high-perf.pdf

high-performance

spectrum analyzer

A spec tr um

ana lyzer

is a

rad io

rec eiver wi th

a swe pt

local

oscillator

that

allows

conti nuo us tu n ing over a

speci fied

freq u ency

ran ge. Th e receiv ed sign a ls a re d is-

play ed

o n

a con vent iona l oscill oscope as

p ips.

Fig. 1

sho ws th e radio spect rum

bet ween

100 k Hz and 100

MHz

th e

Sa n

Fran cisco

area

as d isp lay ed

o n

t he

These instruments

have been limited mainly

to professional labs -

now you can

build your own

spectrum analyzer

from the information

furn ished here

spect ru m a na ly zer d escr ibed in t h is arncte .

Because o f t hei r co st and co m ple xit y. goo d spec tr u m

analy zers

ha ve

been

lim ited

p rim arily t o r esear c h and

developm ent

act ivit y.

So m e

m ilit ar y

ins t ru ments h ave

sh ow n

o n

t he su rp lus ma rket , bu t po o r sens itiv ity

and

se lec tivity.

ima ges ,

sp uriou s

responses ,

and

po or

d y nam ic range have lim it ed th e ir use fu lness.

Co nsiderable l ime was spe n t in the d esign o f t he

spectr u m ana ly zer described h ere . Eve n more t im e was

requ ired to assu re c ircui t re producibility . min im ize t he

v a ri e t y o f pa r ts , a nd sele ct th e least-e xpen sive

co m po ne n ts. No PC boards are u sed in Ih e de sign no r a re

any p lanned . PC board s in t he rf and i·f stri ps. unless

very ca re full y made, wo u ld p ro b ab ly d egra d e t he pe r-

fo rmance o f th e inst ru m en!. Com plete d esign, co nst ruc-

t ion , a nd tes tinq d e t ails ar e incl u de d fo r t h ose w ishing 10

bu ild th e spec tr u m a natv zer .

applications

Th e spec tr u m an alyzer can be used 10 observe

ha rmonics, para sitic os cilla ti on s , and side ba nds o f CWo

a-m. fm or

ssb signals.

Propagat io n conditions in ter ms

Way ne

Ryd er,

W6URH,

115

Hedge

Road,

Menl o Par k, Californ ia 94025

16 rtM june 1977

o f sta t io n act ivity can be observed by looking a t the

num ber of signa ls on an ama teu r band. Th is suggests

ano t he r use fo r O X and contest o perati ng : Yo u can

immed ia te ly see wher e th e pileups are wit hout cra n king

the rece iver t-.n ing dial back an d fo rt h. Th e spect ru m

ana lyzer d escr ibed in these pages was b uilt to lo ok for

t he so u rce o f bird ies in a n ex pe rime n t al gen eral·co verage

rad io rece ive r.

The co nve nt io nal way to o bserve rad io signals is in

t he t ime d om a in on an oscilloscope . Th is is a good

me th o d when no harm o nics, par asit ic osc illat ions, Of

o t her signals are pre sent . Ho wever, ampl if ier s can osc il-

la te , mod ulato rs or mixers m igh t not re ject signa ls beinq

modulat ed, oscillat ors ma y be oscillatin g on mo re t han

o ne freq uen cy , o r d et ectors may be passing the signal

being detected . These signa ls might appe ar on a n osctuo-

sco pe in th e l ime d o ma in as co nfusing pictu res, as sho wn

in fig. 2A. Fig. 28 shows a signal a nd be hind it t he

seco nd ha rmo nic. This is th e freq uenc y d o ma in. If a

signal fundam e nta l is on 5 MHz and th e seco nd har-

mo nic in o n 10 MHz, th ese appe ar on a spec tr u m anatv-

zer as pips at 5 an d 10 MHz. Instead o f see ing a comple x

wave for m in th e ti me d omain as on an osci llosco pe, t he

signal is viewed in the frequ enc y do main. The a mpli tude

and fre qu ency of eac h co mponent can be seen (f ig. 2e ).

