Homebrew_Spectrum_Analyzer.pdf

Number t6 on your F~ eat'Cl

Home-brew

Spectrum Analyzer

Project for DXers and experimenters alike.

by Gregory R. Mcintire KE0 UV

I f you've always wanted a spec-

Theory and Con..truction

trum a nalyzer, but fig ured il

was ...... ay o ut o f you r bud get , here ' s

the an swer! All the compone nts re -

quired to bui ld your own HF spec-

trum analyzer may be found in a

we ll-stocked j un k box. In the wo rst

case , it'll ru n you less tha n ten dol-

lars . as long as you alrea dy o wn a n

oscillosco pe and a n ine xpensi ve ,

gene ral co verage short wave (SWj

rece iver,

S in ce it ' s unlikel y many o f

you have the sa me sco pc and SW

receiver I use , I g ive relat ively

gene ralized instructions here. Wi th

a little ca re . however. you shou ld

be a ble to easily a pply this idea

to most models of $W receivers .

and to a lmost any oscilloscope . If

you

C" p ;L

have

particula r

se tu p.

tho ugh .

co nta ct

me for help on

find ing

the speci fic

con nections

Overview

on the

DX-360.

Please

se nd an

This system is surpris ing ly sim -

ple . Ima gine a "scanning receiv-

e r" whose IF output is rect ified

into DC. This DC signal is then fed

to the vertical input of a n oscillo-

scope. The sc o pe's trace beam

scans the face of the CRT at exactly the same

rate as that o f the sca n ning recei ver. Thus.

whenever the rec ei ver sca ns across a re-

ce ived s ignal. a DC voltage proportionate to

the st re ngth of the recei ved signa l ca use s the

trace on the scope's CRT to de flect upward.

As soon as the scanner passes by the signa l.

the trace deflects back down. If the sca n rate

is fast enoug h. a continuous trace appears o n

the C RT with ve rtical spike s or deflections of

va rying magnitud e whi ch co rrespo nd 10 t he

RF sig nal s in the path of the scan ner.

Th is spectrum a nalyzer interface contains

its own HF receiver. Simply tu ne the SW

receiver 10 the po rtion o f the HF spectru m

SASE.

First, o btain almost any s imple,

low-cost sco pe . I use a forty-dol -

lar, 2 M Hz . used scope .

Next . obtain a simple. LC tuned.

s hort wa ve re c ei ver. M y Radio

Shack Realistic DX-360 works great. Ho w-

ever, since I couldn't see the component s ide

of the board , it wa s tough figuring out what

was what. Afte r a lot o f trial and er ro r, I

found what I wa s looking for. It 's easiest if

o ne terminal of the tuning capaci to r is con-

nected d irectly to g rou nd .

Now see the sc hematic in Figure I . The

receiver provides the sig nal ( DC voltage) for

the osc illoscope to display . All yo u have to

build is a very simple device that wi ll (I)

ca use the rece iver to "scan" a portion of the

HF spectrum and (2) provide a" sy nc " signal

10 the scope so it will also scan the C RT at the

sa me rare .

Photo A . Top \';~w of the spectrum onalvz er interface ,

you want to view. o r feed the a ntenna inpu t o f

the S W receiver with the wid cband IF signal

o f your HF transceiver . Using the latter sys-

tem. any s igna l o n the freq uency to which

you r transceive r is tuned will sho w up in the

ce nte r o f the C RT . This sig nal. 100. marks

the middle ofthe spectrum range the analyzer

is looking at.

The amount o f spect ru m you can view de-

pcnds on the amou nt o f bandw idth available

at the IF (be fo re the final IF filtering) of your

transceiver. This also requires that the IF

frequ e ncy be wuhin the tu ning range o f the

$W rece ive r. If your IF is 455 kHz , you can

usc a s imple AM broadcast band receiver.

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FiguTl' 2. Foil side ofboard . actual sae.

Figur~ I . Schematic f or Ihf" $ 10 or II'SS home-brew spectrum anaiyier.

