Poor_Mans_Spectrum_Analyzer.pdf

Boor Man's Spec rum Ana

-- another 73

- breakthrough

analyzer. Commercial ver-

sions of this useful ri instru-

ment start at $2500, which

is a little steep for most of

us. lt is possible for you to

build a simple spectrum an-

alyzer for about $150 that

works with a low-cost oscil-

loscope. The analyzer can

be used to check HF trans-

mitting equipment, among

other applications. Its use,

theory of operat~on, and

construction are discussed

in this article.

you to view the frequency

components of its input sig-

nal on an oscilloscope CRT.

The spectrum analyzer re-

~eatedlvtunes across the

krequency band you have

chosen with its center-fre-

quency and frequency-span

controls. For example, if

you set the center-frequen-

cy control for 20 MHz and

adjust the frequency-span

control for a tuning range

from 10 MHz below to 10

MHz above the center fre-

quency, the analyzer will

repeatedly tune the

10-MHz-to-30-MHz band.

As the analyzer tunes

from the low end to the

high end of the band, it

moves the CRT trace from

left to right. The S-meter

output from the analyzer

moves the CUT trace up-

ward from the bottom of

the CUT screen according

to signal strength. A spec-

trum analyzer display usu-

ally looks like a number of

spikes. The farther to the

right a signal (spike) ap-

pears on the CRT, the high-

er its frequency; the

strength of the signal is indi-

cated by its height. There

usually appears to be some

"grass" along the bottom of

the CRT display. This is due

to noise. You probably have

seen spectrum analyzer dis-

plays in ham gear sales liter-

ature and some magazine

articles.

To appreciate how useful

a spectrum analyzer can

be, let's first look at Photo

5, an rf signal on a normal

oscilloscope. To me it looks

like a clean sine wave.

What do you think?

Now let's look at Photo

C, the same rf signal on our

spectrum analyzer. The

half-spike on the left is our

zero-frequency reference.

The next signal to the right,

which is the tallest, is the

fundamental component of

our rf signal. The three sig-

nals to the right of the fun-

damental are the 2nd, 3rd,

and 4th harmonics.

If the spectrum of our

transceiver or linear ampli-

fier output looked the same

as this photo, we would not

be complying with FCC

Regulation 97.73, even

though our fundamental

signal was properly within

an HF amateur band.

To understand what's

wrong, compare the height

of the 2nd harmonic signal

to the fundamental. The

second harmcnic is about

2.6 CRT divisions shorter

than the fundamental. With

a 10-dB-per-division vertical

calibration, the second har-

monic is 24 dB below the

fundamental.

FCC Regulation 97.73 re-

Frank H. Perkins WB51PM

Box 13642

Arlington TX 76073

ams enjoy making all

types of electrical

measurements. In fact, it's

one of our favorite pas-

times and topics of conver-

sation. Fortunately, good,

low-cost oscilloscopes,

DVMs, and other instru-

ments are available to us

for measuring voltage, cur-

rent, power, swr, frequency,

and so on.

There is one instrument,

however, that has been be-

yond the reach of most of

our budgets - the spectrum

Spectrum Analyzer

Operation

A spectrum analyzer is a

special receiver that allows

Photo A. High frequency spectrum analyzer covers 0 to 60

MHz.

73 Magazine 9 August, 1982

as vrewed on an ord~naryoscrlloscope Is

Photo C. Same rf signal on rhe speiiriini analyzer. Seconci

harrilonic 1s only 26 dB beloiv the iundan?ei~ta/.

~iial

, ,

D'? 5

L2ori't QLII

,i;nal?

riil

the air!

this signal ~II

., ,

yiii;e~ oi,:-,;oshier transmit-

t2r5 ;I>

your favorite net or club) to

have one of your owlt!

Overail Circuit Operaiie~n

Let's first discuss Fig 2,

the spectrum analyzer

bloc!< diagram. We will

then look at the circuits in

each block in detail. Notice

that the analyzer block dia-

gram looks similar to that

of a s~ngle-conversionsu-

perheterodyne receiver.

The i-f frequency of the

spectrum analyzer is 90

MIHz.

I-iext rnixed with the 90-Mlir

to 150-MHz voltage-con-

trolled oscillaior- (vco) in

the double-balanced mixer.

I he,differ.ence output from

the mixer, which is the de-

sired I-f signal, is then fil-

tered by the 90-MHz band-

pass filter. The bandpass fil-

ter provides the necessary

selectivity lor the spectrum

analyzer. The 90-MHz sig-

nal from tiit; bandpass filter

is preamjsiitied and applied

to the Iog ampiifier. The

output ot the log amplifier

is logarithmic signal

strength video for the oscii-

loscope vertical (Yj axis.

The voltage-controiled

oscillator frequency is con-

trolled by the sweep gener-

ator, which simultaneously

controls the horizontal (or X

axis) of the oscilloscope.

Note that when the vco is

L, Watts to sup-

, .

pre:~ ;i: .iii,nal frequency

coin;:c.r;:?i;:5 (spurs) outside

.';-'

il!e TIT

---:

Spectrum Analyzer. Hookup

Fig. 1 shows how to hook

up the high frequency spec-

tru~ndnalyzer for rnonitor-

rng the cutput spectrcrni of

a trarlsrnitter or linear am-

plifier. Kemernber, the ana-

lyzer is a receiver. it re-

quires a very srnall sample

of power for operation. This

is done with an L-pad Sam-

pler. 1-he sampler will not

interfere with normal trans-

mitting or transceiving op-

eration The output frorn

the L-pad is further reduced

with a step attenuator to

match the full-scale input-

power requirements of the

analyzer (114 to 1/10 of a

milfiwatt). The spectrum is

displayed on the oscillo-

scope being used with the

spectrum analyzer.

