Modifications Needed to Improve MIL-STD-462 Test Method CS03

M i l i t a r y r a d io r e c e i v e r s a r e r e q u i r e d by MIL-STD-461 t o comply w ith r e q u i re m e n ts CS03 ( In te rm odula tion) , CS04 (Rejection of Undesired Signals) and CS05 (Cross Modulation). Each of these requirements appears to t e s t a p o t e n t i a l l y important s u s c e p t i b i l i t y c h a r a c t e r i s t i c o f radio r e c e iv e rs . The implementation of Test Method CS03, p rescribed in MIL-STD-462, does not accomplish the intended t e s t d e m o n s t r a t io n of in t e r m o d u la t i o n c h a r a c t e r i s t i c s . In the ea r ly days of rad io , rece ivers used p a ra l le l tuned c i r c u i t s to e s ta b l i s h s e l e c t i v i t y and d i r e c t de tec t ion of the amplitude modulated signal in to an earphone. These "c rys ta l se t s " as they were known were s t i l l popular in to the middle of t h i s century among boys and hobbyists , but the poor s e n s i t i v i t y of the c ry s ta l se t soon made signal am plif ica t ion p r io r to envelope d e tec t ion d e s i ra b le . For many y e a r s , the prede tec t ion am plif ica t ion was performed a t the tuned frequency of th e rad io and some rece ive rs had as many as th ree or four tuned am p lif ie r s tages in cascade. The complexity of tuning these rece ive rs was inc re d ib le and often a bewildering array of "trimming" or f ine tuning con tro ls occupied the receiver f ron t panel. These tuned radio frequency or TRF rece ive rs performed well but had complex in terna l mechanical coupling sys tem s t o keep a l l o f th e tu n in g s t a g e s in synchronism. In 1918, Edwin Armstrong revolu t ion ized radio receiving with the invention of the superheterodyne p r in c ip le . In a superheterodyne, the received signal i s immediately combined or "mixed" with a tuneable o s c i l l a t o r s ig n a l . The output of the "mixer" i s passed through a s e r ie s of fixed tuned am plif ie r s ta g e s , which s e le c t the mixing product to be am plif ied . These fixed tuned s tages operate at what i s known as the intermediate frequency, or IF, and c o l l e c t iv e ly form what has come to be ca l led the "IF s t r i p " . The ouptut of the IF s t r i p i s d e tec ted , am p l i f i e d and p ro c e s se d in w ha tever manner i s appropr ia te to the rece iver type . The f a c t t h a t the IF i s a f ix e d f requency makes p o s s i b l e th e o p t i m i z a t i o n of g a i n , bandpass f i l t e r c h a r a c te r i z a t i o n , s e l e c t i v i t y , e t c . In a d d i t io n , the "down conversion" of high frequency s ig n a ls , makes p o s s i b l e th e r e c e p t i o n o f s i g n a l s a t frequencies much higher than might have otherwise been poss ib le s ince the IF i s normally much lower in frequency than the received frequency. The superheterodyne p r in c ip le i s probably the g re a te s t s ing le advancement in radio technology and i s used in almost every radio and radar system in ex is tence today. The problem with the "superhet" however, i s spurious responses or simply "spurs" . The basic formula governing the heterodyning of two s igna ls is f IF B < mfR ± nfL < f IF + B Where f R i s the input s ignal frequency f L i s the local o s c i l l a t o r frequency f j F i s the immediate frequency B i s the bandwidth of the IF m and n are p o s i t iv e in te g e r s . Any values of m and n which s a t i s f y the ineq u a l i ty will r e s u l t in an output from the r e c e iv e r . In f a c t , one of the most s ig n i f i c a n t source of spurs are due to those mixing products which s a t i s f y the in e q u a l i ty where the values of m and n are something o the r than the d es i re d . This type of spur i s the sub jec t of MIL-STD-461 t e s t requirements CS04. Another se r ious spur i s the fo r tu i to u s mixing of two or more s igna ls simultaneously presen t a t the antenna p o r t . These s ig n a l s , will combine in the mixer stage in much the same manner as the local o s c i l l a t o r and des ired signal al though normally with f a r l e s s e f f ic ie n c y . The a b i l i t y of the mixer and consequently the radio to r e j e c t these fo r tu