73 Amateur Radio

•

February,1990

•

terminal. (See below if your scope doesn't

have an exte rnal m gger.)

feeds a paralleled capacitor and resistor. The

pulse charges the cap very rapidly , and then

the resistor discharges the cap slowly. This

creates a "concave" sawtooth waveform of

about 25 Hz.

We need this waveform 10 create " lineari-

ty" of the scanni ng frequency of the SW

receiver , The nonlin ear characteristic of a

vaructor d iode , and the nonlinear fashion in

which (he LC tuning circui t of'the SW receiv-

er operates, requ ires a nonlinear volt age 10

be fed to the varactor. but with its nonlireari-

ty inverted in order to end up with linear

tuning.

This DC vol tage feeds a varector d iode

(tu ni ng diode) that is connected in parallel to

the existi ng mechanical tuning capaci tor o f

the short wave receiver. The sawtooth wave is

also fed to the scope's external trigger or sync

Two Bird.. W ith O ne Sto ne

The 555 timer Ie takes care of boIh of the

aOO... -e. II is config ured as a square w ave gen-

e rato r, but with the ON pan of the .....ave being

very short rela tive to the OFF pan. This is

essent ially a pulse wave. The pulse signal

Optimizin~ the Sawtooth waveform

Select values of Cz a nd R5 to give a smooth

concave sawtooth waveform as measured on

the cathode of D3. This is a trial and error

process. If R5 is too large. the scanning range

is narrower and the min imu m capacitance of

VC I will never be reached . The maxi mum

capacitance range o f the varactor-c ard thus

the maximum possible spectrum scann ing

range- is never exploited since R5 will not be

able to discharge C2 to zero volts. If R5 is 100

small, however , the scanning range will be at

its maximu m but will scan too fast. In this

case, the waveform drops off very rapidly 10

zero volts befo re the cycle fi nishes .

You may want to make slighl variations on

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Figure 3. Pans placement on foil, enla rged.

Th e 8-pin Ieis in a If>.pin dip socket with pin

I 0 1 I in th(' drawing , Leave p in 5 position

undrilled to amid breaking the trace from pin

4 to + 12 WI/ H . Note thut C3 and VCI plug

into the 16-l'ill DIP socket for easy removal

for experimentation. C3 and vel call also

each be paralleled wilh other values via the

ullfilled DI P socket holes.

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Figure 4. A typical S W receiver RF and focal oscitkuor tuning circuit. Thr leadfrom CJ from

lhe sp ectrum unatyzeranaches to point A ; the orher spectrum analvzer Iead (from VCI , attaches

to point B.

Photo B. The 0 'scope CRTshowing sig nals in

a 150kH:. piece ofspectrum 0 11 10m .

Photos C ,D. Greg K£0UV getting ready 10 to spot some DX ( In 10", using th~ 110 Spectrum Analyzer. Photo D is a close-up ofthe svuem.

comp rised of th(' 0 'scope, th~ scanning receiver (th(' DX-3 60) and the interfa ce, ",hieh sits atop th~ DX-3 60.

13 Amateur RadiO

February, 1990

these two valu es if the fini shed spectrum ana-

lyzer' s d isplay has wider spikes o n o ne s ide

o f the CRT tha n the othe r. You ' 11 likel y need

a lleronly R5"s value ,

If your sco pe does not have a n ex te rnal

trigger input, yo u may be able to tap into the

linea r sawtooth wave inside the scope. In this

case , you ca n eliminate the 555 timer ci rcu it.

Simply feed this sa wtoot h. wh ich may need to

be voltage divided, 10 obtain a level ofaround

0 -12 vol ts , to the varactor. Couple the sa w-

tooth to L I th ro ug h a ca pacitor. (Experiment

to find the proper value of thi s cap-Q.OO I Il F

is a good sta rt ing po lnt .) T h is will c reate the

c o ncav e

O p t imiz ing t he S ) PC Frequency

I se lected the frequ ency of the 555 timer

c irc uit to g ive a smooth. sha rp di splay o n the

C RT . II should ge nerate pulses in the 20-30

He rtz range . Much below 20 H z g ive s a n ick-

e ri ng CRT di splay . Much above 30 H z

widens a nd s mea rs the di splayed sig na ls ,

p robably due to the SW receiver's AGC re-

sponse time .

