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PARSTAT 2273 Hardware User’s Manual
WARNING To reduce the risk of potentially dangerous electrical shock, only a qualified
service technician should perform this work, and then only with the instrument
disconnected from all sources of power.
NOTE Keep the filter capacitor leads short! Long leads establish sizable ground loops and
could additionally act as radiating antennae.
At frequencies above 10 MHz, these measures might not suffice to prevent radiation from being a
problem, particularly at VHF frequencies. Additional measures will then be required. Shielding is
generally effective. A suitable shield can be constructed using metal foil, wire screening, or similar
materials.
Once the instrument is completely surrounded by the shield (taking care not to unduly restrict
ventilation), the only additional requirement is to install low-pass filters where lines pass through
the shield (all openings through the shield should be as small as possible). A capacitor between a
line and the shield can function as a suitable low-pass filter. The leads of the capacitor should be
as short as possible. We suggest using coaxial feedthrough capacitors.
In the case of a signal lead, it is essential that the capacitor’s value be such as to attenuate the
interference frequencies without unduly attenuating critical frequency components of the signal
itself.
Coaxial cables are a special case in that the cable shield acts as an extension of the enclosure
shield. This being the case, the filter can be counted in a shielded box fitted with coaxial
connectors without undue concern for keeping the box extremely close to the enclosure. If more
convenient to do so, it can be located at some distance from the enclosure as long as the integrity
of the coaxial shield is maintained.
The preceding techniques are extraordinary measures that should be required only in unusual
cases. If they are applied with care, RF interference should be reduced to an acceptably low level
in all but the most critical applications. Contact the Customer Service Department for advice in the
case of a problem that does not yield to these measures.
2.4.3. Transient Sensitivity
Princeton Applied Research instruments are designed and constructed to ensure normal
operation in the presence of moderate transient levels. Although these provisions are sufficient for
operation in most places where this equipment is used, it is certainly possible for transient levels
in particular environments to be so severe that they make reliable operation uncertain. There are
three general types of high-level transients:
1.
Static Discharge — Transients from this source generally affect input or output circuits. Input
circuits that include MOS field-effect transistors to achieve a high input impedance are
particularly susceptible to damage from this source. Damage typically occurs when the charge
built up on a user’s body discharges into an input or output connector as a connection is being
made. Among the factors determining the tendency for charges to build are the kind of
clothing fabrics worn, shoe materials, and the materials in the floor or floor covering.
2.
High-Level Transients Generated Internal to the Place of Use — Such transients almost
always enter the instrument via the line cord. Possible sources include heavy-duty electric
motors, RF equipment, lasers, diathermy machines, arc welders, spark chambers, and
others.
3.
Lightning — Transients caused by lightning almost always enter the instrument via the line
cord.