The most recent type consists of 
20,732 turns of No. 18 HF-insulated wire 
taper wound on a core consisting of 35 
Telcon 79 strips, each 0.015" x 0.75" x 
72", about one-half square inch in cross- 
section. All wire splices are welded. 
A copper shield with an insulated lap 
encloses the winding while this in turn 
is enclosed in a section of plastic pipe 
with watertight end fittings and cable 
connectors. The resulting resistance is 
47 ohms and the inductance about 210 
henries. In service these coils and 
their connecting cables are buried. 
AMPLIFIER SYSTEMS 
Two types of amplifier designed and 
built at PNL have been successfully used; 
a DC amplifier for frequencies below 
3 cps and an AC amplifier for frequencies 
above 2 cps. Each type together with its 
auxiliary equipment is described. 
1. DC System 
(i) Input filter - The input filter 
contains two bridged-T, 60 cycle re- 
jection sections having a total DC 
resistance of 18 ohms inserted between 
the antenna and amplifier. In some 
locations an RF filter is also re- 
quired. Since the frequencies below 
1 cps have much larger amplitudes 
than higher frequencies, and a 
chopper amplifier is used, a low pass 
section is not required. The input 
filter includes the capacitors for 
series tuning the detectors and a set 
of resistors which, if required, 
could reduce the "Q" of the detector 
circuit. A line-balance potentio- 
meter is incorporated. The filter 
chassis also incorporates a 1 ohm 
precision copper resistor in series 
with the detector circuit for cali- 
bration when an air-cored antenna is 
’ 
used. Low thermal solder is used in 
the filter and DC amplifier input 
stage. 
(ii) DC amplifier - The low frequency 
system uses a chopper type DC ampli- 
fier operating at 60 cps. An elec- 
trostatically shielded, specially 
built input transformer is used to 
match the chopper to the first stage 
of the amplifier. The chopper noise 
has been materially reduced by lower- 
ing the chopper-drive voltage and 
using a "bucking coil", fed by cur- 
rent which can be adjusted in phase 
and amplitude, to cancel out pick-up 
in the contacts introduced by the 
drive coil. Each chopper is care- 
fully adjusted in the lavoratory for 
minimum noise. With a source imped- 
ance of 40 ohms the maximum DC 
voltage gain is about 10”. An atten- 
uator alfows the gain to be reduced 
in 10 db steps to -80 db. A properly 
adjusted instrument has a noise level 
of about 0.005 microvolts rms in the 
frequency band 0.02 to 3 cps with a 
40 ohm source resistance. 
Figure 3 depicts the amplifier 
chassis, Figure 4 the block diagram 
of a three component-two station 
installation, and Figure 5 shows a 
family of frequency response curves. 
A monitor point for the amplified 
60 cycle signal is provided in the 
circuit before demodulation. The 
signal at this point can be displayed 
on an oscilloscope to determine if 
unwanted 60 cycle pick-up is entering 
the amplifier through the detector 
circuit or if a steady DC bias such 
as would be produced by a thermal EMF 
is present. The latter may be coun- 
teracted by an adjustable bias in the 
amplifier whereas the former cannot 
be balanced out in the amplifier. 
Experience has shown that the ampli- 
fier is exceedingly sensitive to cer- 
tain environmental conditions and 
surprisingly tolerant toward others. 
A few of the precautions which seem 
to be well worth observing are 
detailed as follows: 
(a) Installation - Heaters of the 
tubes must be supplied from a DC 
source. The amplifier should be kept 
as far as possible from electrical 
apparatus that might produce 60 cps 
magnetic fields (minimum distance 
5 ft). The amplifiers in a multiple 
installation should be spaced at 
least 2 ft apart. 
(ob) The Grounding System - It is 
desirable to obtain a grounding 
system such that there is no appre- 
ciable pick-up of unwanted signals. 
The line balance potentiometer in the 
input filter will have no effect if 
the pick-up is negligible. Exper- 
ience has shown that the interference 
pick-up is very much reduced if a 
high resistance, at least 20 megohms, 
is maintained between coil and shield, 
and between shield and ground. The 
ground connection should be made only 
at the amplifier with the detector 
connected as a balanced input. 
(c) Thermal EMFts - An inspection 
of the signal at the AC monitor point 
Pay, show a constant LIL 6 signal 
cps) which cannot be eliminated 
with the line balance potentiometer. 
This may be due to thermal EMF's in 
the input circuit, and can be 
Superior numbers refer to similarly numbered references at the end 
of this paper. 
325 