Make the co nnectio n to the AGC circuit of

the SW recei ver at a po int be fore th is AG C

voltage is act ed upon by a ny " t imed decay "

circuitry. If you r SW receiver ha s a n Scmc-

rer. try lapping into the AGC voltage there .

Connect you r scope to the s-mercr. then man-

uall y tune the receive r across so me sho rt -

wa ve signa ls and determine how fast the

AGC volta ge r ises and fa lls . It should be vt'r)'

fast. If it is not , then work bac kwards fro m

the S- mete r till you get to a po int w here the

AGC voltag e is fast.

There' s a point j ust after the IF filte r where

a port ion o f the IF is rectified. This is the

AGC voltage . You may also be able to tap

into the AGC vo ltage at the same point that it

is fed back to the RF amp.

T he aim is 10 probe aro und wit h the sco pe

until you li nd a point w here you get a DC

voltage proportionate to the stre ngth of a re -

cei ved s ig na l with insta nta neous response as

you tu nc . When you find it. co nnect a wire to

it. Bring the wire out of the receiver' s cabinet

and connect it to the

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driving the varactor.

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The varacto r tuning d iod e (VC I l l usc is

rated at 15 p F , but th is c a n vary accordi ng

to your system. You may need to cha nge

th is value, andlor the value of C 3 , depending

o n how much sca n ning ra nge o r bandwidth

yoo want . A 33 pF varactor ga ve too much

tuning ran ge: too much of the spectru m was

di sp layed on the CRT. which ca used a loss

o f re so lut io n a nd mel di ng o f the d is played

sig na ls. I sugge st us ing a l6-p in DIP pac ka ge

fo r the 555. so th at you c a n use the extra ei ght

sockets to try out d ifferent VC I /C3 combos.

Remember that VCI and C3 art simply two

capacitors in serie s , so you ca n calculate what

amounts of change will have what a ffects

on the total ca paci ta nce, using the sta nda rd

se ries

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T he IF fil ter o f the sca nning receiver limits

the width of each di splayed s ig na l. T he na r-

rower the filt er. the better . I temporarily

inserted in cascade (se ries) a 4 kHz filter

with the 6-8 kHz wide filters in my DX -360,

and got much na rrower spikes on the CRT

d isp lay . A 2 to 3 kHz fit ter would be ideal.

I re co mme nd using a metal e nclosure 10

keep o ut stray R F . Also, usc sh iel ded cable

from the IF o f the transceive r to the antenna

input o f the sho rt wa ve receiver. to keep

e verything but the IF out o f the SW receiver.

Mod<> Bue 2 5..

TR·75t

AlI Mode M_

25..

+ C2.

If you are tuned 10 the high end ofthe dial o f

the SW receiver. you will want a s ma ll varac .

tor. and vice versa. The value s I chose g ive a

d isplay o f about 325 kHz wid e . The wide-

band IF of my Yaesu FT - IOI Z O is also about

this wide, so this means I ca n view about 150

k Hz of spectrum on bot h sides o f the frequen -

cy 10 wh ich the Yaesu is tu ned .

Connect C3 and VC I . in se ries, across the

Iocal oscitkuo r (LO) tuning ca paci tor in the

SW receiver. Si nce we are tuning o nly a

re lativel y sma ll part of the spectrum, we

don ' t need to tune the RF ta nk circuit. See

Figures 4 a nd 5 .

