HP 416A - Page 44

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b. Set the SELECTOR switch at the SWEEP position.
2. Perforrn steps 2. through 7 of the broadband calibration procedure
given fo r ope ra ti on in the X band. These instru c tions n1ay be found
under Calibration {100% Reflection, Broadband}. above. Controls on
the Model 670HM and the Model 67 OSM are similar in designation and
function.
Beam Voltage and Current, Model670 SHF Oscillators ~-
Beam
Beam (Cathode)
Model
Volts
Current
67 0H
J. ,. 000
20
ma.
670 HM
1,0 00
20 ma. I
670S
80 0
- 900
14
-
16
ma.
670SM
800
- 900
14
-
16
rna.
Factors Which Affect Accu r acy of Refl~ctometer M_easurements --
Ge ne ra l - In any measu re rnent systern operated at micro wave frequencies ,
there are a number of pl aces w he re err ors may be introduced into the system .
Differences in character i stics b e tw ee n co rresponding components in the incident
and reflected c h a nn e ls , l oss e s , a nd spur i ous signals all contribute to po ss i ble
inaccuracy of the ratio ind i c ate d by the M odel 416A meter.
In practice , probab le er ror generally i s not a total of th ese possible er-
rors because all e rro r s a r e no t in th e same direction and are independen t, and,
therefore, the probability of err o rs canceling is large. However, in a ny m icr o- -
wave measure1nent system some er ror will be present.
The essence of good practic e in using any systern is to ernploy te ch n iques
(and utili ze available acces so rie s ) to n1inin1ize o r remove the effec t of th e error
inherent in the system . F o r tun a t eLy t he reflec to r neter l ends itse lf admirab ly t:o
techniques w hich min imize er2r ors considei·ably. The SET TO FULL SCALE
control on the Mode l 4 16.A , fo r exan1ple, provides a me a ns by w hich loss e s in
the system may be overc ome in general at the tirne of calibration, by i ns erLon
of suffici e nt :::; aini .c ~:_::.e ::cefl ec te d cha nn el to bring the reflecte d v ol tag e up to th e
level it would h ave in a l oss le ss syst e rn. Anot h er technique that rnay be us e d ,
which includes the use of a vailab le a u xilia ry equipment (a slide scre w tun e r a nd
moving loa d) , v..rill canc el out th e e rror signal which is due to finite di rectivity
oi the re fl ected d irecL on ; ;i_ ;_ c oup -e r. ·r h e s e a nd other a pp li ca b le techn i ques h ave
been :i nd ica t ed as p e rt:me nt th r oughout the ope rabng inst r uctions.
T he rnagnitude of e ~r.ror in th e syst e rn , as indicated in Fig. 10, is r- e-
l a ted to the rn E .gn i tud e o :t th e reflE ::c ti on co· ~ ffi c i ent and i s much _e s s w hen n1eas -
uring reflection coeff i ci e nts of a l ow magnitude than when me a suring high re -
fle ction coefficients.
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