low-cost.pdf

low-cost spectrum analyzer

with kilobuck features

Build WA2PZO's

MHz on the UHF TV == 500 MHz - 800 MHz. When

the sweep width is set at zero, each of these bands

of frequencies may be manually tuned just as in a

single-frequency receiver. Both wideband FM and nar-

rowband FM signals, and surprisingly, even amplitude

modulated signals are detected quite well by the FM

IF amp /detector IC and amplified by the audio IC.

Figure 1 is a block diagram of this spectrum analy -

zer using a Sanyo varactor tuned TV tuner. A ten-turn,

10k pot with + 35 VDC across it is used to adjust the

center frequency of the tuner. A low varactor bias

yields low frequency and a high varactor bias provides

higher frequency on the TV band to which the tuner

is set. The sawtooth sweep voltage that is capacitive-

Iy coupled into the tuner's varactors is the horizontal

sweep voltage from the oscilloscope. If your scope

doesn't have the horizontal sweep output (the Heathkit

OL-1 does not), just mount an RCA phono jack on the

front of the scope and bring out the horizontal sweep

from the scope 's horizontal multivibrator to this point.

The 100 kilohm load across the horizontal sweep out-

put should have little or no effect on the scope's

operation.

Besides the TV tuner and scope , the rest of the cir-

cuit consists of only three integrated circuits. The

second mixer /oscillator chip is a Siemens S042P. The

ceramic filter is a Murata SFJ two-section filter at 10.7

MHz. The combination IF amp /detector/ AGC amp is

a National LM-3089N, and the 1/2-watt audio amp is

a National LM-386N.

useful instrument

for operating position

and lab bench

Although laboratory-grade spectrum analyzers

cost $4500 or more, you can build a spectrum analy-

zer offering many features of its costlier cousins for

about $50. How can such amazing capabilities be had

at such incredibly low cost?

• Through the use of a commercially mass-produced

varactor tuned TV tuner that covers the VHF low, VHF '

high, and UHF TV bands. It will tune down through

the 2-meter and 6-meter Amateur bands without

modification, with better than l-microvolt sensitivity.

By using a crystal-controlled converter and a narrow-

er IF filter, anyone of the HF Amateur bands could

be viewed as well, by simply upconverting to a TV

channel.

• Through the use of consumer-grade integrated cir-

cuits in the oscillator/mixer, dual ceramic filter, IF am-

plifiers / detector, and audio amplifier (offering audio

as well as scope output it is really a spectrum monitor).

• Through the use of your own oscilloscope. Just

about any scope may be used; I used a 1951 Heathkit

Model OL-l with its original cathode ray tube.

construction, testing, and alignment

Figure 2 is a schematic of the analyzer. A printed

circuit board is available from WA2PZO; I recommend

using this and WA2PZO parts kits . (Although the pots,

S-meter, and speaker aren't furnished, most are avail-

able from Radio Shack .I Figure 2A shows the com -

ponent layout on the WA2PZO printed circuit board

(foil side down). Alternatively, you could use perfboard

and point-to-point wiring.

Figure 3 is a schematic diagram of the interconnec-

tions between the Sanyo tuner, the three-IC printed

circuit board, and the scope , Once everything is con-

nected as shown in fig. 3, alignment can begin. (It

should be easy, since there are only four adjustments,

spectrum analyzer applications

Spectrum analyzers allow the user to observe in real

time an adjustable/variable bandwidth of radio fre -

quencies. One of the earliest spectrum analyzers for

the ham bands was the " Panadaptor, " manufactured

by Hallicrafters in the early 1950s. I used one to check

for F2 propagation 6-meter band openings in the

mid-1950s and credit the panadaptor with my earning

the IARU 6-meter WAC award (phone) issued by the

ARRL.

The band of frequencies swept by the spectrum

analyzer described in this article may be varied from

zero up to about 38 MHz on VHF low TV = 50 MHz

- 88 MHz, zero up to about 85 MHz on VHF high

. TV = 135 MHz -

By Robert M. Richardson, W4UCH, 22 North

Lake Drive, Chautauqua, New York 14722

220 MHz, and zero up to about 300

82 UI September 1986

the ferrite cores in Ll , L2, L3, and L4.)

With all the parts on hand - and assuming you're

using the PCB - assembly ti me is at most an hour

or two. I t uned up my uni t using only a grid dip meter

as a signal source in about 20 minut es.

Construction of the PCB proceeds as follows:

1. Install the jumper at the lower edge of U3 as illus-

trated in fig. 2A.

2. Install the 11 resisto rs as shown in fig. 2A .

3. Install the 16 capacitors as indicated in fig . 2A.

Since C6 and C7 are electrolytics, be sure to observe

the polarity indicated in fig. 2A.