.. -

three kinds o f spect ru m ana lyze rs

Fig. 3 shows a real-t ime a na lyzer. The inco ming signal

is co n nec ted to a series o f filte rs fo llo wed b y de tectors.

A sca n o r swee p ge nera tor d rives t he hc eizo n ra t plat es of

an osc illoscop e and co nt ro ls an elec t ron ic switc h, which

selec t s th e p rop e r d et ect o r o u t p ut . The freq uency range

is limite d by t he nu mbe r o f filte rs and t hei r ba nd wid th.

T his ty pe o f ana lyze r is q uite ex pe nsive beca use of t he

la rge n u mbe r o f fi lters req uired t o co ve r a spec t ru m. Its

ma in ap plicat io n is in t he audible o r su ba udibt e range.

An o th er t ype is the t u ned rad io -fre q uen cy analy zer

show n in f ig. 4 . The in p u t filte r is a t u nable bandpass

fig .

2 . A f u ndlml ntal· fr e qulnc y

olg n al a n d Ito oi con d

fI..monl c

II oho w n

o n

ooc iliooc o PI

Ii'll

II m l

d o m a in .

A , T h l

u m e

olgn a l a n d

Ito oe c o n d

I'oarm o n lc a 'i Ol'oow n

In I i'l l l re q u l n c y

d oma in

m u " ..11 I fil l.

rel a ll o n"·"po.

T I'o I

1p lcl . u m a na ly u

diopla~1 a

fu n d a m l n l a l

019na l

I n d

Ito

OIcond

fI.,mo nl c

I he

l re q u enc y

d oma In

II 11'o 0wn

A mpli ludl an d

f req uln cy

0 1

c fl c a n III n e n .

filt er, whic h is tun ed by t he scan gen erato r. The TR F

anal yze r lac ks resolu t io n and sensitivi ty . Also , t u nab le

filte rs usua lly d on ' t have constant band wid th . The TR F

analyz er is used ma inly fo r mic rowave ana lys is. There

are ot her t yp es of analy zers, suc h as a d igit al p rocesso r

th at

perf o rms

Fo u rie r

ana lysis,

but

th ese

are

very

com ple x.

The most co mmo n ty pe o f spec t ru m ana lyze r is the

su perhe te rod y ne , as shown in fig. 5 . It is simila r to a

sta nd a rd su perhe terody ne receiver with th e fo llowing

e xcept io ns: There is no t u ned circuit in the fro nt end,

because it wo uld be d iff icult t o ma ke o ne c o n t inuou sly

t unable from 100kHz t o 100 MHz, an d t he loca l oscine-

to r is e lec t ron ica lly tuned by th e scan gene rator fro m

th e oscillosco pe. The su pe rhe terod y ne analyzer has the

advan tage of a wid e lu ning ran ge and co n t rolled ba nd '

wid t h . Also t he sensitiv ity c an be mad e un ifor m. Image

problem s are min imized by tr iple con ve rsio n.

Fig. 6 shows t he spec t ru m ana lyze r d esc ribed here.

No mo re t ha n 1 volt r ms sho uld be app lied to t he inp ut

att enuat o r. The o u t pu t f ro m the ane oueto r sho uld be

limite d to abou t 10 m V rms ma xim um . The fu nc t io n of

eac h assem bly

li g . I. Dlopla~ Ol'o owln g 11'0 1 Sa n Francloco ·aru ra d io Opl cl,um o n

I fle opect , u m anal~u,. A T V a n len n a wll u nd t o ..ce .... I fle

Oi9na l0 o n I he .lg l'll I'oa ll 0 1 Ifl i diopll ~, a nd 11'0 1 T V a n len na In d

w ll u oe d II a lo n g· w l.. anlenna 10 ..c e i.. oignalt o n 11'0 1 li lt flail.

is d iscussed

with

refe rence to Ihe block

d iagram .

l ate r,

theo ry

o f

ope rat io n,

circuit

d esign,

june 19 77 UAl17 '

construction ,

a nd

che ck o ut

ar e

described

for

each

asse m b ly .

