Highpass Filter Block Symbol
Definition: The Highpass Filter Block symbol represents a two-terminal frequency-selective network that passes signals above its cutoff frequency (f_c) while attenuating signals below f_c, depicted in block diagrams and schematics as a rectangular block labelled 'HPF' with an input pin (In) and output pin (Out), consistent with functional block diagram conventions in IEC 60617 and IEEE/ANSI signal-processing notation.
Also known as: HPF, high-pass filter, high-frequency pass filter, low-frequency reject filter, treble filter, DC-blocking filter.
What the Highpass Filter Block symbol means
The Highpass Filter Block symbol denotes a functional circuit block that selectively passes high-frequency signal components while attenuating (reducing) low-frequency and DC components below the cutoff frequency. In schematics and system block diagrams the HPF block represents the complete filter network — which may be passive (RC, LC, crystal) or active (op-amp-based) — without exposing internal component details.
Highpass filters are essential in signal processing, audio electronics, communications, and measurement systems wherever DC offset removal, noise reduction at low frequencies, or frequency band separation is required. The symbol's clean block representation allows engineers to describe system-level signal flow without cluttering the diagram with individual R, C, and L component symbols.
How to identify the Highpass Filter Block symbol
The highpass filter block symbol is drawn as a rectangle labelled 'HPF' (or 'High Pass Filter') with a single input line entering from the left (labelled In) and a single output line leaving from the right (labelled Out). Some variants include a frequency-response curve inset showing the characteristic rising slope below f_c and flat passband above f_c. The block may also show the cutoff frequency value (e.g., 'HPF 1 kHz') as an annotation.
Function and usage boundary
A highpass filter attenuates signal frequencies below the cutoff frequency f_c at a rate of −20 dB/decade (first-order) or −40 dB/decade (second-order) per pole, while passing frequencies above f_c with minimal loss. The simplest passive HPF is a series capacitor and shunt resistor (RC highpass); active HPFs use op-amps to provide gain and improve roll-off steepness without loading the source. The filter's transfer function is H(s) = s·RC / (1 + s·RC) for a first-order RC design.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 does not define a unique glyph for a highpass filter block; functional blocks are represented as rectangles with descriptive labels per IEC 60617-02 (symbols for diagrams — Part 2: symbol elements, qualifying symbols and other symbols having general application). The label 'HPF' or 'f > f_c' identifies the function. |
|---|---|
| ANSI/IEEE 315 | ANSI/IEEE 315-1975 and IEEE 91-1984 use rectangular functional block symbols for signal-processing elements including filters. A highpass filter is represented as a labelled rectangle; the designator used in system block diagrams is typically FL (filter) with a type qualifier (HP). |
| Key difference | IEC and ANSI/IEEE both use identical unlabelled rectangles for generic functional blocks; the filter type is identified solely by the internal label (HPF / High Pass / HP). There is no visual glyph difference between the two standards for functional block diagrams. |
The displayed SVG is the live editor implementation, not a licensed reproduction or certification of an IEC or IEEE database glyph. Verify the exact official entry and project edition before issuing work.
Logical terminals
| Logical ID | Editor terminal name |
|---|---|
| in | In |
| out | Out |
Where the Highpass Filter Block symbol is used
- Audio amplifier coupling: series capacitor highpass filter (f_c ≈ 20 Hz) removes DC offset at amplifier input stages while passing the full audio band (20 Hz–20 kHz)
- Subwoofer/tweeter crossover networks: highpass filter routes frequencies above the crossover point (e.g., 80 Hz or 3 kHz) to the midrange or tweeter driver in a loudspeaker system
- ECG and biomedical signal conditioning: highpass filter (f_c ≈ 0.05 Hz) removes slow baseline wander caused by patient movement or respiration while preserving the cardiac waveform
- RF communications receiver front-end: highpass filter rejects out-of-band interference and image frequencies below the operating band in superheterodyne receivers
- Vibration and seismic analysis: highpass filter removes low-frequency DC drift and mechanical sway from accelerometer signals before FFT analysis
- Phase-locked loop (PLL) feedback path: highpass filter in the error path of a PLL improves transient response by emphasising high-frequency phase error components
Key facts
- The Highpass Filter Block symbol is a rectangle labelled 'HPF' with two terminals: In (input, left) and Out (output, right), representing any circuit implementation that passes frequencies above the cutoff frequency f_c.
