The high pass filter symbol in electrical schematics takes two primary forms: a block diagram (a rectangle containing a frequency response curve that attenuates low frequencies and passes high frequencies) standardized under IEC 60617 and IEEE 315, or a discrete component arrangement (a series capacitor followed by a shunt resistor or inductor). If you are reading a system-level block diagram, look for the rectangle with the rising curve; if you are reading a component-level schematic, look for the series capacitor.
Schematic Reference Table: High Pass Filter Symbols
The table below maps the most common high pass filter (HPF) symbols to their governing standards and practical meanings. Use this as a quick-reference when parsing unfamiliar schematics.
| Symbol Type | Standard / Origin | Visual Description | Practical Application & Meaning |
|---|---|---|---|
| Block Diagram (System Level) | IEC 60617-11 / IEEE 315 | Rectangle with an internal graph: x-axis is frequency, y-axis is amplitude. The curve starts low on the left (stopband) and flattens high on the right (passband). | Used in RF, DSP, and audio block diagrams to denote a filter stage without detailing the internal discrete components. Often includes a cutoff frequency ($f_c$) annotation. |
| Discrete Passive RC | IEEE 315 / Universal | A capacitor in series with the signal path, followed by a resistor connected in parallel (shunt) to ground. | The most common 1st-order analog HPF. The cutoff frequency is determined by $f_c = 1 / (2\pi RC)$. Used for AC-coupling and DC-blocking. |
| Discrete Passive LC | IEEE 315 / Universal | A capacitor in series with the signal path, followed by an inductor connected in parallel (shunt) to ground. | Used in high-frequency RF and power electronics where resistors would introduce unacceptable thermal noise or power loss. |
| Active Filter (Op-Amp) | IEEE 315 | An RC high pass network feeding into the non-inverting input of an operational amplifier triangle symbol. | Provides gain and a sharper roll-off (2nd-order or higher) without loading the previous stage. Common in precision audio and sensor signal conditioning. |
| Audio Crossover HPF | Industry Convention | A capacitor symbol with a diagonal slash through it, often accompanied by text like "-12dB/oct" or "2kHz". | Specific to loudspeaker crossover networks. The slash denotes a specific filter slope or order, directing high frequencies to a tweeter. |
Regional and Standard Variants: IEEE 315 vs. IEC 60617
While the physics of a high pass filter are universal, the way engineers draw them depends heavily on regional standards and the era of the schematic.
- North America (IEEE 315 / ANSI Y32.2): The IEEE 315 standard governs graphic symbols in the US. It heavily favors drawing out the discrete components (the series C and shunt R) even in higher-level diagrams, unless the filter is a monolithic IC or a sealed module. If a block symbol is used, IEEE permits a simple rectangle with the letters "HPF" inside, bypassing the need for the internal curve graph.
- Europe and International (IEC 60617): The IEC 60617 standard strictly enforces the use of the rectangular block symbol with the internal frequency response curve for system-level diagrams. Under IEC rules, drawing discrete components inside a functional block boundary is generally discouraged; the block is treated as a black box.
- Legacy UK / Australian Drafts: On older schematics (pre-1990s), you may encounter the legacy "horseshoe" capacitor symbol (a curved plate and a flat plate) used in series. While not explicitly an HPF symbol, veteran engineers used the curved plate to denote the side that should face the higher impedance or ground side in electrolytic AC-coupling applications, which inadvertently acted as a high pass filter.
Symbols and Rows People Get Wrong
Even experienced bench technicians misread specific filter symbols or misinterpret their intent. Here are the most common pitfalls corresponding to the rows in our reference table.
Row 1: The Block Diagram Curve Confusion
The most frequent error is confusing the High Pass block symbol with a Low Pass Filter (LPF) or Band Pass Filter (BPF). The fix: Look strictly at the left side of the internal graph (the low-frequency/DC region). If the curve starts at the bottom left and rises, it is a High Pass. If it starts high on the left and drops, it is a Low Pass. If it starts low, peaks in the middle, and drops, it is a Band Pass. Never assume based on the block's position in the signal chain alone.
Row 2: The "Coupling Capacitor" Trap
A single capacitor placed in series with a signal path to block DC is, mathematically, a 1st-order high pass filter. However, schematics rarely use the formal HPF block symbol for this unless the $-3\text{dB}$ cutoff frequency is a critical, calculated design parameter (e.g., rolling off subsonic rumble below 20Hz in a phono preamp). If you just see a series capacitor labeled C14 with no HPF designation, treat it as a DC-blocker first and a filter second. For a deep dive on how these passive networks interact, All About Circuits provides excellent foundational math on AC filter behavior.
Safe Interpretation When Markings are Faded or Missing
On older PCBs, the silkscreen HPF or F1 designator may be burned off, obscured by flux, or faded by UV exposure.
How to verify safely:
1. De-energize the board and discharge all filter capacitors.
2. Set your multimeter to continuity/diode mode.
3. Trace the signal path from the input. If the signal hits a capacitor in series before reaching the load, and there is a resistor (or inductor) connecting that same node to ground immediately after the capacitor, you are looking at a passive RC (or LC) high pass filter.
4. Measure the components. If $R = 10\text{k}\Omega$ and $C = 100\text{nF}$, you can calculate the hidden cutoff frequency: $f_c = 1 / (2 \times \pi \times 10000 \times 0.0000001) \approx 159\text{Hz}$.
Frequently Asked Questions
What does the high pass filter symbol look like on an audio crossover schematic?
In loudspeaker crossover schematics, the high pass filter is almost always represented by a capacitor in series with the tweeter. To denote the filter slope (which dictates how aggressively bass frequencies are blocked), a diagonal slash is drawn through the capacitor symbol. A single slash or the text "6dB/oct" indicates a 1st-order filter (one capacitor). Two slashes or "12dB/oct" indicates a 2nd-order filter (a capacitor in series followed by an inductor in parallel to ground before the tweeter).
Why does my schematic show a high pass filter block but no capacitor?
If you see an IEC 60617 HPF block symbol but the surrounding schematic shows no discrete capacitors or inductors, you are likely looking at a Digital Signal Processor (DSP) or a Switched-Capacitor IC (like the LTC1068 series). In DSP systems, the high pass filter is a mathematical algorithm applied in firmware, requiring no analog filtering components. In switched-capacitor ICs, the capacitors are integrated directly into the silicon die, so the schematic only shows the IC block, a clock input, and the signal I/O pins.
How do I identify a high pass filter on a PCB if the silkscreen symbol is faded?
Ignore the faded silkscreen and trace the copper. A hardware high pass filter requires two things: a series reactive component (capacitor) and a shunt component (resistor/inductor) to ground. Use a multimeter in continuity mode to find the series capacitor. Then, check the node immediately after the capacitor; if a resistor connects that node directly to the ground plane, you have found an RC high pass filter. If the silkscreen is completely missing, map the component values and calculate the $-3\text{dB}$ cutoff frequency to confirm its purpose in the circuit.