The in p ut attenu a tor is fo llo wed by an LC 130 MHz

lowpass f ilt e r. Its pu rpose is to prevent inc o m in g signal

harmon ics fro m mi x ing wi t h the f irst lo ca l oscillator and

producing sp u r io us responses. The inco m in g signal, 0.1 -

100 MHz, is mixed with th e vco to produce a 200-MHz

first i-f. The vco is driven by a swee p sh ap e r that predis-

torts th e sa wt ooth w ave f rom the oscilloscope to com-

pensate fo r tuning

INPUT

nonlinea r it ies of th e var a c tor tuning

diod e in t he vco .

Th e 200-MHz i-f ou t p u t fr om th e f irst m ix e r (all

m ixe rs a re doubl e bal anced) goes to an ampl ifie r, w h ich

c o m pensates fo r th e m ix er lo ss. Next is a 20a-MHz

bandpass f ilt e r followed by the sec on d m ixer. Th e

200-MHz i-f signa l mixes wi t h th e second lo c al oscillator

output to p rov ide a 50-MH z i-f signal. Th is signal is

amplified and applied to a 50-MHz bandpass filter. The

output of th is bandpass filter is mixed with 39.3 MHz to

p roduce a third i-f of 10.7 MHz . The 10.7 MHz output

fro m th e t hird m ixer is then am pli fi ed a nd fed through a

250-kHz bandpass ceramic f ilt er . Th is stage is followed

by an i-f attenuator.

Next ar e t w o identical c rystal filters to p rovid e 1- and

10- kH z ba nd w idths. Th ese fee d a ga in eq ua lizer , whi ch

compensates fo r the d iff e renc es in gain b etween the

var iou s b a nd w idths. Compl e t ing the i-f se ct io n is an

ampl if ier that d rives a second 250-kHz bandpass ceram ic

filter.

The loga r ithmic amplifier compresses the output sig-

nal so that a la rge -amplitude signal can b e displayed on

an osc illoscope. Two d ivision s on t he sco p e correspond

to a change of 10: 1, o r 20 dB . Six d ivis ion s represent a

change of 1000: 1, or 60 dB. Without th e log amp, a

CRT abo ut 30 inc hes (76cm ) t a ll wou ld be required to

d isplay th e same info rmation w ith th e sens it ivity ava il-

ab le at th e to p of th e di sp lay . A vid eo f ilter is included

to re m ove h igh ·f reque ncy n oi se in narrow-ba ndwi d t h

operation. Th e performa nc e specifications f or the spec-

trum analyzer ar e listed in table 1.

FREQ U EN CY

fig.

real-time

spectrum

analyzer.

Filters

are

spaced

provide continuous coverage of the frequency spectrum.

Th e d ispl ay scope fo r the spectrum analyzer should

be dc coupled, have 0 .1 V/ di vis io n ve rt ical sensit ivity,

and h ave six d ivisions of vertical ca librat ion by ten

divis ions o f horizontal cal ibration. Also app rox imately

6V of ho rizontal sawtooth voltage should be available to

drive th e spe ct ru m analyzer from the scope. Scope input

im pe da nce shou ld

b e

1 megohm and in p u t capacitance

10 ·40 pF .

circuit description

Th is

secti o n

presents

the

theory

operation

and

des ign

d etails of each o f th e major circu its in the spec-

trum

analyzer.

rec o m m e n d ed

that

the

following

text

read

before

attempting

bu ild

the

c ircu it

modul es

as much

information

is provided that w ill be

helpful w he n

construction

is started.

Especially im p or -

tant

a re

t he

suggestions

decoupl ing,

shielding, and

bypassing.

Sweep shaper. The sw eep-shaper schematic is shown in

fig. 7. The resistors in the input ci rcuit, which are

switched by 5601 A, determine the sweep width. In the

1a -MH z/d ivision posit ion , the sweep c enter f requency is

set to 50 MHz by R 101. In all other posit ions the sweep

center f requency is set by R601. R 104 sets the low-

freque ncy lim it , 0.1 MHz , and Rl02 sets the h igh-

frequency limit, 100 MHz. Rl03 is a f ine frequency

adjustment w ith about 500 kHz of range. Ul01 provides

isolation

.s p e c t r u m

table

Performance

specifications

the

W6 U R H

analyzer.

frequency r a ng e :

se n si tiv i t y :

selectivity:

0.1

MHz - 100 MHz.

