HP 61 - Calibration Procedure

Appl. Note 61
For coaxial testing use the % 780 series Directional
Detectors* (shown in Figure 2) to complete the level-
ing-feedback-loop from RF to the sweep oscillator
leveling amplifier. Frequency response of these de-
vices for 1 to 2 and 2 to 4 Gc ranges is +0.2 db.
All % 752 series waveguide couplers have a RF fre-
quency response of better than £0.5 db. To minimize
this frequency error in making transmission meas-
urements use the coupler curve matching setups
shown in Figures 3 and 4. Figure 3 shows how the
detected RF response of one coupler is used to adjust
sweep oscillator leveling. This compensates for the
detected RF response of the coupler driving the oscil-
loscope, if the two sets of coupler curves are identical.
Frequency response curves of 752C couplers usually
match within one or two tenths of a db. The 752C/
424A system used as a detector presents a main guide
SWRof 1.05to the device under test and therefore re-
duces measurement mismatch error. The 424A De-
tectors have a response of +0.2db through H band and
+0.3 db in X band. The response of M and P band
units is 0.5 db.
For making high-loss transmission measurements,
Figure 4 shows an arrangement for removing coupler
attenuation between the device under test and the detec-
tor-indicator. The result is a 10-db improvement in
sensitivity over the Figure 3 setup. This system also
employs coupler frequency response curve compensa-
tion, but needs the factory matched E01-752C arrange-
ment in order to hold coupler matching limits within
one or two tenths of a db. Knowing your measurement
frequency range, select the proper sweep oscillator,
directional coupler or directional detector, and other
instruments from Table IV.
For the oscilloscope detectors shown in Figures 2, 3,
or 4, always use the load resistor purchased under
423A or 424A Option 02, because detector and load
have been optimized for square-law voltage response
(linear power). The 423A or 424A used with its indi-
vidually factory-mated load has a square-law response
within +1/2 db below 50 mv DC output as measured
on the oscilloscope. When square-law loading is re-
quired for the detector, use only the factory-supplied
load furnished with each Option 02 detector. Identifi-
cation is easy because load and detector bear identical
serial numbers. The detector used for leveling need
not be square-law loaded because under closed loop
conditions, level always remains the same.
CALIBRATION PROCEDURE
With the instruments set up as shown in Figures 2, 3,
or 4, connect coax or waveguide together, in place of
device under test. Oscilloscope horizontal and ver-
tical amplifiers should be set to respond to DC, and
the vertical attenuator should be set to a convenient
multiple or fraction of ten. Of course, the maximum
power applied to the flat detector should be within the
square-law range. Filters, pads, and adapters with-
in the RF leveling loop will not affect the frequency
response of the leveled RF output.
* The directional detector combines directional coup-
ler and microwave crystal detector thereby avoiding
the SWR ambiguity of connecting the two instruments.
Page 3
Transmission
Attenuation
in
db
1.4 1.5 1.6
Frequency in Gc
Figure 5. Shows 0, 3, 7, and 10 db Calibration
Lines and the Insertion-Attenuation
of a 1200-Mc Low-Pass Filter
With Sweep Oscillator Power Switch in "STANDBY''.
Cut transparent ""db" scale from foil. Place scale
on oscilloscope graticule as shown in Figure5. Use
either static charge or several small spots of rubber
cement to hold scale in place.
Adjust sweep oscillator frequency limits.
Set oscilloscope horizontal gain control to give de-
sired calibration in Ge/em. The sweep oscillator
sweep circuits must be operating for this adjustment.
Set oscilloscope vertical position control to estab-
lish the base line on the CRT screen for « attenua-
tion. For use with the ""db" scale in Table III, the
base line should be exactly 3 cm up from the CRT
graticule center line.
With Sweep Oscillator Power Switch in "RF"'.
- Adjust sweep oscillator for leveled sweep as dis-
played on oscilloscope.
Check flat detector for overloading. Detector out-
put as measured on oscilloscope should not exceed
50 mv. If voltage is excessive, lower sweep oscil-
lator output power or insert an appropriate pad be-
tween RF and detector.
Set zero db calibration line exactly 3 cm down from
CRT graticule center line, using oscilloscope ver-
tical attenuator and vernier.
Recheck zero and « dblines again as outlinedunder
the previous steps. Also, be sure oscilloscope ver-
tical DC balance is in proper adjustment sothe base
line calibration will not change with various oscillo-
scope attenuator settings.