BRUEL & KJAER 4188 - Brüel & Kjær’s Patented Charge-injection Calibration Technique

Chapter 8 —
1
/
2
" Microphone Preamplifier Type 2669
Brüel&Kjær’s Patented Charge-injection Calibration Technique
Falcon™ Range of Microphone Products
Microphone Handbook
Brüel & Kjær
8
−
10
8.8 Brüel&Kjær’s Patented Charge-injection Calibration
Technique
This is a new patented technique for verifying the entire measurement set-up in-
cluding the microphone, the preamplifier and the connecting cable (see Fig.8.11).
The Charge-injection Calibration (CIC) technique is a method for remotely verifying
the condition of the entire measurement set-up
including the microphone
. This
is a great improvement over the earlier insert-voltage calibration method which
virtually ignores the state of the microphone. The CIC technique is very sensitive to
any change in the microphone’s capacitance which is a reliable indicator of the
microphone’s condition.
The technique works by introducing a small but accurately defined capacitance
C
c
(typically 0.2pF) with a very high leakage resistance (greater than 50000GΩ) into
the circuit of the preamplifier, see Fig.8.11 (upper diagram).
C
i
and
R
i
represent
the preamplifier’s high input impedance and
A
its gain (≈ 1).
For a given calibration signal,
V
i
, the output,
V
o
of this arrangement will change
measurably, even for small changes in the microphone’s capacitance
,
C
m
. The CIC
technique is about 100 times more sensitive than the insert-voltage calibration ar-
rangement shown in Fig.8.11 (lower diagram).
In the extreme case where there is significant leakage between the microphone’s
diaphragm and its backplate (
C
m
becomes very large), the signal output will change
by tens of decibels compared with only tenths of a decibel using the insert-voltage
method.
Another important CIC feature is that, unlike the insert-voltage technique, it is far
less sensitive to external electrical fields.
Fig.8.11 Charge-injection calibration (CIC) technique compared to insert-voltage calibration technique
940528e
C
m
C
c
C
i
R
i
Housing + cable
A
C
m
C
i
R
i
V
0
V
0
V
i
Calibration
Signal Source
Microphone
Typical Values : C
m
= 15 – 20 pF C
i
= 0.3 pF C
c
= 0.2 pF A ≈ 1
V
i
V
0
V
i
C
c
C
m
+ C
i
+ C
c
= A
( )
V
0
V
i
C
m
C
m
+ C
i
= A
( )
A
Insert-
voltage
CIC