HP 61 - Reflection Measurements

Page 4
- Using sweep oscillator marker controls, check lin-
earity of your horizontal sweep at major vertical
graticule lines and make any horizontal adjustments
needed.
YOUR AMPLITUDE AND FREQUENCY SCALES
ARE NOW CALIBRATED.
MEASUREMENTS
Place the unknown device under test between the di-
rectional coupler and the flat detector (feeding the os-
cilloscope) shown in Figures 2, 3, or 4. Using the
db scale shown in Table III, the oscilloscope presen-
tation is now a direct plot of transmission loss in db
vs frequency. Use the oscilloscope vertical step-
attenuator to bring the trace as close as possible to
(but still above) the lower 3-cm calibration line, with-
out changing the vernier or vertical position controls.
Note the ratio of steps traversed onthe attenuator, and
from Table I add the related number of db to the cali-
brated oscilloscope scale reading.
Table I. Oscilloscope Scale Correction
with Attenuator Setting
Attenuator Voltage Ratio DB Correction*
1.0 0
.5 3
.2 7
pat 10
.05 13
.02 17
.01 20
* Add this number to transmission attenuation.
Attenuator settings, of course, can be extended be-
yond those shown in Table I using the same system of
correction. If more vertical amplifier gain is needed,
use the 140A Oscilloscope with 1400A and 1420A Plug-
In Units. The photo shown in Figure 5 is an oscillo-
scope picture of a 1200-Mc coaxial low - pass filter
response complete with vertical attenuator calibration
lines. The setup of Figure 2 was used for this picture;
arrows show sweep oscillator marker pips for exact
calibration. Ahigh degree ofaccuracy can be attained
on bothfrequency and amplitude axes. Note the trace-
splitting precision with which amplitude and frequency
calibration lines fall on the graticule in the example
given.
It is reasonable to expect similar performance in day-
to-day service with truly square - law/frequency - flat
detectors, with the sweep oscillator well leveled, with
a modern internal -graticule oscilloscope, and with
stable circuitry throughout the system. The @ instru-
ments shown in Table IV, when connected as shown in
Figures 2, 3, or 4, incorporate all these features.
REFLECTION MEASUREMENTS
Probably one of the most useful functions of a leveled
sweep oscillator is in making rapid accurate reflection
measurements. Typical leveled reflectometer setups
for coaxial and waveguide testing are shown in Figures
6 and 7. The forward-detected signal is used to level
Appl. Note 61
the sweep oscillator. The detected signal reflected
from the device under test is displayed on the oscil-
loscope which is calibrated in SWR vs frequency. The
unknown load on which the reflection measurements
are being made is used as atermination on the through
arm of the coupler system, opposite the end fed by the
leveled sweep oscillator.
For coaxial testing, use the & 770 series Dual Direc-
tional Couplers shown in Figure 6. Coupler arm fre-
quency response is specified not more than +1 db.
However, if the response of forward and reverse arms
match, the sweep oscillator will be leveled to the for-
ward arm response curve in a manner that will com-
pensate for reverse-arm response. Typical forward-
and reverse - arm frequency - response matching for
these devices in the 1 to 2and 2 to 4 Ge range is better
than 0.2 db.
For waveguide testing, two 752 directional couplers
can be placed together to form a waveguide reflectom-
eter. Waveguide coupler matching, as in dual coaxial-
coupler arm-matching, uses the forward-arm response
to compensate for reverse-arm response. Coupler-
arm frequency - response matching for the 752C ar-
rangement shown in Figure 7 is usually within one or
two tenths of a db. Knowing your measurement fre-
quency range, select the proper sweep oscillator and
directional coupler from Table IV.
Always use the square-law load resistor purchased
under Option 02 for loading 423A or 424A detectors as
indicated in the previous section on transmission
measurements. Detector DC output should be held
below 50 mv.
CALIBRATION PROCEDURE
With the instrument set up as shown in either Figure
6 or 7, oscilloscope horizontal and vertical amplifiers
should be set to respond to DC, and the vertical atten-
uator should be set to a convenient multiple or frac-
tion of ten. Power applied to the flat detector driving
the oscilloscope shouldbe within the square-law range.
Filters, pads, and adapters within the RF leveling loop
will not affect the frequency response of the leveled
RF output.
Be sure reflectometer coupler directivity is high
enough so that unwanted forward signals do not inter-
fere with reflected signals. Table II shows the mini-
mum quality of coupler directivity requiredfor meas-
uring reflections. The higher the coupler directivity
above the minimum value, the lower the area of meas-
urement ambiguity.
Table II. Coupler Directivity Required for
Measuring a Given SWR
SWR Measured | Coupler Directivity Required
>1.10 >30
>1.05 >35
>1.03 >40