You may have to mount a t rimmer capaci -

to r, se t at approximately the same value a s the

varactor diode (VC I), ac ross the RF sect ion.

o f the main tuning c apacito r o r the shortwave

recei ve r. This is because the LO "scans" a

s lig htly lo we r range of frequenci es than the

RF sec t io n o f the tuning cap is cente red on. I

tried it without a t rirncap and it wo rked , but I

g el high er AGC voltage to the ve rtical input

of the c'scope with this tri m mer insta lled ,

and thus taller spikes o n the CRT d isplay .

If room permits. mount C3. VCI and 1I

inside the receiver ca binet , as cl ose as pos si -

ble to the LO tu ning cap. T here is very lillie

room inside my D X-360, so I s im ply so ldered

wire s 10 each te r mina l o f the tuning cap. (o ne

of these wires is the receiver' s ground) a nd

brought the two wires to the outs ide o f the

radio v ia a ~ ~ stereo phone jack . I used (he

third conductor of the ste reojack to bring out

the AGC vo ltage o f the rece iver 10 feed the

scope 's vertical input.

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Parts List

v. w

{may be 22k-100kJV. W

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Rl .R2

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0 .05

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disc ceramic

electrolylic

(any temp stebre cap)

mica or any stable cap

(olh er val ue s may be

substltul ed. depending

00 desired scan range)

15pF varaclor dlOde MV2105

15010 300 llH inductor 43LS 154

or 43LS564

01 ,02, 0 3 l N914 or any small signal

silicon diode

SPECIAL

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330pF

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ut LM555 or NE555 timer IC

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the paris lor th is p rojec t . Si nce they r e-

quire a minimum $15 order, yoIJ may want to gel

several val UM 01 verector diOdeS ,

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213·390·8003

•

Fe bruary, 1990

73 Amateur Radio

If the IF of your transceiver is 455 Ufz .

replace the SW receiver with a simple AM

broadcast band receiver. Tu ne the AM re-

cei ver to the low end of the dial, and you may

have to use a 100pF or higher varacror diode .

(Simply parallel two or more varactors to get

higher valucs .)

Ofcourse, you can analyze chunks of spe'C~

trum the SW receiver itself tunes through.

Just unhook the transceiver from the setup,

and repl ace the SW receiver ' s original anten-

na (or something with higher gain .) Tune the

receiver to the band of interest and view the

activity on the scope CRT. One disadvantage

o f this is that, to maintain the same spectrum

viewing width, you would ha v e to swap in

and out different values ofVC I fo r the lower,

midd le, and upper sections of the dial on the

SW receiver .

KENWOOD

Lei the Fun ~in!

After co nstructing the simple interface.

power il up with a 12 volt supply and list en to

the audio of the SW receiver. It should have a

buzzing so und as it very rapidly scans. Tune

the receive r to a busy shortwave band. You 'll

note that the added capacitance of the varac-

tor diode caused the d ial calibration on the

receiver to be a bit inaccurate , and that you

will have to tune the receiver higher than

normal .

If all checks out so far, turn off the power

and co nnect the AGe wire to the scope's

vertical input. Set the trigge ring fu nction of

the scope to external and connect the external

trigge r o r syrc inpu t of the scope to either the

concave sawtooth waveform , or to the pulsed

wave d irectly from pin 3 of the 555 Ie. Tum

th e power back on and adjust the scope's

input ancnuaror until you get a display on the

screen. Now set the scope's sweep speed until

it sweeps the enu re C RT at a rate slightly

faster than the 555 's frequency. I se t my

scope's umebasc at I millisecond per divis ion

and usc the variable sweep control to slow it

down.

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So Now Th ai You've Ruilt It , , ,

.. . what do you use your spectrum a nalyz-

er fo r? Imagine sining in front of you r rig in

the wee hoursofthemoming, hoping to work

some rare OX. As you sit and listen, slowly

tuning across a seemingly dead band (which

you know will soon be openi ng!), you have

your eyes focused on the CRT . You see a

small p ip about three-fourths of an inch to the

left ofcenter: you watch it for a moment, just

to be sure it isn't noise. Sure enough , it has

the rhythm ofa CW signal! Quickly . you tunc

downward and watch as it moves to the right .

