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Eliminate Interference from the Ether
To eliminate broadcast transmissions and other ether-borne signals try the
following:
Ensure all measurement system interconnections employ minimal length
transmission lines of good shielding efficiency, terminated correctly using high-
integrity RF connectors. Where direct connection of the Leveling Head to the
measurement load is not possible, it is likely that rigid or double-screened coaxial
line will be necessary. All RF connectors should be screw-thread-captured
against precision mating surfaces (e.g. SMA, PC3.5, N-Type, TNC and better).
These connectors must be torqued correctly.
Eliminate Interference from System Clocks – Common Mode and Ether Borne
Small signals will have to be measured in a narrow noise bandwidth implying a
tuned measurement (e.g. Measuring Receiver or Spectrum Analyzer). To ensure
accurate tuning of the measurement it is likely that a Reference Clock will pass
between or be fed to all of the instruments involved. This clock will be a relatively
large impure signal (>1 V pk-pk), typically at 10 MHz, possibly a square wave.
Such a clock is likely to interfere with low-level measurements at the clock
frequency and its harmonics.
To minimize interference at harmonics of the clock, use a sinusoid clock or a
filtered digital (square wave or pulse) clock.
Reference Clock distribution connects the source and measurement instruments
by two paths: the signal path (small signal) and the clock path (large signal). The
following design features of the Instrument minimize common-mode coupling of
the clock to the signal:
•
Attenuation in the Leveling Head, close to the Load
•
Floating RF Common
•
Transformer coupling of the Reference Clock, input and output
Another way to reduce common-mode coupling at the measurement instrument
is to route the Reference Clock signal through a common-mode choke (a suitable
ferrite ring over its coaxial cable).
Other signal paths between the source and measuring instruments may also
exist. For example, it may be necessary to isolate a GPIB connection at the
measuring instrument; use either a bus isolator or a common-mode choke.
3
Local Operation
Instrument Operation
3-119
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