poor_mans.pdf

PRACTICALLY SPEAKING ... /~v

building the

"po o r man' s

sp ectrum analyzer"

amplified i-f ou t p ut and di sp lay it o n

a 'sc op e, and yo u have a spectrum

analy zer . Shee r qe niu s! I b ou gh t bo th

fo rm s o f tu ner fro m Science Work -

sho p ; f i g . 1 sho ws th e ca ble-read y,

w ide -range modeL '

T he i-f boa rd u sed in th e W 4UCH ar-

t icl e and sold by Scie nce W or ksho p is

shown in fi g , 2, T he term " i-f" used

here act ually m eans a fi xed freq ue ncy ,

sing le-conve rsion superheterod yne fm

receiver tu ned to 45.75 M Hz (t he

tu ner 's i-f ou t p ut fr eq uenc y ), and

d ow n-con vert ed t o t he stand ard 10.7

M Hz used for fm receiver i -f am plifi-

ers . Beca use t he i -f st rip is actually a

sing le-co nv ersio n rece iver , t he ov erall

spec trum analyz er is a dual-co nversio n

su pe rhe t. In fa c t, it ca n be used as a

V H F receiv er if th e sw eep is turned off

(see S weep Onl Otl in f ig . 31.

Th e literatur e that ca me wi t h the i-f

boa rd suggested th at it be we ll shield -

ed , and th at f ced thro ugh capaci tors be

used o n all lead s exc ep t th e i ·f input.

T he shie lded enclos u re is a standard

c hassis box w ith Icld o ve r flanges. Be-

wa re of ma ny "shield ed boxes" now

on the m arket . T he fl anged type

shown in fi g . 2 is minimally accepta -

bl e f or sh ield ed p rojec ts. The t ype of

bo x th at doesn ' t hav e overlappi ng

flanges isn ' t acceptable at all , So me

lM B boxes use little d im p les o n each

edge for su ppor t , so th ey w on ' t p ro -

vide adequate shield ing for m ost rf

pr oject s. W hi le t hey 're fine for audio

and DC project s, th ey leave a great

deal to b e desired at rf .

Bei ng an " o lder g uy " in rad io, I still

ca lled th e teedthr ou gh capaci to rs by

tha t name an d had a diffi cult t im e find ·

ing th em locally; it seems that t hey' re

now ca lled " EM I filt ers," Lu ck ily , a lo-

cal num ber for Newa rk Elec tr onics was

listed in th e Yello w

In Sep te m ber 1986 an exciting a rti-

cle on spec trum analyz ers appeared in

ham radio I " Low- Co st Spec t rum Ana -

lyzer With Kilo buck Features. " page

82 1. Ha ving bee n in bo th c om m unica -

tio ns servicing and engi neering school,

I'd used spectru m analyzers, bUI never

owned one . (Most professionals can' t

afford th em .1 I o nce cons ide red pu r-

chasing a plu g -in spec trum analyzer to

fi t our existing biom edical electro nics

laboratory oscilloscope ma inframe. but

it cos t ove r $ 12 ,0001 Then ca me

W 4UC H and his art icle on th e very at -

fordable WA2PZOt Science W orkshop

" Poo r M an ' s Spec trum A nalyzer . "I I

d ec ided

fig . 1. W id e rllng e . l -VHF / H-VHF /UHF,

voltage-t u ned

b uild my ow n spectrum

" c a b le

r oa dy"

t u nor

analyzer .

The W A2 PZO concept is based on

t he fa ct th at m od ern TV tuners, espe -

cially the "cable- read y " variety , are

varac to r-nm ed . The f am iliar switc hed

inductor tu ner is replaced by a vo ltage -

tu ned ve rac to r oscillator. T w o typ es

are avai lable: one, w hi ch wa s used in

th e W 4UC H article, has separate low-

VH F, h igh ·V HF , and U H F band s. A

swi tch is used to selec t ba nd coverage.

T he second is a wi de -range "cab le-

ready " tuner t hat tunes from low V H F

t h rough UHF television ba nd s in one

0-35 volt [so m e arc 0-30 vol t I ra ng e.

Obviously, if yo u can mod ul ate t he

tu nin g vo lt age w it h a sawtooth wa ve-

f o rm (see " Prac t ically Speaking , "

J an uary , 1987, page 89) , th en yo u

have a sw ep t tu ner. Dem odu late it s

u s e d b y tho a Ul h o r .

fig . 2 . W A2PZO / S cie n c o W orkshop i-I

bOll rd b u ill io l o s h ie ld o d b o_ .

' W A1PZO. Sc,.."CI' W ","hUI' . 1'.0

!lo. 393. f1" lh

Pages , so I wa s

1"' ''1''. N,>.... Vur k 11714

M arch 1987 ~1 99

able to

bu y th em d irectl y from

th e

OL-l oscilloscope t o swee p the tuner.

