Potomac Instruments FIM-21 - Page 20

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SECTION J. THEORY OF OPERATION (Cont.)
Block Diagram Description ( Cont.)
If the user were t o hold the field meter at a precise point in a
radiated field !mown to have a st ren gth of 200 MV/M , and adjust the receiver
gain to read this value on the met er, the instrument would then be calibrated
at this frequency and it would read field strength in volts pe r meter at all
meas uring points, provided that the gain does not change, and that the re -
ceiver tuning does not change.
To calibrate the field meter, then, it is only necessary to have
some reference signal in the receiver to indicate the proper setting for the
receiver gain control. This signal is provided by the calibration oscillator.
When the FULL SC.ALE switch is positioned to CAL, and the FUNCTION
switch is positioned to FI, a precisely-determined fraction of the oscillator
output voltage is injected into the receiver antenna circuit and is detected
by the receiver. At the same time a detector fed directly by the oscillator
output voltage provides a de voltage for comparison with the receiver detec-
tor output.
In the Fil-1 - 21 and FIM-41 the two detector outputs that must be
equalized are compared using a null circuit, T he null circuit produces a
meter reading that drops as the receiver detector voltage approaches the
oscillator detector voltage from above or below; the reading is minimum
when the voltages are equal. The receiver gain value for which the detector
outputs are equal is determined by the relation between the RF voltage at the
oscillator detector and the RF volta ge injected into the antenna circuit. The
c
ir cui try that determines this injection voltage thus determines the calibration
accuracy. The circuitry is factory - set during final calibration for correct
field readings. Since the vol tage induced in the antenna by a field varies
with frequency, the RF injection voltage mus t also vary with frequency in
exactly the same way in order for the calibration to be correct across the
full frequency range. The desired variation with frequency is produced by
a simple reactive network.
It is the double function of the calibration oscillator, as a
signal source for the receiver and as a reference level for the null circuit,
that provides the very important self-compensating feature of the calibration
of this instrument. For example, if the output of the oscillator were to be
reduced by 20% for any reason, the reference level for the null circuit -would
be 20 % lower, but the signal fed into the loop would also be 20% lower, so
that no change in the gain control would be required to calibrate.
3.3
Circuit Descri p tion - Refer to Figure 3-1
In t he recei-ver the RF signal is induced by the magnetic component
of the incide nt field in a shielded, unbalanced loop antenna . The loop is
part of the first tuned circuit of a double - tuned bandpass filter that is
tuned over the RF frequency range. The input can also be supplied from an
external source; selecting thi s mode switches the loop antenna out of the
circuit while maintaining the same RF b andpass characteristics.
3-3