10 IlV or l ess in the I -kH z posi t io n .

Approxi m a te ly

1 ,

10,

o r

25 0

k Hz (s w itc h-

abl e) .

sweep wid th :

10,

100,

500,

1 0 00,

an d

10,000

for the in p ut attenuator and some ga in . U102

divi sio n.

th e

10,000

kHz

provid es

isol at io n

fo r th e frequency control. The out-

M Hz/divisio n p os i t i o n,

t h e cente r f req ue nc y

puts

U101

and

Ul02 are resistively combined and

i s

set

MHz. Otherwise,

f r eq u e nc y

controlled

the

center

c o n t r o l

the

drive

U 103,

which

nonlinear

amplifier

that

front pan el.

f irst LO frequency jitter: '" 5 k Hz

signal separation : 10 kH z f o r 40-dB r eso lu ti on i n I -kHz p o sitio n .

complements

the

nonl inearities

the

tuning

diode,

CR203, in the vCO.

As the frequency is inc reased , more voltage is re-

qu ired. If a change of 1 volt is required to go from 260

to 270 MH z, a change of 1.3 volts may be requ ired to go

from 270 to 280 MHz. Fig. 8 gives an id ea of the

distorted wavefo rm ava ilable f rom U 103 and the actual

wav ef orm req ui re d

input impedance : 5 0 o hms

response 100 kHz - 100 MHz : ± 2 dB

frequency accu racy : ±3 M Hz.

vick!o filter : 1 0 a n d 1 k Hz.

shape factor 3 - 60 dB:

t o

compensate fo r

no n l ine ariti es of

2 5 0 k Hz 2.5 : 1

1 0 kH z 50 :1

1 k H z 30:1

the

tun ing

d iode

the

vco.

Compensation

is usually

req ui red

between

270 and

300 MHz . When the voltage

18 ~ june 1977

' ""'

-~.,,"'~ - -

Rf sec t io n . Th e rf sec tion (fig. 10) ccn tams th e first t wo

j.f stages o per a t in g at

200 and

50 MHz . Th e th ird

i-t,

L r---1

o pe rating at 10 .7 MHz, is contained in th e i· 1 section,

Th e in pu l s igna l is applied 10 the first m ixer. MX301 ,

th rough an rf auenue tor and a lo wpa ss IiIter. The

artenuator is required to keep th e input t o th e first

m ixer bel o w approllima tely 10 mV . MX301 eombines

the 0 .1 · 100 MHz in pu t WI th th e 200 · 300 MHl signal

from th e vee t o pr oduce the f irst i·f 0 1200 MHl . 0 301

pr ov id es ga in 10 compensate fo r t he lo ss th roucjl

MX3Q 1. l305 th ro ugh l307 lo rm a 2oa-MHz bandpass

filt e r. Ou tput from t he 150-MHl local oscittetcr , 0305.

m ix es WI t h th e 200·MH z signal in MX302 t o produce a

5O·M Hz i-t , Th e 5O·M Hl signa l is am p lif ied by 0302 and

0 304 t h en appl ied to th e th ird m ixer . MX303, th ro ugh a

tou r-sectio n bandpass

' ''. .. . TII" a "

•• d io

h aou,nc)'

IT A "I

eel."m . ... I)'U' . T un '...

,••ccom p.l"" l d

II )' '''lullln,

1111 •• ' .. q ..... e,..

11>..."

on pin 3 of U103 re aches the voltage on t he right side o f

R 115. gain will be incr eased by th e set ting 0 1 A115 . and

so on up th e line, 10 R l 06. R11 5 se ts Ih e nonlinearity at

10 M Hz and A l06 at 100 M Hz. The l uning di od e used in

th e vee I bu il t .....as l inear u p to abo ut 280 M H z.