That approach works if yo ur oscilto -

scope prov ides th is wavefo rm . Bu t

m odern osci lloscopes rarely have th e

sawtooth available on th e front or rear

panels. A lso, ma ny don 't have a

horizont al input. Loo k at yo ur ow n os-

cilloscope's fron t penel. Som e tw o-

channel oscillosc opes have an " X-V"

mode on the vertical selec tor. If yours

does, then one of th e vertical channels

can be re-c onfigured as a horizon t al

channel at th e fli ck o f a sw itc h.

If you don't have a horizont al inpu t.

or X-V capability, yo u can still build the

" Poor M an 's Spectr um Ana ly zer" if

you have either an " EXTERNA L TR IG-

GER" inpu t {m ost 'scopes dol or a

" TRIGGER GA TE" output. T he form -

er allow s an ex ternal signa l, such as

the falling edg e of an externa l saw -

t oot h, to tr igger the sw eep. The latter

outputs a narro w pu lse every time the

oscillosco pe triggers. By allowing the

'scope to sett-tnq qer. yo u get a st ring

of pulses th at can be used t o trigger

cert ain types of sawtoot h gen erat ors.

Science W orkshop makes a board

available l l ig. 3 1tha t can be used fo r

generating and controlling an external

sawtoo th . A lt houg h it suffic es at this

poin t, I' m not t ot ally happy w ith the

design . As I see it. th ere are two prob -

lem s (see fi g . 4): first . t he sawtoo th

isn 't very linea r (see fi g . 4AI. and it s

fall ti m e is too long . Second , th e saw-

t oo th cl ips at vario us sett ings o f the

center fr equenc y and sw eep rate con -

trois. Perhaps in the fut ure I'll find time

to re-design these circuits, but lor now

the saw tooth b oard IS satisfact ory .

source called for in th e arti cle.

In retrospec t, " next time" I might try

using a singl e connect or for all leads

o ther than the i-f , and O.OO2-IIF disk

ceram ic capac it ors on each lead at th e

c onnect or. A good chassis-m oun ted

connector cost s about $5 (or less), and

high quality di sk capacit ors c ost on ly

about 80 cen ts each an d even less per

un it in bargai n pac ks. The EMI f ilt ers

ca lled for in the article are abo ut $4

each; ab ou t 12 are requi red .

a d d ing a s weep ci rc u it

A sig ni ficant problem with th e

W 4U CH article for ma ny readers is the

lack of a sawtooth circuit . W4U CH

used the saw tooth outpu t of his Heath

' '''''''

d c p ower s u p p ly

The Poor M an's Spectrum Analyz -

er requi res a two-vol t age, single-

polarity dc po we r supply : + 12 VD C

and + 24 VDC . The schem atic diagram

o f a po wer supply that m eets these re-

quirements is sho w n in f ig . 5.

I used a pair o f small 12.6 -VA C

transf ormers ITl and T2), wi th the

primaries co nnec ted in parallel and the

secondaries co nnected in series, to ob -

t ain th e req uired vol tag e. I used avail-

able com ponents - a pair of brand-new

Radio S hack. pc -m oun t tra nsfo rm ers.

You can use instead eit her a 25 .6-VAC

tra nsfo rmer or a oual-secondarv trans-

former

<C" NC'

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stoc ked

Digi-Key , '

Dick

fig . 3. Sweep boa rd SW-600 1 cOlllle c l io n:> lavaila ble fr o m S cien ce W o,ks h opl ,

' 0 1\1' K~ CO'POIan",,,. P 0

8 0> 617. Th.·! R"," ' fal"'.

M,,,,,,,so!.. 5670 1.

fig _ 4 . Wa vef o r m s h o rn S W 6001. IAl M id -' <I llg e :>;lw IOOl h . I BI a n d leI IIle $Ilw l OOl hs ta ke n a l e~lIeme5 o f C ENTER FREQUENCY

a nd S W EEP RATE co m -e ta .

100 ~1 M ar ch 1987

,"'

"0"""

f ig . 5A . DC power s upplv : top view 0 1 pe r f ·

m o unted assem b lv.

Smith He ctroruc s. " 0 and o t he r di st ri -

butors . Th e c urre nt req uirem ents fo r

t his proj ect aren ' t crit ical, so almo st

any tr an sformer wi th a rating of 300

rnA o r more is acceptable .

Two th ree- term inal lC regulators are

used in th is project. Th e 7824 talso us-

able : LM-340T-24 J provide s t he need -

ed + 24 V DC , w hile a 7812 lor

LM ·340 T -121 pr ovid es th e + 12 V DC .

Both regulators are standa rd, but I

f o und th at t he 7824 was a littl e hard

to find loc ally. Th e NTR line of replace-

ment sem ico nd uc t or s, stocked by

m any lo cal distribu tor s, ca rries a good

replacem ent num b er .

