Table of Contents
Advertisement
Model 415E
3-39.
SMITH CHART CALCULATIONS.
3-40.
Use of the Smith Chart
for calculating
imped-
ance is outlined below.
Followingthe generalized рго-
cedure
is a numerical example.
Other
methods
are
possible for first enteringthe Smith Chart, but the one
suggested here is practical and easy to use.
a. Determinethe guide wavelength, A gas 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:
ах
Ad
^g
f. Start at center of Smith Chart and draw a circle
with a radius equal to the swr.
g. Enterthe Smith Chart at the top, move inthe 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 а line from the AA 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 70, the line char-
acteristic impedance.
Note
The convention of enteringthe chartas stated
in step g applies only if the minimum
is lo-
cated first with the load onthe line and relo-
cated whenthe line is shorted.
If it is neces-
sary to first establish the shorted minimum
point, the direction of ДА would be opposite
to the direction of probe movement required to
relocate the minimum withthe load concerned.
3-41.
The following example will clarify the above pro-
cedure,
Figure 3-10 showsthe important steps involv-
ing the Smith Chart. The assumed characteristic im-
pedance is 50 ohms.
Тһе
distance
between adjacent
minima
is 15 cm, therefore A,
= 30cm.
Тһе swris
measuredas 3.3. А minimum i$1ocatedat22 ст.
The
loadis shortedandthe minimum shifts to 19 cm, toward
the generator.
02152-2
Section ІШ
Paragraphs 3-39 to 3-49
Аа = 22 cm - 19 cm = 3 em
АА
AW rg = 3 ст/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 0.12 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 70 = 509) is then:
50 (0.44 + 10.63)
= 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 0to 1 volt DC with internal resistance of 1000
ohms and provides a convenient
means of obtaining а
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 10K ohms 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 willresultin a 0.3
volt RMSout-
put signal, anda minimum scale reading (10 db) will ге-
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 ina
0.5
volt RMS output signal. А zero input signal results in
a zero volt output signal.
3-49.
The Model 415E is especially useful as a tuned
amplifier ina measurement setup using an Oscilloscope
andaSweepOscillator.
Sweep speeds may be increased
(overthe speeds using
a ratio meter in a reflectometer
system) and the Model 415E, used as a high gain amp-
lifier, provides the required sensitivity.*
Тһе 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.
3-9
Advertisement
Table of Contents
