Optoelectronics M1 Owner's Manual page 8

USING THE COUNTER
CONTINUED
USEWITHAPSI04
Allbroadbandcountersare subjectto two fundamentallimitations in their sensitiv-
ity.The firstis the noiseof the electronsmovingthough the.circuitry,of.the
cou~tcr
input circuitry. Fora typical3GHzbandwidth front
end,
this results
In
input ~se
floor of about ·'7l)jBm. S inceany desired signal
to
be counted must exceed
l~
level
by 10-15dB so the countercanreliablycount zero crossings, the limiting
sensitivity
is -44 to -60dBm. This
figure
is approached by Optoelectronics' counterswhen
operated in a laboratoryenvironment,but there is another.more limitingfactor
when attemptingto countradiatedsignalsusing an antenna.
Unlessyou find yourselfon a desertedPacificisland,the signalyou intend to
measureis
not
the
only one reachingthe counter.Oncean antenna is attached
to
the counter,
every
RFsignalbesidesthe one of interestbecomesa
so~n:
of
interferenceand the
secondsensitivitylimitation.The level of these incidental
signalscan be quite large,in fact, and usually is the limiting factorin frequency
counterlow levelperformance. LocalAM, FMand TVtransmittersbroadcastwith
hundreds of kilowattsand yieldan RFbackgroundthat obscuresall measurements.
Again,sincethe desired signalmust exceedall othersby 10-15dB,this
~ifference
in
levelcanonly
be
achievedby gettingthe counter muchcloserto
the
desired source
than to the interference.
The most effectiveway to increaserange is
to
use the Optoelectronics' Active
Presclector. This unit addressesboth of the limitationsdescribedaboveby imple--
mentinga narrow bandpass filtertunable over an extremelywide ~uency range.
Wheninsertedin the signalpath between the antenna and counter,
this
has two
effects:
- The
narrow4MHzbandwidth passband removesmost interferingsignals,
allowingonly signalsat the centertuned frequencyto pass through
to
the counter.
The apparent increasein sensitivity can be 40dBor moredepending on the levelof
the interferingsignal.
-Since
the bandwidth of the measurementsystemis reduced,the 3GHzbandwidth
noise floorlimitation is also reduced.
Gain
in the ActivePreselector correspond-
inglyboostssignallevelto take fulladvantage of the bandwidth reduction.
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The4MHz bandwidth of the filterwascarefullychosenas the bestcompromise
betweensensitivityimprovementand frequencyagility . As coveredabove,the
reductionin bandwidth eliminatesinterferingsignalsand broadband electrical
noise. Therefore. narrowingthe bandwidth generally resultsin greater sensitivity
(thisconcept . carried
to
an extreme,is whatmakes typica l communications. .
receivers
very
sensitive- with bandwidthsof 15kHzor so). Unfortunately, if this
narrowingis carriedtoo far, the signalcan be extremelydifficult
to
find unlessyou
alreadyknow what frequency it is. Thisofcoursedefeatsthe
purpose
of usingthe
frequencycounter altogether . The 4MHzbandwidth of the APSI04representsa
good balanceof sensitivityimprovementand easeof use.
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