BRUEL & KJAER 2203 - Reference Oscillator

.
oooooooooooooooooooooooooooooooo
dB
+1
-1
- 5
- 10
- 15
-2 0
20
50 100
_F
4145:- p
F,
200
freefield response
pressure response
free-field response
with
UA 0055
p
500
1000
2000 Hz 5000
F
10000 20000
169112
Fig. 4. 3. Typical frequency response of Microphone Type 4145
The input amplifier follows the attenuator. Its high input impedance
(> 1 0 MO) ensures negligible loading of the attenuator, and its low out-
put impedance (<50) prevents loading effects of the weighting net-
works or external filters. It should be loaded, however, by an imped-
ance
of at least 500 n.
The calibration potentiometer (ADJ.) is placed in the feedback of the
input amplifier and varies its gain over a range of at least -2 to+ 4dB.
4.4. REFERENCE OSCILLATOR
The reference oscillator operates at approximately 1 kHz and gives a
square
wave whose amplitude accuracy is better than ± 0,2 dB. Since
this reference voltage is inserted after the input stage, it must be re-
membered that the input stage and microphone is not included in any
calibration using the built-in reference voltage.
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