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Model 415E
3-39. SMITH CHART CALCULATIONS.
3-40. Use of the Smith Chart for calculating imped¬
ance is outlined below. Followingthe generalized pro¬
cedure is a numerical example.
Other methods are
possible for first entering the Smith Chart, but the one
suggested here is practical and easy to use.
a. Determine the guide wavelength, A
as explained
in Paragraph 3-28.
b. Measure the swr by the method in either Para¬
graph 3-26 or 3-28.
c. Locate a convenient minimum with the load still
in place.
Record the probe carriage reading.
d. Replace load by a short, relocate the minimum
and record the probe carriage reading.
Determine
A d, the difference between this reading and the one
from step c.
Note whether the minimum was moved
toward the load or toward the generator.
e. Calculate the shift of the minimum, in terms of
wavelength:
f.
Start at center of Smith Chart and draw a circle
with a radius equal to the swr.
g. Enter the Smith Chart at the top, move in the di¬
rection of probe movement noted in step d and a dis¬
tance AA, computed in step e.
Use wavelength scale
at the periphery of the Smith Chart.
h. Draw a line from the A A point to the center of
the chart.
i.
Locate the normalized impedance as the inter¬
section of the swr circle and the line drawn in step h.
j.
The actual impedance is the product of the nor¬
malized impedance from step i and Zq, the line char¬
acteristic impedance.
Note
The convention of entering the chart as stated
in step g applies only if the minimum is lo¬
cated first with the load on the line and relo¬
cated when the line is shorted. If it is neces¬
sary to first establish the shorted minimum
point, the direction of A a would be opposite
to the direction of probe movement required to
relocate the minimum with the load concerned.
3-41. The following example will clarify the above pro¬
cedure. Figure 3-10 shows the important steps involv¬
ing the Smith Chart. The assumed characteristic im¬
pedance is 50 ohms.
The distance between adjacent
minima is 15 cm, therefore Aff = 30 cm.
The swr is
measured as 3.3. A minimum is located at 22 cm. The
load is shorted and the minimum shifts to 19 cm, toward
the generator.
Section III
Paragraphs 3-39 to 3-49
Ad = 22 cm - 19 cm = 3 cm
AA = Ad/aD. = 3 cm/30 cm = 0.1 wavelength
3-42. The following numbered steps refer directly to
Figure 3-10.
(1) A circle for swr = 3.3 is drawn.
(2) A line is drawn from the 0.1 A point (toward the
generator) to the center of the chart.
(3) The normalized impedance at the intersection
of the circle and the line is 0.44 + j 0.63.
The impedance of the load (for Zq = 50ft) is then:
50 (0.44 + j 0.63) = 22 + j 31.5 ohms
3-43, SPECIAL APPLICATIONS.
3-44. The Model 415E is equipped with outputs which
allow applications other than as a meter indicating de¬
vice for swr or attenuation.
3-45. RECORDER.
3-46. The rear panel recorder output furnishes an out¬
put from Oto 1 volt DC with internal resistance of 1.000
ohms and provides a convenient means of obtaining a
permanent record of measured data.
For proper op¬
eration, the recorder output ground (BNC shell) must
be connected to a floating ground. Adapters are com¬
monly available to float the ground of grounded input
instruments at the power cord (see Paragraph 3-11).
3-47. AMPLIFIER OUTPUT.
3-48. The rear panel amplifier output furnishes an
output from 0 to 0.8
volt RMS into lOKohms or more.
The Model 415E will supply up to 126 db of voltage gain.
For proper operation, the ground terminal (black) must
be connected to a floating ground (see Paragraph 3-11).
With the 415E EXPAND switch set to NORM, a full
scale meter reading will result in a 0. 3 volt RMS out¬
put signal, and a minimum scale reading (10 db) will re¬
sult in approximately 0.03 volt RMS. With the 415E EX¬
PAND switch set to any position except NORM, a full
scale meter reading results in a 0.8 volt RMS output
and a minimum scale reading (2 db) results in a
0.5
volt RMS output signal. A zero input signal results in
a zero volt output signal.
3-49.
The Model 415E is especially useful as a tuned
amplifier in a measurement setup using an Oscilloscope
and a Sweep Oscillator. Sweep speeds may be increased
(over the speeds using a ratio meter in a reflectometer
system) and the Model 415E, used as a high gain amp¬
lifier, provides the required sensitivity.*
The AM¬
PLIFIER OUTPUT (AC) is often more useful for this
purpose than the RECORDER OUTPUT (DC) since the
DC output is filtered to reduce ripple and its response
is too slow to make full use of maximum bandwidth.
* See hp Application Notes 54, 61, 65, and 66.
02152-2
3-9
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