Low Pass Filters


Low-pass filters allow signals from DC to a specified frequency to pass unimpeded with no attenuation while rejecting signals higher than this frequency. They are essentially the opposite of high-pass filters that allow higher frequencies to pass and those below the cutoff frequency to be rejected.
While in an ideal low-pass filter the cut-off would be dramatic slope, in practice it declines gradually from the transition region to the stopband region as shown in the figure. Specifically, the cutoff frequency is the point at which frequency drops by -3 dB or 70.7% from the passband. The transition region is the area where this falloff occurs. The simplest low-pass filters consist of a resistor and capacitor but more sophisticated low-pass filters combine series inductors and parallel capacitors.

Caption: The gain-magnitude frequency response of a first-order (one-pole) low-pass filter.

Key specifications that must be considered by specifying low-pass filters include:

  • Cut-off frequency (1 or 3 dB)
  • Band of operation
  • Rejection in the passband
  • Insertion loss at the cut-off frequency
  • Insertion loss in the passband
  • Rejection frequency
  • Attenuation at the rejection frequency
  • Ultimate rejection frequency
  • RF power handling ability
  • Type of connectors, package style
  • Size restrictions
  • Custom requirements

Low-pass filters, such as those from Anatech Electronics, can meet stringent environmental conditions such as DO-160, Mil-STD 810, and others needed for the aerospace and military applications. Connectors can also be changed in many cases and cutoff frequencies can be shifted to better match the desired cutoff frequency. Most designs our lumped-element (sometimes called Lumped Constant (LC) types, distributed elements, or both. In addition to many standard models custom designs can be quickly accommodated. Overall specifications for the company’s low-pass filters are shown in the table below.

  • Frequency range

    500 kHz to 15 GHz

  • 3 dB bandwidth

    0.3 to 100%

  • Number of sections

    2 to 16

  • Typical VSWR


  • Impedance

    50 or 75 ohms

  • Average input power

    50 W

  • Package styles

    Connectorized, PC mount, drop-in, surface mount

  • Operating temperature

    range-40 to +85° C

  • Shock

    20 G (11 ms)

  • Vibration

    10 G (5 to 200 Hz)

"We are what we repeatedly do.  Excellence then, is not an act, but a habit"

Will Durant

"We are what we repeatedly do.  Excellence then, is not an act, but a habit"

Will Durant

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