You Meet This in Practice (and Where You Don't)
You won't often see simple RL high pass filters in basic audio crossovers. Inductors large enough to handle audio power (like 10 mH) are physically bulky, expensive, and suffer from high DC resistance (DCR) which wastes amplifier power as heat. Instead, audio engineers use RC or LC topologies.
However, the RL high pass topology is a staple in RF and high-speed digital design. A common application is separating high-frequency switching noise from a lower-frequency control signal on a shared trace, or acting as a phase-shift network in oscillator circuits. It is also heavily utilized in sensor interfaces where a slow-moving DC bias needs to be bled off through the resistor to ground, while the fast AC sensor signal is routed through the inductor to the amplifier.
The Silent Killer: Self-Resonant Frequency (SRF)
The biggest trap for hobbyists and junior engineers is assuming an inductor behaves ideally at all frequencies. Real inductors have parasitic parallel capacitance between their wire windings. This creates a parallel LC tank circuit. The frequency at which this parasitic capacitance resonates with the intended inductance is called the Self-Resonant Frequency (SRF).
According to the Coilcraft Tech Center, once your operating frequency exceeds the SRF, the component stops acting like an inductor and starts acting like a capacitor. If you use a standard 10 mH drum-core inductor, its SRF might be as low as 2 MHz. If you try to use it as an RL high pass filter for a 50 MHz RF signal, the inductor's impedance will actually drop as frequency increases past 2 MHz, completely destroying your filter's high-pass action and potentially passing unwanted VHF noise. Always check the manufacturer's SRF spec sheet and select RF-grade chip inductors (like the Coilcraft 0402HP series) for anything above a few megahertz.
RL Filter High Pass vs. RC High Pass Selection Matrix
When deciding between an inductor-based (RL) and capacitor-based (RC) high pass filter, the choice usually comes down to DC handling and physical constraints. Here is how they stack up on the bench:
| Criterion | RL High Pass (Output across L) | RC High Pass (Output across C) |
|---|---|---|
| DC Blocking | Fails. Inductors pass DC freely (0 Hz = 0 Ω reactance). | Excellent. Capacitors block DC entirely. |
| High-Frequency Limit | Limited by SRF (parasitic capacitance ruins performance at VHF/UHF). | Extremely high. Ceramic caps maintain characteristics well into GHz ranges. |
| Component Size (Low Freq) | Massive. Requires large, heavy iron/ferrite cores for sub-1kHz operation. | Compact. Electrolytic or film capacitors handle low frequencies easily. |
| Current Handling | High. Can handle significant DC bias current without saturation (if sized right). | Low. Capacitors do not pass DC current; AC current limited by ESR and ripple ratings. |
Frequently Asked Questions
Why is my RL filter high pass attenuating ultra-high frequencies?
If your filter works perfectly at 1 MHz but starts attenuating again at 50 MHz, you have hit the inductor's Self-Resonant Frequency (SRF). Past the SRF, the parasitic winding capacitance dominates, and the inductor effectively becomes a capacitor. In a high-pass topology, a capacitor in the output leg will form an unintended low-pass filter with the source impedance, rolling off your ultra-high frequencies. To fix this, swap the component for an air-core or RF-grade ceramic chip inductor with an SRF well above your target frequency band. You can verify this on the bench by sweeping the component with a NanoVNA and looking for the impedance peak.
Can I use an RL filter high pass for DC blocking between amplifier stages?
No. This is one of the most common mistakes when transitioning from RC to RL designs. Because inductive reactance is zero at 0 Hz (DC), an inductor acts as a short circuit to DC. If you place an RL high pass filter between two amplifier stages with different DC bias voltages, the inductor will pass the DC directly, likely destroying the input stage of the second amplifier. For DC blocking (AC coupling), you must use a capacitor-based (RC or LC) high pass filter.
What happens if I swap the resistor and inductor positions?
Physically swapping the components in a series circuit doesn't change the current flow, but it changes where you measure the output. If you keep the output probes across the inductor, it remains a high pass filter. However, if you move the output probes across the resistor, you have built an RL low pass filter. At high frequencies, the inductor's massive reactance chokes the signal, leaving almost no voltage to drop across the resistor. At low frequencies, the inductor acts as a short, passing the signal directly to the resistor. As noted in Electronics Tutorials, the component you measure across dictates the passband: measure across the L for high pass, measure across the R for low pass.