Care must be tak en so Ihal no low-frequ en cy co m-

porten ts , inc luding po we r-su pplv h u m , a re a d de d t o the

inc o m ing saw too t h wave by t he swe ep sha pe r. Added

lo w -freq uency components will resul t in l irst loca l-

oscillat o r inSia bili t y . Th is sho ws u p as jitter in th e d is-

pla y wh en look ing at narrow sca n w idths.

Vo llage-eon tfolled osc,lIator. 020 1 and 0 20 2 together

wit h l:101 , l 20 2. and CR20J 101m a 200-300 MHz

vollaqe -<:ontroll ed os ci llat o r j vcc - f jg . 9 1. 0203 is a

buffer amplif ier . 0204 and 0205 p r ov ide o u tputs to the

f jrSI m iller and a secon d ou l l)U t .

Several co m pr omises wer e made in th e d es ign of the

vco. To acmeve h igh frequency stab ilit y. th e c scurator

should ha ve a h igh ell ra t io ; h owever. to tu ne it wi t h a

varector. it low e l l rat io is required. Va raClor circ uus

wilh rea sonably h igh 0 at th ese fr equencies h ave a

relal ively sma ll lUning range. When conside r ing t uning

range , line a rit y . and 0 th e MOlorola Ep icap t un ing

diodes see m t o be about the best . Care ful d es ign o f the

o sc illator an d am pl ifiers was necessary 10 p rovide a

co os te n r output leve l to t he l irst m ixer . Th e 2 N2 369A

d id no t have su fficient qain -ban dw idt h p rod uc t 10

p rovi de constant o u tpu t whe n used as an oscillator n an-

sist or , b ut beca use o f m e low ga lll requu ements o f th e

am p lifiers, the 2N2369A IS sa tistec tc rv in these ci rc ui ts.

filte r.

Th e

signa l

th en m ixed

with

th e 39 .3 ·M Hz o u t pu t from 0306 to pr oduce 10 .7

MHz.

Seve-a! st eps were tak e n 10 m in imize s p uri o us and

image respo nses. Th e in p u t to the pa ss fIlte r a tten ua tes

harmon ics t hat would o th erwise m ix w it h the f irst local

oscillator sign a l, producing spur ious signalS. Using a f irsl

i-f twice, th e h ighest in pu t frequen cy m in im izes ba sic

im age problems . In te rmed iate fr eq ue ncies se par at ed 5 :1

,~~r>-o-1&·~·;·"~Eh

r ,..

I U " L.- ~

II..... ··~I -

.J.

fI, . 5 . SI"' llhfl'd iliaCI< " " , •• m 01 •

, .. p ..... I.'od.,".

, pe et, u m

."., ." ••• Loul oHlIIlIo. It , .. " . "

II., I " .

H. n , . " ...10'.

o r less Iro m th e ne xt l.I fur ther red uce any images .

Double-bala nced, fo u r-diode m ixers p rod uce th e fewe st

unwant ed p roduc ts an d have retativelv h igh toc at-

osci lla to r iso la t io n . Sh ie ld ing and by pass ing pr even t the

t hree local oscilla tors and th e ir harmomcs I rom m ix ing

and p roduci ng spu rio us sign als. For example , With

inadequat e shie ldi n g. th e 39.3 ·MHl local osci lla to r signa l

would lea k ba c k Ihr ough th e Iront end . A loc al signal on

6 meters would be p icked u p by th e sec o nd i·f 150

MHz!. Id ea lly each sec t ion should be in a die -c ast bo x

w it h an rf gask el. but th e construction w ith copper-clad

board , d escr ibed lat er.

adequate

if care is tak en

en sure an rf -light enclosu re .

A temte bead is used on all dc leads on each s ide o f

th e feedthrou!fl. PIst o n mmmers are required for the

2OQ·M Hl ba ndpass fill e r fo r mechan ical r igid ity . l in ks

l 3 09 and l31 1 sh o uld be construc ted so t hey can be

mov ed from 0 to Y.. inc h 10 to 12 .5mml awa y from th eir

respe ctive coils .

