Quantcast Bandstop Filters

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MIL-HDBK-232A  Bandstop Filters.  Bandstop filters operate opposite to bandpass
filters in that a high impedance is present over the specified band of
frequencies, while a low impedance is offered above and below that band.
Bandstop filters may be used to block selected frequencies for EMI.  Filter parameters.  Regardless of the type of passive, filter used,
the facility designer must consider certain characteristics in selecting
filters.  The ideal situation would be to design and procure filters for each
application.  However, time and cost normally dictate the use of general
purpose, readily available filters.  Parameters to consider are: insertion
loss, voltage drop, overcurrent rating, and maximum temperature rise at rated
current.  For power lines and VF signal lines, the following values are
Insertion loss
Not less than 100 dB, 14 kHz to I GHz
Voltage drop
Not greater than 1% at full load
Over current
140% for 15 minutes
Temperature rise
Not greater than 25 deg. C above ambient at 100 A
(proportionally lower for less current)
Not greater than 0.5 mA between the case and ungrounded
For highpass, bandpass, and bandstop filters the designer should select
filters with an insertion loss of 100 dB for unwanted frequencies.  All other
parameters apply as stated.  Filter installation.  Filtering at the point of egress is
accomplished in the EM vault (if the facility is so equipped), or on the
facility entrance plate.  The filters are circumferentially bonded to the
plate.  Within the facility, filter cases for signal lines are bonded
directly to the equipotential ground plane.  Power-line filters are bonded to
the equipment chassis and the FPSS.  Neutral filtering.  The need to filter a neutral conductor in a
facility may be dependent upon the adequacy of its ground system and the
physical plant of that facility.  A decision to filter the neutral conductor
should be delayed until the facility is installed in order to evaluate the
ground system.  Measurements of neutral voltage to ground should be made at
different points between the source and the load.  If a difference of
potential of less than 1 volt is consistently found, then neutral filtering
may not be indicated.  If a greater potential exists and cannot be corrected,
then neutral filtering may be indicated.  Strict compliance with the criteria
in MIL-STD-188-124 and the guidance in MIL-HDBK-419 should eliminate the
neutral filter requirements.  This handbook assumes such compliance and
assumes dedicated source power as described in paragraph (i.e.,
that the power service transformer and first service disconnect are totally
within the CS and that appropriate physical security has been applied to the
CS).  Small facilities that are installed in existing structures (e.g., wire
rooms in embassies, consulates, etc.) cannot be expected to meet the above
assumptions.  In these cases, neutral filtering is indicated.  When neutrals
of polyphase systems are filtered, the neutral filter must be capable of
passing the third harmonic of the combined frequencies of the phases (e.g.,
450 Hz for 3-ph 50 Hz, 540 Hz for 3-ph 60 Hz).


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