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9640A
Instruction Manual
Eliminating 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.
Eliminating 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:
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•
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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.
Avoid Grounding RF Common on the Instrument
While the Instrument reference clocks are transformer coupled, the External Modulation
and Sweep Trigger I/O connections are DC coupled to floating RF common. Be aware
that making connections to these I/O ports can ground the RF Common (e.g. via an audio
signal generator, oscilloscope, or spectrum analyzer). Common-mode chokes, as
described earlier, may reduce interference, but they may not be compatible with very
low-level measurements.
Verifying the Level of an Interfering Signal
Having tuned a low-level measurement, determine the level of interference to that
measurement by breaking the signal connection and terminating or shorting the
Instrument and its measurement ports. Re-establishing connection of the Leveling Head
ground to the measurement ground (touch contact of the two grounds is often sufficient,
but a back-to-back terminator or short will improve the verification). Any signal now
detected will be interfering with the measurement, adding or subtracting according to its
phase.
De-tuning the Interfering Signal
For many low-level measurements, it is good practice to re-tune the measurement away
from any interfering transmission or coupled clock.
3-58
Attenuation in the Leveling Head, close to the Load
Floating RF Common
Transformer coupling of the Reference Clock, input and output
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