The field strength is then read by switching off the calibrating oscillator and rotating the instrument's loop for an
accurate reading.
1. Signal Input: Refer to the schematic diagram for circuit details. The loop, L -1, is of the unbalanced,
shielded type, with one end connected to L -3, and the high end loaded by a high -Q adjustable inductance, L -2.
The loop has only a few turns, so its fundamental frequency is above the highest frequency to be used. Additional
inductance is provided by L -2 to give a wide enough tuning range when adjusted by section C -2A of the tuning
capacitor which is operated by the front panel RECEIVER control. This form of antenna minimizes the effects
of distributed capacities, reduces antenna effect, and requires no balancing. The Q of the loop circuit is about
100 at one megacycle. This high -Q factor makes for high sensitivity and selectivity and provides high image re-
jection through the use of an RF amplifier stage.
2. R. F. Amplifier: The loop is connected through a conventional capacity divider which forms one section
of the RF attenuator system to the RF amplifier, VT -1. The attenuator circuit is controlled in six steps by sec-
tions S -lA, 1B and 1C of the FULL SCALE RANGE switch, and works in conjunction with an IF attenuator controlled
by section S -1D of this same switch. Together these attenuators provide six steps of receiver output voltage, each
progression providing ten times the receiver output of a preceding value.
The attenuator is arranged so that reduction takes place first in the input to the IF amplifier. On the 100
pV /M position, the 1 MV/M position, and the 10 MV/M position of the FULL SCALE RANGE switch the RF attenu-
ator is out of the circuit. On the 100 MV/M position, the 1 V/M position and the 10 V/M position the IF attenuator
is maintained at full attenuation, and loss is introduced progressively in the RF attenuator. With this arrangement
noise originating in the front-end circuits has no effect on output meter indications except on the 100 µV/M position.
Even on this position its effect is negligible and does not materially affect the accuracy of the indication.
Provision is made in the attenuator to avoid detuning of preceding circuitry. The RF amplifier tube, VT -1,
operates with a fixed bias derived from the resistor network R -22, R -23, and R -24.
3. First Detector and Oscillator: The output of the RF stage is coupled to the first detector tube, VT-2,
by a transformer, T -1. This same tube, through its associated transformer, T-6, operates also as a local heter-
odyning oscillator. Transformers T-1 and T -6 are tuned by capacitors C -2B and C-2C respectively. These
capacitors are ganged with each other and with the loop tuning capacitor, C -2A. The tube acts as a conventional
oscillator -mixer to develop an intermediate frequency of 455 kc. Linearity of output of this detector is good, since
the range of signal inputs over which it functions is reduced.by the fact that there are three steps of attenuation
in the input to the RF amplifier, VT-1. The output circuit of VT-2 contains the fixed inductance which is tuned by
C -20 to resonance at the intermediate frequency. The COARSE GAIN control is in the signal grid of this stage.
4. IF Attenuator: The IF Attenuator circuit functions on the 100- µV/M, 1 MV/M, and 10 MV/M positions
of the FULL SCALE RANGE switch S -1 (positions of greatest sensitivity). This circuit is fed by a capacitance
voltage divider formed by C -21 and C -22. This prevents changes in the attenuator capacities from affecting the
tuning of the choke L -12 in the first IF amplifier.
5. First IF Amplifier: This is a conventional IF amplifier stage operating on fixed bias, when the LOG-LIN
toggle switch S -4 is in the LIN position to feed linear output. With S -4 in the LOG position an AVC bias is derived
from the DC output of the metering crystal, X -2, which varies the gain in VT -3.
-5-
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