I·f sec t io n . The ttiird .mix er outpu t , 10.7 MHl. is am pfi-

fied

by 0401 lfig. 11 I. 0402 p rov ides driv e at 300 oh ms

F l 4 0 1.

Ne xt ,

e mi t te r -folln wee,

0 4 03.

pr ov ides

Clo,. u p " •• w

0 1 I '"

00 111 ' . ": 0"1' 011' <1 o 'cllI.lo • . M"" c l.cu ll Ii

50 o h ms to the i·f at te uue to r. Th e two c rys lal

dr ive at

, ...

co mpl " m ' ''1 10

I".

","I ; c o "n. Clon ' 0' OUIo.. 1 10 , .. .

filt e rs

ar e

id en t ic a l, and

first , Y4Ql , is de scribed.

the

1i~1 m l'"

. " " I , . c k l", , . " . ,.10 ' ••• In c o m p ••l m.'"

0 " 1.11 .

june 19 77 ~ 19

CR402 switches in R401 to broaden the crystal filter for

10-kHz bandwidth. R402 adjusts the gain in the l -kHz

bandwidth posit ion , and R403 adjusts gain in the

10-kHz bandwidth position to the same as in the

250-kHz bandwidth positions. CR403 switches in R403

the

10-kHz

bandwidth

pos ition .

0408

and

0409

prov ide

gain

about

40.

Q409 provides

300-ohm

dr ive to F L501 in the log-amp section.

FL401 and FL501 prov ide 250-kHz bandwidth. After

manufacture, these filters are separated int o groups and

are color coded as to the ir actual center frequency,

wh ich are around 10.7 MHz. Because of spurious reso-

nances above the crystal-filter frequencies, orange and

only orange color-coded filters maybe used. With the

orange-colored ceramic filters , crystal-filter spurious

responses are down - 50 to - 60 dB . With opposite-end

ceramic filters, they may be down only 15 dB.

On e of the more challenging aspects of designing a

spectrum analyzer is selectivity . In its widest position,

the spectrum analyze r should have a bandwidth to

obs erve signals us ing a sweep w idth of 100 MHz. In its

narrowest pos it ion, it should be able to resol ve signals

close together, such as a carrier an d its sidebands. Spec-

trum ana lyzers w ith elaborate sets of carefully matched

crystal filters can ach ieve bandwidths as low as 10Hz at

th e 3 -dB po ints. These crystals are especially ground to

F RE OUE NC y

fig. B. Top curve shows wave

shape available from Ul03 in the

sweep shaper; bottom curve ~

shows actual wave shape required

compensate

for

the nontinear- ~

diode, CR103, i n ~

ity of the tun ing

the VCo.

LF -r-o-r-NC-'-

0404 pro vides a paraphase output, which drives Y401

and neutral izes its capacitance. 0405 prov ides a high

input impedance to the crystal -filter output. CR401

provides a bypass around Y401 fo r 250-kHz bandwidth.

5 0 MHz

BA NDPA S S

FI LTER

IN Pu T

T TENUATOR

TH I RD

M I XER

IN PUT

J V A C

MA X

\~---------

FI G.IO

---

S ECO N D L 0

U50 MH z

S WE E P INP U T

FROM SCOPE

FIG ,9

\~--------------

S£CT I ON ---------------~I

R-F

EQUALIZER

S ECOND

FI RS T

C RY S T A L

F I L TE R

C RYS T A L

F ILTER

GA I N

AND

AMP LIF IER

SHO RT F OR 2 5 0KHz

BAN D W I D T H

S HO RT FOR 2 50KHz

BA N D W IDTH

\~-------------- I - F

S EC TI ON

r--------;:----;:::---::-----;=-----;==J-~.!?.::~!..:!.~--_____7 OU T P U T

S COPE

250 KH z

BANDPA S S

F I L TER

OFF/

1...

l OltHz

-.J

\~-----

LOG

AM P

S ECTIO N ( FIG , / 2 )

f ig . 6. Complete superheterodyne spectrum analyzer described here. Each block represents an Individually sh ielded section or assembly.

20 rnJl june 1977

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