Measurement Of A 50-Ohm Line Terminated In Its Characteristk Im- Pedance At50 Mc - GENERAL RADIO COMPANY 1606-A Operating Instructions Manual

TYPE 1606-A R-F BRIDGE
-137 - 1932 - (-137)2
x, = -16,400 -16,400 = -136 ohms
0.984
5.4 MEASUREMENT OF A 50.OHM LINE TER-
MINATED IN ITS CHARACTERISTIC IMPEDANCE
AT 50 MC. At very high frequencies, lead correc-
tions are very important. I1 is also desirable, if
possible, to bring up the outer conductor of the co-
axial line over
the
panel and make contact either
with the panel directly or with a clamp placed un-
der one of the panel scre'xs. (One of theblack panel
screws suppliedmust be replacedwitha" unpainted
lo-32 screw for this application.) (See Figure 9.)
a. Connect thegenerator and detector, andcheck
for leakage as outlined in paragraph 3.5. At high
frequencies, reliable measurements cannot be made
unless both the generator and detector are fitted
with coaxial connectors.
b. As indicated in paragraph 3.7, either the short
clip lead or a short length of No. 2.0 bus wire can
be usedfor connectionto the unknown. Assume that
the short clip lead is
used
for this measurement.
Screw the lead into the ungrounded bridge terminal
and clip it to ground directly at the end of the co-
axial line under test. (See Figure 4b.) The react-
ance of any groundconnection used is therefore in-
cludedin the initial balance and is notmeasured as
part of the unknown.
c. Since the line is terminated in its character -
istic impedance, the measured reactance will be
low. Therefore, the REACTANCE dial should ini-
tially be set in the lower part of its range, say at
500 ohms, with the switch at LOW. Establish ini-
tial balance using the INITIAL BALANCE controls.
d. Transfer the connecting-leadclip to the cen-
terconductor of
the
coaxial line andrebalance with
the RESISTANCE and REACTANCE controls. Sup-
posethe readings are 40.5 ohms and 350 oh&s,re-
spectively. Before corrections, the indicated rc-
sistance R,
and reactance Xm are:
R,
= 40.5 ohms
X
350 - 500
"1 =
50
= -3.0 ohms
For a slightly more precise reactance reading. re-
peat the measurement, with the REACTANCE dial
initially set closer to zero.
for a dial reading of 40.5 ohms at 50 MC. It 1s 1.23.
The corrected value of resistance then becomes:
RI"'
= 40.5 . 1.23 = 49.8 ohms
f. To correct for the capacitance to ground of
the connecting lead, determine from Figure 6 the
corresponding reactance (X,) of the short clip lead
at 50 MC. It is -838 ohms. Applying equations (1)
and(2)to determinetheactual line inputimpedance,
ZP
RX
49.8
= 0.996 = 50.0 ohms
X x =
-3.0 - !E$ - k&
0.996
= Oohms
g. This example is
cited as an extreme case,
in which failure to correct for the inductance of the
resistance capacitor leads to a" error in resisr-
ante measurement in the order of 20 percent.
5.5 MEASUREMENT OF BALANCED CIRCUITS.
The Type 1606-A R-F Bridgewillnot measure bal-
anced circuits directly. However, the measurement
can be made by a" indirect method. In the balanced
circuit shown in Figure 8a, the following three im-
pedance measurements are required:
Zl = impedance between A and ground, B
grounded,
Z2 = impedance between B and ground, A
grounded,
Z3 = impedance between A and B connected
together and ground.
The effective components of the balanced ner-
work can be calculated from the following equations:
221
zJJ$=
Zl Zl A
1+Y?-iq
222
Z BC =
Z2 Z2
1+y--
3 Zl
223
7
Z
AC=l+Z3 z3
Figure 90.
Zl z2
15