Potomac Instruments FIM-21 - Page 21

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SECTION 3. THEORY OF OPERATION ( Cont.)
3.3
Circuit De s cript i on (Cont.)
A capacitive voltage di vider RF attenuator follows the RF filter.
It is switched by the FULL SCALE switch in three 20 db steps to provide part
of the total measurement range. The attenuator output drives a broadband RF
amplifier and mixer. The oscillator driving the mixer is ti.med over the RF
range to produce an IF frequency of 455 KHz.
The 1F signal passes through a ceramic filter that, in conjunction
with the RF tuned ci rcuits, provides receiver selectivity. The signal is then
fed to a resistive attenuator having two 20 db steps. Following the attenuator
is the IF amplifier, made up of broadband stages with tuned circuits at input
and output. IF gain is controlled by a front panel control, and also, in the
LOG mode, by a voltage fed back from the detector. The IF detector at the
amplifier output supplies voltages to the meter, the audio circuit, and cali-
brating circuit, and to a simple de amplifier that provides the RECORDER output .
The
audio amplifier drives a speaker or an external load. A switch
on the front - panel AUDIO control removes power from the amplifier when audio
is not needed, in order to reduce battery drain.
A calibrating oscillator, gang- tuned witn the receiver, provides
a signal at the selected frequency for self - calibration. A front - panel con-
trol provides a fine frequency adjustment for exact tuning. The de voltage
from a detec t or at the oscillator output is compared with the voltage from
the receiver detector in a null circuit, which produces a dip in the meter
reading when the two voltages are equal. A portion of the oscillator RF
voltage is i njected into the receiver in such a way that, when the receiver
gain is ad j usted for a null circuit dip, correct field readings will be ob-
tained when the oscillator is switched off.
DC power is supplied by six 1.5 volt cells in series and a sirnple
voltage regulator that has an output of -6.0 volts. A battery check circuit
indicates battery condition on the meter. It is adjusted to indicate battery
replacement at a total voltage of 6.6 volts.
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