HP 416A - Page 32

-
rd
.....
'""
Q)
Ul
.qi
I.!)
-
r-
-
-
N
-
NOTE : ( Cont ' d)
on the oscilloscope. Referring to t he reference li ne a s a
"go - no go" mark, non - technical personnel ca n rap i dly run
equipmen t through test.
c. Recheck the zero reference {step 8c on p ag e 1 7}.
Sin gle - F requency Operation - -
In
almost all microw a ve measurement syste ms cer tain err or signals are
prese nt . W hen a reflectometer system c a n b e opera te d at a si n gl e fr e quency ,
most of th ese error signals can be compe n sated for by the use of certa in add i tional
equipmen t. Even without the use of such additional equip m en t, som e advantage in
accuracy i s ga i ned in single - frequency operation, bec a u se th e M od e l 416A c an be
calibrated exactly to the value of a k nown reflection coefficie n t instea d of th e b es t
obtainable average, as in broa d band operat i on.
On e of th e error signals presen t in a reflecto me ter s ystem is tha t du e to
imp erfect d i rectivity of the d irectional coupler. The Hewlett-Pac k ar d 752 s eri es
of dir e ct io nal couplers have a high directivity {ratio of forw a r d t o reve r s e p ow e r )
of 40 db o r better , but even with such high directivity the m agnit ud e of the reverse
signa l is important when measuring low reflection coeffic ie nts . By t he u s e of a
slide s crew tuner and a moving load, the directivity of t he reflec ted di re ctional
c ouple r c an be made infinite at a single frequency. Wh en th i s t ec hn i que is used ,
it ma y b e performed before or after calibration. With the s li de sc r ew tuner prope rl y
adjusted, the reflected directional coupler becomes a coupler wit h infin i te directiv i ty
at the si ngle frequency of interest. The procedure for c anceli n g o ut th e directivity
er r or is d e s cribed under Adjustment of Slide Screw Tu n er, below.
N OTE: (l} Setup procedures for single-freque nc y o pera ti on are the sa rne
as that described above for broadba nd op er at i on e x cep t that no
c onnection need be made between the M od el 41 6A and the ver-
tical input on a d - c oscilloscope.
{2} Equipment specified in the followin g proce d ure s i s u sed when
operating in the X band. For operatio n in the S ba nd , procedure
is the same; equipment to be used is specifie d in Setup (S Band },
below •
Calib r a t io n at Single Frequency with Fixed Short
T o calibrate with a fixed short ( 100% r eflection), procee d as de s cr ibed
und er C a l ib rat i on (100% Reflection, Broadband), above, mo d ifyi ng s tep s 1, 3,
and 6 a s indicated bel o w.
NOTE : Calib r a ti on with a sl i d i ng short i s the more a c curate me thod .
1. On the Model 670H (or HM) osci ll ator :
a. Set the FREQUENCY - KMC dia l for the d esired frequency.
b. Set the SELECTOR switch at the MANUAL position.
3. On the Mode l 670H {or HM) , adjust the ATTENUATION co n tr o l wh il e
watching the tuning eye on the Model 416A. The ATTE N UA T I ON control
should be adjusted to obtain an approximately 1 /8
11
gap in th e tu ning
eye shutte r .
- 20-