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GR's automatic bridge t a k e s about one-half
second to achieve balance.
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5.1.9 COAXIAL-LINE INSTRUMENTS.
The Slotted Line
The upper-frequency limit of conventional bridge
circuits using lumped-parameter elements depends on
the magnitude of the residual impedances of the ele-
ments and leads.
The corrections for t h e s e usually
become unmanageable a t frequencies above a few
hundred megahertz,
and circuits based on coaxial-
line techniques are more satisfactory.
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One of the b a s i c methods of measuring the im-
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pedance of a coaxial device i s the measurement of the
standing-wave ratio i t introduces in a uniform line.
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The measurement i s b e s t made by a slotted line, an
instrument consisting of a length of coaxial air line
wlth a longitudinal slot in i t s outer conductor and an
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electrostatic probe, which enters the line through the
slot. The probe is moved along the length of the line,
sampling the field inside.
Thus are the magnitudes
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and positions of voltage maxima and minima determined
and, from this information, the impedance of a n unknown
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connected t o the line. In t h i s instrument the impedance
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standard i s the line itself, and i t s accuracy depends
primarily on i t s physical dimensions.
General Radio offers two slotted lines:
t h e
Type 874-LBB, for general impedance measurements,
and the highly accurate Type 900-LB, the most ad-
vanced slotted line available commercially.
The Admittance Meter
T h e GR Type 1602 UHFAdmittance Meter u s e s ad-
justable loops t o sample the currents flowing in three
coaxial lines fed from a common source and terminated,
respectively, in the unknown, a standard conductance,
and a standard susceptance. The loops are adjusted s o
that the combined output from them is zero (a null bal-
ance). S c a l e s associated with the three loops give the
value of the unknown directly, in terms of admittance.
The Transfer-Function and Immittance Bridge
T h e GR Type 1607 Transfer-Function and Immit-
tance Bridge i s similar t o the Admittance Meter de-
scribed above but a l s o permits four-terminal measure-
ments, s u c h a s those of forward and reverse transcon-
ductance and transsusceptance, transimpedance, and
input-output voltage and current ratios.
T h i s bridge
i s widely used t o evaluate the transfer characteristics
of transistors and tubes in the vhf and uhf ranges.
5.1.10 GENERATORS AND DETECTORS.
Several GR bridges include both generator and
detector.
Some others
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the Types 1615, 716-C, and
716-CS1 Capacitance Bridges and the Types 1632 and
1633 Inductance Bridges
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are available a s complete
measuring assemblies, with generator, detector, inter-
connecting c a b l e s , relay rack, and other accessories.
Unless one obtains s u c h a complete system, he must
carefully choose generator and detector t o ensure
satisfactory measurement results.
(Even with a com-
plete system, the user may a t times wish t o connect a
different generator or detector t o the bridge, and almost
a l l GR bridges include panel connectors for such use.)
T h e chief generator requirements are good fre-
quency stability, adequate power output, and low
harmonic content.
A wide choice of GR oscillators is
available, covering the frequency range from audio t o
microwave.
Desirable detector characteristics are;
1. .High sensitivity, preferably the ability to
d e t e c t a few microvolts.
2. High selectivity, to reject harmonics, noise,
and other interfering signals.
T h i s is particularly
important in measurements on iron-core c o i l s and other
nonlinear elements.
3. Logarithmic or nearly logarithmic response,
t o minimize gain adjustment during the balancing
procedure.
4. Good shielding, t o prevent errors from extran-
e o u s pickup.
At audio frequencies, GR's Type 1232 Tuned
Amplifier and Null Detector is recommended for i t s
high sensitivity and for i t s general versatility in the
lab. The Type 1212 Unit Null Detector is useful up t o
s e v e r a l megahertz.
Crystal mixers are available for
both the detectors, extending their frequency ranges t o
about 6 0 MHz.
At t h e s e and higher frequencies, the
heterodyne type of detector is preferred, because of i t s
wide frequency range and excellent shielding. The GR
Type 1241 Detectors operate from 70 kHz to 2000 MHz.
One of the most popular generator-detector com-
binations, the Type 1311 Audio Oscillator (50 Hz t o
10 kHz) with the Type 1232 Tuned Amplifier and Null
Detector, is available in a single assembly a s the
Type 1240 Generator-Detector Assembly.
5.1.11 CONNECTIONS
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SHIELDING.
Adequate ground connection and shielded genera-
tor and detector leads are always important, but they
are particularly s o a t high frequencies.
At audio and
low radio frequencies, electrostatic shielding of leads
is usually enough; above a few megahertz,
coaxial
l e a d s , securely grounded to the detector, generator, and
bridge s h i e l d s , are necessary.
GR's patch cords and
c a b l e s (Table 1-1) are recommended for bridge con-
nections.
PRINCIPLES O F OPERATION 5-4
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