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6.
CHARACTERISTICS
6.1. CHARGE AMPLIFIER NOISE CONSIDERATIONS
The compressor and vibration meter sections of the 1 050 have a low noise charge
amplifier input stage. The amplifier itself produces noise of less than 0,01 pC. However,
the noise from the charge amplifier is also dependent on the capacitance of the cable
and transducer connected to the charge input. The total input noise can be calculated
using the following relationship:
Total input noise
=
inherent noise (1
+
1
F
. C
)
n x mput sens.
where C
=
transducer capacitance
+
cable capacitance
The capacitance of Bruel
&
Kjrer
Uni-Gain®
accelerometers is typically 1 nF, and the
capacitance of a low noise cable is about 90 pF per metre of cable length. Table 6.1
shows the maximum cable length that can be used, for each sensitivity setting, whilst
maintaining the specified accuracy of the level control and meter reading. In general
cable lengths should be kept as short as possible. The use of cable lengths above 100m
is not recommended and should be avoided.
Selected Input Sensitivity
Maximum Cable Length
1 ,00 pC/ms-
2
10m
3,16 pC/ms-
2
30m
10,0 pC/ms-
2
100m
31 ,6 pC/ms-
2
100m
4,00pC/N
40m
T01198GBO
Table 6. 1. Maximum cable lengths for each input sensitivity selection
If a combination of cable length and transducer sensitivity, exceeding the limits stated in
the table above, is unavoidable; the use of a Charge Amplifier (e.g. Bruel
&
Kjrer Type
2635) is recommended. The output from a charge amplifier should be connected to the
voltage input of the 1050 and the input sensitivity should be set to 1 V /ms-
2
or 1 mV
/N
as
appropriate.
6.2. COMPRESSOR SPEED CONSIDERATIONS
The compressor regulation speed is controlled by the combination of the Compressor
"Rate" and "Max. Speed". The resultant compressor speeds are shown in chapter 1,
Fig. 5.
The choice of the compressor regulation speed is dependent on stability and distortion
considerations. The stability is affected by the sweep rate, test object resonance
a-
value, tracking filter bandwidth etc.
65
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