- A first-order highpass filter attenuates signals below f_c at −20 dB per decade (−6 dB per octave); each additional pole adds another −20 dB/decade of roll-off.
- The cutoff frequency f_c of a first-order RC highpass filter is f_c = 1 / (2π·R·C), where R is in ohms and C is in farads.
- At the cutoff frequency f_c, a highpass filter's output is −3 dB (approximately 70.7%) of the passband output level — this is the standard definition of the −3 dB point.
- Highpass filters block DC (0 Hz) and low-frequency signals; a simple series capacitor is the most basic passive highpass filter used to remove DC offsets in amplifier coupling.
- Active highpass filters use op-amps to achieve steeper roll-off and gain without the impedance loading problems of passive designs; common active topologies include Sallen-Key and Multiple-Feedback (MFB).
- In IEC 60617 and ANSI/IEEE 315 block diagrams, the highpass filter is represented as a labelled rectangle; the designator FL (filter) with a type qualifier HP identifies it in system-level schematics.
- Highpass filters are the complement of lowpass filters: the two types are used together in bandpass and band-reject filter designs, or in loudspeaker crossover networks to divide the frequency spectrum between drivers.
Frequently asked questions
What does the highpass filter symbol mean in a schematic?
The highpass filter symbol represents a circuit block that allows signal frequencies above the cutoff frequency f_c to pass through while attenuating lower frequencies. It is shown as a rectangle labelled HPF with one input (In) and one output (Out), and may be implemented as a passive RC network or an active op-amp filter.
What does the highpass filter block look like?
The highpass filter block is a simple rectangle with the label 'HPF' or 'High Pass Filter' inside. A signal input line enters from the left and an output line exits from the right. Some diagrams include a frequency-response curve sketch inside the rectangle showing a rising slope at low frequencies and a flat passband at high frequencies.
What is the cutoff frequency of a highpass filter?
The cutoff frequency f_c is the frequency at which the filter output falls to −3 dB (70.7%) of the passband level. For a first-order RC highpass filter, f_c = 1 / (2π·R·C). Frequencies above f_c pass with minimal loss; frequencies below f_c are progressively attenuated.
What is the difference between a highpass filter and a lowpass filter?
A highpass filter passes frequencies above f_c and attenuates frequencies below f_c; a lowpass filter does the opposite, passing frequencies below f_c and attenuating those above it. The two filter types are complementary and are combined to form bandpass or band-reject filters.
What standard defines the highpass filter block symbol?
IEC 60617-02 defines general rectangular block symbols for functional elements including filters, with the filter type identified by a label. ANSI/IEEE 315-1975 uses the same rectangular block convention. There is no unique glyph for a highpass filter in either standard — the HPF label is the identifying element.
How steep is a highpass filter's roll-off?
A first-order (single-pole) highpass filter has a roll-off of −20 dB/decade (−6 dB/octave) below the cutoff frequency. A second-order filter has −40 dB/decade; a fourth-order filter has −80 dB/decade. Higher-order filters provide sharper frequency separation but require more complex circuits.
What are common uses of a highpass filter in electronics?
Highpass filters are used to remove DC offsets in audio amplifier coupling stages (f_c ≈ 20 Hz), separate high-frequency signals from low-frequency noise, route high-frequency audio to tweeter drivers in loudspeaker crossovers, remove baseline wander from ECG signals, and reject out-of-band interference in RF receivers.
Related symbols
- Bandpass Filter Block symbol
- Bandstop Filter Block symbol
- Differentiator symbol
- Ferrite Bead symbol
- Lowpass Filter Block symbol
- Pool Pump symbol
Physical breadboard, PCB, and panel mapping
No universal physical profile is published for this symbol yet. Create a versioned custom part from the exact manufacturer dimensions and terminal map before fabrication.
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