As it becomes centered on the C RT, you hear

it. Whi le working this station, you see anoth-

er signal appear on the C RT. You quickly

make your QSL info exchange and tune till

this new signal is centered.

l ts also useful for those who take an ex-

plorer's interest in what goes on on the E· M

wave spectru m. For e xampl e.1 am fascinated

at watching and trying \0 analyze the myriad

of sweepers, or " runners," on 10 meters as

they go racing left and right across the CRT.

As they pass the cente r, you hear epeep- A lot

of strange stuff goes on on this band! lf you

are a birdwa tcher, the only place you will

ever SEE the woodpecker is on your spec-

trum analyzer. (And the woodpecker is in-

deed a strange bird to seen

[u]ffi[D~~rn[(]

Electronics Supply

" All the components

required to build your

own HF spectrum

analyzer may be found

in a well-stocked

junk box. "

C:I

~'CNII£Il SlICE 1956

3621 FANNIN

HOUSTON . TEXAS 77004 ~

Number 25 on you. Feedback card

E:~. ! WBB£L~""

VIDEO 1.0. BOARD

Now disconnect the antenna from the SW

receiver , and connect the antenna input o f the

receiver to the widcband IF OUt put of your

transceiver. If your transceiver does not have

an IF output jack , look for a source of wide-

band IF immediately before the IF fil ter , and

bring it out via a wire. This signa l is used only

to fe ed the antenna input o f the shortwave

scanni ng receive r, so you can loosely couple

it to the antenna input, with a resistor in line.

Choose a resistance that will give max imum

IF signal to the scanning receiver w ithout

overloading its from end . You may fi rst want

to use a pot with a kno..... n range 10 determine

this, and then replace it with a fi xed resistor

with the appropriate

' Custom Graphics with your Call Sign

' 4 Screens (2 Hl-res/Z color bar)

' 12 VDC Operation

-tnsten t Video 10

' Video Relay for switching in Live

Camera vdec

-Built-ln Automatic Sequencer-Timer

(steps through all four screens)

Co nclusion

The most lime consuming pan of this pro-

ject usually is trying to locate the AGC circuit

and the local oscillator LC tank circuit of

the SW receiver . Co nstruct ion technique

isn' t critical. although the varactor should

not be located too fa r from the LC circuit.

I breadboarded this c ircuit using 12-i nch

long connecting wires, and it worked just

finc. For dis play stability, however, keep

connecting lengths as short as possible .

Now , you can build from the junkbox a

feature for w hich avid OXers spend addition -

al thousands of dollars in comme rcial ama-

teur gear! FJI

VQG·l with pre-programmed call s:

$99

C~II or writf' for catalog ofavailable 9raph~

value. I used a

lOOk

resistor for the OX·360 's inpu t.

T he last IF frequency of my Yaesu

Ff-IO IZO is 8.9 MHz , so I simply tune

the SW receiver slightly above that setting

on its dial. Then I tunc the Yaesu to a jlmng

CW station and watch the spikes o n the C RT

until I identify the one that I am hearing.

Next , I tune the SW rece iver unt il that

part icular spike is in the center o f the C RT

dis play.

Now. as I tunc the Yaesu. the signals on the

display move left or right in such a manner

that the signal I'm tuned to is always dis-

played in the cemerof the CRT screen.

ELKTRONICS

125 36 T.R. 77· Findlay, OH 45840

(4 19) 4 2 2 ·8206

ElKTRONlCS

12536 T.B. T1

r,ndIIV_ 011 'lSB'lO

(410) 4~

Grt'go ry R. Mcintire KEa U v can be reached

Of Hillsview Tr. Cn ., Lot 92, B~IJ~ Fourche

SD 5nJ7. Greg K£ OUV has been licensed

since May

'8 7, and has S WlLd since

'8 /.

Other hobbies include beekeeping,

CIRClE 8 ON READER SERVK:£ CARD

73 Amateur Radio

•

February, 1990

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