T here's nothing c ri t ica l abo ut t he

parts layo ut, and pe rf board can b e

used fo r co nst ru ct io n . T he d iodes

I CRl and CR2) are used to pr event th e

charge in th e output c ap aci tor s fr om

damag ing t he vo ltage reg ulator s at

t urn -off . Do n't d elete t hem , even

though yo u may see m an y ci rc u its u s -

ing the se reg u lato rs witho ut c harge

du mp d iod es. Th e shie lde d co nst ru c-

t io n and t he O.1-/cF o ut p ut c apac ito rs

are needed becau se o ne might be us-

ing th is device in clos e pr oximity to a

hiqh -po wer transmi tt er. Th e capaci tors .

m us t be m o un ted o n t he o u t pu t ter -

minai, o r at least as close as ph ysic al -

ly possib le .

fig . 58. D C power s u p p ly sc hemot ic ,

photo o f t he sp ec t rum analyzer d is-

play. The ce nt er frequ ency wa s adjust-

ed to th e lo w end o f th e fm broad ca st

band . Th e larg e ce nter spi ke is the sig -

nal from m y M easur em en ts M o del 80

sig na l g ener ator set to approxima tely

85 M Hz. The small sp ike t o its rig ht is

WAMU -FM (88 .5 M Hz), my favo rit e

pu bli c rad io sta t io n; the o th er spikes

are o the r frn ba nd sig n als . The large

sig nal barely visib le o n the left side is,

I believe, Channel 5 TV in W ashington ,

DC.

Those who d o n ' t have a hor izo n tal

in put m u st use th e sawtoot h t o tr ig .

ger th e swee p t hro ug h th e EXTERN A L

TRIGGE R inp u t. I reco m m end u sin g

t he neg at ive t raili ng ed ge of t he sew -

toot h wavefo rm f or this pur pose (set

TRI GGER S LO PE - or eq u iva lent

switc h - 10 th e negat ive positionl.

Al so, be su re t o mak e th e sweep t ime

ac ro ss th e entire horizon tal aspect o f

t he 'sco pe g ralic u le eq ua l to th e peri-

od o f th e sawtoot h leading edge .

Otherw ise, th e 'sco pe and sawtoot h

won 't s weep in sync .

g iv en m e a few ideas f or c ha nges o r

im pro v em en ts. First , I plan to re -

desi g n th e sawtooth gene rator tpossl-

b ly gene rat ing th e sawtoot h d igi t ally l .

Second , I pl an to add an am p fifi-

er / attenua tor based on Mini-Circ uits

fixed arten uato rs and a Sig netics

NE·5205 amplifier.' T he range wi ll be

·60 t o + 19 d B , T hird , the re may be a

co nverter for h f, an d t uners to ba nd -

lim it th e spectr u m analyzer at will to

certain V H F Amateur ba nds . This

m odification wi ll punch o ut certain lo -

ca l sig nals t hat t end t o dr ive recei v ers

in to in terrnod pr oblem s at my OT H,

Fo urt h , WA2PZ O is w o rki ng on a

tr ack ing os c illator c irc u it , and in f act

has a t ent ati ve ap pr oach to it s desig n.

A track ing osci llator produces an o ut -

put at th e spe ctrum anal yzer's center

fr eq uency . Besides its obvious use as

a signal sour ce, it 's also usefu l for dr iv -

ing a frequenc y co un ter . Presen tl y,

tu n ing ind icat ion is by seat -of -t he -

pants calibration o f the v o ltage co n -

tr ol. I plan t o bu y t he W A 2P ZO t rac k -

ing

p er form a nce

Figu r e

if,

oscillat o r

k it

and

when,

becomes available.!

varector t uners are inheren tly no n-

linear

sho w s

os cilloscope

future p roj e cts

T he spectrum analyz er project has

--Oock S",,1I, Elect1 onlCs . P.O flo. 80:'1. H"'l....."od

cav . Cdhl" """ . 94063 1302 \

thei r

v on aq e-v s-treouencv

M a r ch 1987 ~ 103

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SY'STE lifl

charac teristic and the result ing curve

looks parabolic in shape . Digitally

generat ing the sawtoo th signal is a

w orthwhile c ons ideration. If you w ant

10 try it you rself, write to m e and I'll

send yo u a brief on th e met hod .

(Please enclose a No . 10 SA SE. ) A

very brief discu ssion of the digi tal

linearization me thod is given on pages

3<Xl-302 of m y book, Ho w to Design

and BUIld Electronic Instrumentation . Z

A llhough my me thod is based on dis -

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your spectrum analyzer.

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concl usion

WA2PZO deserves accolades (and

our busin ess ] because o f the Poor

M an' s Spec trum A n alyzer project ,

w hich o ffers oppor tunit y for ex -

perim en ta tio n in areas pre vio u sly

closed to Amateurs so lely for reasons

o f cost . If yo u have an idea for it s use.

a new or dilfelen t m od ifi cat ion . 0 1 a

particularly well-b uilt version o f the

W 4UCH /WA2 PZO project. send me

the details.

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