HP 416A - Page 47

b , In b r oadba nd operati on : b eca se of the s qua n ~-Ja.v r n atu :re of the de· ·
te e to r i nput / output c ha.r ac t er ... st ic, variat ions wit h :f re que n c y :i n the :d
inpu t to the de t ector will re s ul t i n n otic e ab ].e v; ?. :::· i a t::io:. H!S (:ripp:i::) :i n the:
fovel oi t he de t e cted out p ut voltag e"
Spurious Signa l s Due to De tect or Mi srnat c h --
Re flec t io ns -
The d e te c t or s ca nn ot b e perfe ctl y matched to the t r a nsm i ss i on sys ten1, .a n d.
therefore th e re will be so n1e reflec tion . Frm n t he in ci de nt dete <C t o:r- , the re fle ct io n
is re -re flected at t he ge n era tor and co u. p1e d b y th e di r e c t i on a l c oup le r ba c k ~i. n to
the incident d etecto:r. From the reflecte d detector, th e re fl ection is re - reflected
at the load a n d co up led by th e dire c t i ona l cou p le r ba c k in to th e r eflec te d d et e ct c r.
N OTE: T h e s e error s i gna l s (F
3
an d R
3
, respe c t ive l y } a re d i s cu ss ed :~ n
g reater d e t ail und e r ~_pu~· ).ou~ _?jgn~ I . ~'..! b el o w.
Op e_::.ati ng C ~ ~ d era t ~o ns _ -
D i :rectio na.l Coupler s : Wh e n a 20-db d irec t io na l c oup ler i s u se d· :m fr .e
incident c h a. nn er;--r h. e'"er:r.ord ueto d etect or :r efle cti o n w i ll b e sm all b ecau s e the
spur i ous s ig na l '.xri.H b e atte n uate d 1 0 0: 1 whe n pa ss i ng th r ou gh th e c o u p ler t wic e.
Since opera ti ng co n diti on s i n t he re flec ted c h a. nn el li m i t th e a mo \: n t of a t te n u at io n
which can b e to lerated, . co mpa rativ e ly little atte n uat i on i s a v aila b le i n t he refl ec t;;:;d
channel to redu ce t he arnpHto..ide of the d et e ct o 1~ m i sm a t ch e r r or s i gn a l. Wr .ce n a ·
10-db coup l er is us ed, th-i; •:.:rrcn:· s :i. gn 2.l vrlH b e atte nu ated on ly 10 : 1 a n d the :-.:f;.fore
will be gr ea t e n ou g h to c ause a not icea b le ri pple in the det e c tor ou t pu t s i n ce t he
error sign al is ou t of phase; ·wi th th e s a mp l e d reflec ti on a nd a t t i mes a d d s, .a.t other
times subt r act s.
Si gn al Ge n erat o;:-; To rn1n1r.ai z!=' r;e£1ections , t he s i;;na.l ;.1,et1er<:ito ::' and
the dir ectio na l caup l er sEC: ;D~-Ld be rea so n ab :i .y 'V e l.7 . matche d. Wh en a si . gnal S'.J urce
such as th e l'vfod el 670 HM. o s cilla tor i s u se d$ th e l ev el of the o utp ut sign al ca n b e
made suf fici e n t l y h i gh to pe! Tn it _;_ ns er t io n of a 6 - to 10 - db pad b etvvee n th e s y stem
and the s i gn a l s ourc e. Wh e n a pa d ( su ch a s a H e-,vl e tt- Pac k ar d Mo de l X3 82 .A
Broadband Prec ision Wa ve g u:de Att e n uator, w hi c h i s d e s i gn e d t o p re s e n t i m.pedance -
matching cha racteri s tic s to t he refle cto meter } i s u s e d, spu ri ous s i gn als r e l ate d to
generator reflection s ( s ee Spu riou s S i gn a l s , be l ow) can be minimized.
Errors Due to Differences in Re c tification Efficiency Between Detectors
The a c tual d etected output from a crysta l detector will vary from unit to
unit and also with frequenc y .
'
NOTE : To min i m i ze the wide range of differences in characteristics
which can exist between one crystal detector and another, the
Model X421A detectors scheduled for reflectometer applications
are c a refull y tested and matched at the factory in order to provide
so f a r as possible a pai r of detectors with similar characteristics.
In single -frequency operation, the variation between the levels of output
voltage from two d etector s operated at the same input power can be overcome by
adjustment of the SET TO FULL SCALE control on the Model 416A. The range
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