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APPENDIX 1
Checking the Sensitivity of
Counters
Introduction
The sensitivity of a counter is normally specified as the
minimum signal voltage on which the input of the counter
will trigger correctly.
When you use a signal-source with an output-impedance
of 50
Ω
, constant-output-amplitude, and the counter has a
50
Ω
input-impedance, the input signal of the counter is in
theory independent of the cable length. However, if the in-
put impedance deviates from 50
Ω
there will be standing
wave reflections which will cause changes in the ampli-
tude of the signal between the signal-source and the
counter input.
Two factors determine the magnitude of the changes, i.e.
frequency and capacitive load.
EXAMPLE: For a 1 M
Ω
//35 pF input, the 35 pF parallel
capacitance is approximately equal to a 50
Ω
ca-
pacitive load at 100 MHz.
Consequently, it is of the utmost importance to know how
sensitivity is measured.
Recommended Instruments
–
Signal-source with a 50
Ω
output impedance.
–
>350 MHz oscilloscope with a 50
Ω
input impedance.
–
BNC T-piece.
–
Two BNC-cables, one short and one long.
High Impedance Inputs (1 M
Ω
)
Preparations
Connect the instruments as illustrated in the figure above.
Turn off AUTO and set the counter to maximum sensitivity.
Method 1
–
Adjust the amplitude of the signal-source to the minimum
level accepted by the counter.
–
Read the amplitude on the oscilloscope.
–
Check that the reading is the same as, or less than, the
sensitivity level in the counter specifications.
Method 2
–
Adjust the amplitude of the signal-source until the oscil-
loscope indicates the sensitivity limit in the counter
specifications.
–
Check that the counter is operating correctly.
Signal source
Counter
under test
Oscilloscope
350 MHz
T-piece
Attenuator
(if required)
Shortest cable
possible
directly
on counter
50 ohm
>
50 ohm
Figure 9.
Connect the instruments like this.
APPENDIX 1
Page: 43
PM 6666 - OPERATORS MANUAL