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t (rise)
= less than 0.2 us
t (fall)
= less than 0.2 us
I (ampl)
= Current amplitude 133mA 5%
30.2.2 Linear envelope detector. The linear envelope
detector shown in Figure 4 consists of diode D1, capacitor, C1,
and resistor, R3. When the excitation signal pulse goes low (to
zero) the excitation circuit appears as a high impedance with
respect to the coil. The collapsing field of the coil is a
negative pulse, as shown in Figure 5, which forward biases D1. C1
charges to the negative peak of each cycle of the excitation
signal. When the diode is not forward biased the time constant
formed by R3 and C1 allows C1 to discharge through R3 so that the
output at TP3 follows the test signal envelope (10 Hz input drive
signal) of the modulated (1 KHz) wave.
30.2.3 Filter and gain. The output of the linear envelope
detector is first buffered by a voltage follower to ensure correct
circuit separation. The signal is then input to the filter and
gain section to remove the 1KHz excitation field and to amplify
the low level 10 Hz output voltage of the magnetometer (core under
test). Finally, the overall signal gain of 250 is set by the
amplifier stage after the filter.
The filter used must meet the following parameters:
Type:
Butterworth Bandpass
Six
Order:
Ga in:
250
Ten Hertz (tuned to the drive signal)
Center Frequency:
Five
Q:
Figure 6 shows one implementation of the desired filter; it is a
biquad design.
30.3 Calibration
30.3.1 Test circuit frequency calibration. The biquad filter
of Figure 6 is tuned by Inputting a 40 millivolt peak to peak 10
Hz ac test signal across TP3 and ground. The output voltage at
TP4 is monitored. The frequency of the signal at TP4 will be the
same as that of the input signal. An adjustment is made at the
first stage of the filter (R7) such that a maximum voltage is
attained at TP4 (the exact amplitude being of no importance).
Subsequent adjustments made at R(14), and R(21) will properly tune
the second and third stages of the filter to the 10 Hz input
signal.
11
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