i to u s combinations i s re fe r re d to as the rece ive r two tone intermodulation re je c t io n and i s o s tens ib ly the sub jec t of MIL-STD-461 t e s t requirement CS03. The most w idely a c ce p te d two tone intermodulation t e s t requ ire s two s igna ls applied a t the input of the un i t under t e s t (UUT), spaced such th a t equation 1 i s s a t i s f i e d f spur = f Q = 2 f j f ? (1) where f spur i s the " th i rd order intermodulation product" f i s the tuned or cen te r frequency of the UUT f^ is the tuned frequency of generator No. 1 f 2 i s the tuned frequency of generator No. 2 The t e s t methods normally used in t e s t i n g components such as mixers and am plif ie rs i s e i t h e r a center frequency t e s t or a spot check of a few f requencies . Normally, the r e s u l t s of a two tone intermodulation t e s t are not highly frequency dependant. This ob­ se rva t ion coupled with the d i f f i c u l t y of scanning two generators simulataneously, one a t a r a te twice the o ther ( in order to keep f spur equal to f ) has made swept or scanned two tone intermodulation t e s t s r a r e . Test Method CS03 of MIL-STD-462 s t a r t s ou t exac t ly as described above and r e s u l t s obtained a t the i n i t i a l frequency se t t in g s are an accurate r e f l e c t i o n of t h e two to n e in t e r m o d u la t io n c h a r a c t e r i s t i c s of the UUT. However, the t e s t opera tor i s in s t ru c te d to inc rease the frequency of generator number 2 alone un t i l a given upper l im i t i s reached. The r e s u l t a n t [2 , -1] spur with which we s t a r t e d , now moves down in frequency a t the same r a te generator number 2 moves up un t i l the spur i s ou ts ide the s e l e c t i v i t y of the rec e iv e r . At t h i s po in t , the number of subsequent spurious responses and t h e i r i d e n t i t y (m and n values) becomes complicated to p red ic t and the v a l i d i t y of any data taken i s very doub tfu l . The t e s t ac tu a l ly degenerates to a psuedo CS04 t e s t U.S. GOVERNMENT WORK NOT PROTECTED BY U.S. COPYRIGHT 32 SESSION 2A since most rece ive r responses recorded are s ing le signal responses and are not r e la te d to the two tone intermodulation c h a r a c t e r i s t i c s of the rec e iv e r . I d e a l ly , the frequency of both Generators Number 1 and 2 should be increased a t the same time and at r a te s such t h a t the formula f spur = f = 2 f , f 2 i s continuously s a t i s f i e d . A s im i la r argument app l ies when f^ and f 2 are below band. The symmetry of the s i t u a t io n i s c l e a r and the below band argument wil l not be pursued. P rio r to the advent of syn thes ized , computer c on tro l led s ignal sources , such a t e s t was extremely d i f f i c u l t t o im plem ent. Now, however h ig h ly a c c u r a t e s y n th e s i z e d g e n e r a t o r s can be s e t to sp e c i f ic frequencies via the IEEE 488 (or o ther) instrument bus and a program to s tep the p a i r of generators through the required t e s t range can be e a s i l y w r i t t e n . The maximum step s iz e permissable must be s p e c i f ie d , but i s not t e r r i b l y c r i t i c a l s ince two tone in termodulation responses tend not to be highly frequency dependent. The wording change to MIL-STD-462 Test Method CS03, needed to accom plish th e above s t a t e d o b jec t ive i s given in Figure 1. The second improvement needed in Test Method CS03 i s in the determination of the upper (and lower) frequency l im i t s of the t e s t . C urren tly , the t e s t must c o n t in u e u n t i l G en e ra to r Number 2 r e a ch e s "a frequency equal to ten times the rece ive r tuned frequency or 10 GHz whichever i s l e s s " . This c r i t e r i o n seems r a th e r a r b i t r a r y and ignores the s e l e c t i v i t y of the re c e iv e r . I f one accepts the premise th a t the in t e n t of the method i s to t e s t the n onl inear elements (mixers, sa tu ra ted a m p l i f ie rs , e t c . ) f o r t h e i r c o n t r i b u t i o n to t h e two to n e intermodulation response of the system, one must use only those methods t h a t wil l apply two equal s igna ls to one or more non l i n e a r elements in the system.