4. Install t he Murata ceramic filter , FLl . Because it's

sym metri cal, it can be installed in eit her direction.

5. I recommend carefully installing sockets for U1

(16-pin DIP), U2 (8-pin DIP), and U3 114-pin DIP).

6. Install L1 through L5, carefully bending the pins

slightly so that they fit easily into the PCB's pre-drilled

1----. ~~~~

+3!5VDC

f xce p t as ind ica t. d, dec ima l

~.'u.s of cap.lc Uance . ,. in m ic l O-

'.reds (" Fj; ofha, • • ,. in plco'.,-

ads (p F); , •• i, r. ltCe S .t,. i n oh ms.

O.~

600V

SC OPE

WI DTH

1.000

::::: ' .000.000

HORIZONT A L

SA WTOOT H

OU T

fig. 1. Block diagram of WA2PZO spectrum analyzer.

R,O

120

12 0

+I2V

+ 12V

:Jli

RII

' Ot

111Hz

'0. 7I11H z

c,e

u 2

22 0

S042P

C9 L/Ii~OB9

0.0 4

~60

' 2

CII

0. '

3 3

0.0'1

C,O

+ 12V

R '

12 0

<t, ' 4 ,1 6

Exce pt as in dicated, de c im al

Y"/ues 0' capac/t. nee . ,. In miCro-

farads (I,F); other s iH e In p icO /i1t .

a ds (pF} ; res isfa nce s a t. In o hms.

" = 1.000

1 0 . 0 ' C6 l/OrOJlF_,",--,

.:;::~,,' 'L--r:-~

CI 2

M ::I:

1.000 .000

' OOk

fig. 2 . S chematic of WA2PZO spectrum analyzer.

September 1986 m; 83

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84 ~ Septe m be r 1986

f ig . lA . C o m p o nen t la yo ut o n th e WA2PZO p rin t ed ci rc ui l b o ard Uo il , ide d ownl.

hol es. Because l5, th e 150 p H induc to r. is sy mmetri·

ca l it ca n be ins talled in either d irec ti on.

7. M ost important: lubrica te t he small Iemte COtes of

L'l, L2. LJ, and l 4 with a d rop or lWO o f W D·40 beIOfe

attem p li ng adjustment. These cores are extremely brit-

tl e; use a pl ast ic o r nylon tuning 1001 ma tch ed 10 the

wid th o f t he f err it e slo t s (usi ng a m eta l o ne w ill gua r-

an tee their de st ructi onl.

8 . If you 're not usi ng a 2OO- pA S-m ate r, install a jum-

pe r fr o m ' A ' to g ro u nd .

Testin g an d a lig nment pro ceed a s t allo w s :

1. Connec t th e TV tuner' s V H F inp ut t o yo ur z. m erer

antenna .

2. Se t the V HF / U H F swit ch 10 the V H F p ositio n .

3 . Set th e Hilla sw itc h to Hi.

4 . M ake su re t he to p o f th e ferr it e co re in II is exact -

Iy ev an w ith th e top 01 its shi elded hou sin g .

5 . Se l t he sq uelc h conuol t o ze ro and adjus t the

vol u me cc nuo l fOI a slig h t h iss from t he spea ker WIth

the gai n control at ma ximum res .st ance 110 k ilo h m sl.

6 . Se t the wid t h co nuo t (saw tooth sw eep h o m sc o re !

to zero.

7. Co n nec t a von rneter t o p in

..... .......

_ &1<'1 0

'"'" • • "

..... .....

. 4 441:'''•

. .. . . .

. - '

, . . . . . . ..

. .~ ...,.

... .,

'v,,,

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......

..···c..

o..'~~'

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IC .....

•

_ __ ._._ ...

-.,~~"

~ • • ~ .fO <'.C~" aoU

.. _---

_ -

- __ -

•

, . ,- ~ .. _-

.~_

. ~.

. ~r ••• •

li g . J . S chematic s how in g in !... conn ect io ns bel w e .. n

!u ner . prinled CirCUli b oard. a n d os c illo s c o pe. N Ole ; + 35 .

4 20. a nd

10 o f the

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tve racto r mp u rl . Use the

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frequency

10k ·tu rn

pol

th at

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8 . Turn t he can ter fr equen cy pot slowly dow n treduc -

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scope . If no TV stations are nearby , use a sign al qener-

a lar w it h mo d ulat ed o u tp ut o r a g rid d ip m et er .

9 . Peak the fe rri te slugs in L2 an d l3 f or m ax imu m

signal o u tp u t o n the scope .

10 . Co n ti n ue tu ni ng th e cente r fr equen cy POI un t il a

TV aud io sig nal is f ou nd , th en adjus t l4 for ma xim u m

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~ '"

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