It is important to observe

good safety practices when

using the L-pad, attenuator,

and spectrum analyzer. Be

sure all station equipment,

the L-pad, attenuator, ana-

lyzer, and oscilloscope

cases are properly ground-

ed. Use the proper L-pad for

your power range. Double-

check your hookup before

applying power. If the out-

put of a transmitter was di-

rectly connected to the an-

alyzer by accident, it would

instantly be damaged when

the transmitter was keyed.

of operation at

jCZCt :,;1 .- ;:2.

L, -. ., ._ below the fun-

d~.j.;i~!-;i

?i . :,i a transmitter

'1 -:> .;::(I i/?iatts, this fig-

ui-0 :.: .;., :.:%, For a 1000-

mi;: ;ii..r;i!tter

f rci--!

or linear

..,.

ar-i;~i;ii:l: ..h:e figure is 43

dB. . I:i:.$:-k!~g

our photo

agar, ,d..; :ioiice that the

3:~'; i, ;ri;i?j(lic signal is

abiii:: ,c jilJ below the fun-

dam<-!iir,-i. Jl/e1realso going

to i, :..=

The sampled input signal

from the L-pad is adjusted

to the proper power level

with the step attenuator, as

we discussed before. The

signal is titen taker1 through

a low-pass filter with a

60-MHz cutoff frequency.

The low-pass filter prevents

90-MHz signals from leak-

ing into the analyzer and

"confusing" it. The input is

. .

.; groblern with the

3rd i,?.;-:::>r~ic if we are run-

niu?g ', '!ia:ts oi more pow-

er. 7:: ;it!-I harmonic is no

prcoie.-:-; :,ii~ceit's about 55

dl? .',i?icj>,v ihe fundamental.

5-i

,:t; correct the prob-

letri :?v ddding a filter be-

t\*~~~~-,

e :id(-

- ,

. , transceiver or lin-

- .it,(: ~.$ c antenna. How-

e\:i?:., ,:niess we are able to

checi; rile output spectiurn

of transmitting equip-

rnC17i, .NC? may never I<now

we a pl.oblen3--until

'ieighbors start corn-

or we get a "friend-

I\/ adx:isory" from the local

Fc:C

--is-

OSCILLOSCOPE

Er-

SAMPLED

e Ax's

HF SPECTRUM

:;;:)nitoring

station.

ANALYZER

WlDTH

CENTER

1 ;!;?re are many uses for a

Spec.iiun analyzer besides

mOn!:cring transmitter out-

PQts,!>titt.his use alone can

%a!<? 32 l-iF spectrum an*

lyre7 nonstruction project

"or-ri?whlie. If you build

One. yocr'll probably be the

firs;

Note I. Never hooktransmitteror linear directly to step attenuator or

analyzer. Always use L-pad sampler of the proper power rating.

Note 2. Be sure transmitter, linear, L-pad,attenuator, analyzer, and

scope are grounded.

oil your blo~k

Fig. 7. Typical HF spectrum analyzer hookup.

73 Magazine

(or in

August, 1982

'ill

with a Z-axis (blanking) in-

put. The power supply pro-

tuned to 90 MHz, the ana-

lyzer is tuned to zero MHz.

When the vco is tuned to

120 MHz, the analyzer is

tuned to 30 MHz. With the

vco at 150 MHz, the analyz-

er is tuned to 60 MHz.

The tuning range of the

analyzer is adjusted with

the center-frequency and

frequency-span controls on

the sweep generator. The

sweep generator automati-

cally tunes the analyzer

across its tuning range

about 10 times each sec-

ond. The sweep generator

vides + 24 V dc, + 12 V dc,

and -6 V dc for the spec-

trum analyzer circuitry. The

power supply operates

from 12 V ac supplied by a

wallplug transformer.

Fig. 3 shows the sche-

matic of a 100-to-1000-Watt

L-pad sampler, with alter-

nate circuitry for a 10-to-

100-Watt sampler, a l-to-10-

Watt sampler, and a0.25-to-

I-Watt sampler. Four pairs

of 4.7k, I-Watt resistors

form the series element of

the 100-to-I000-Watt sam-

pler. A 51-Ohm, 112-Watt re-

sistor forms the shunt ele-

ment. The L-pad resistors

are rated for continuous op-

eration. A single hair-thin

strand from an old "zip"

cord provides some fusing

protection in the event of a

component failure or cir-

cuit fault. The series ele-

ments for the other power

ratings are shown in Fig. 3.

Photo D. Bottom view of spectrum analyzer chassis. Log

clamps or "shorts out" the

amplifier is at the top. Power supply and sweep generator

video during the retrace be-

board is directly below the log amplifier. Vco is next. The

tween each sweep to avoid

mixer is directly below the vco. The mixer connects to the

a confusing oscilloscope

low-pass filter at the left. The bandpass filter is at the lower

display. This eliminates the

right. Preamplifier is on the middle right.

need for an oscilloscope

H MIXER

H PREAMP 14 LOG AMP h

INPUT

FROM

L-PA0

VIDEO TO SCOPE

-, IY AXIS)

2.5VP-P

ATTENUATOR

FILTER

SAMPLER

0-to-59-dB Step Attenuator

Fig. 4 shows the step at-

tenuator schematic. Five pi-

style resistive attenuators

are switched in or out as

necessary to achieve the

proper attenuation. Switch-

es are double-pole, double-

throw. Resistors may be 112

Watt or 114 Watt, although

114-Watt resistors are easier

to work with. Note the

shielding between sections.

Resistors must be 5% toler-

ance. (The resistor values for

each attenuator came from

Reference 1.)

RETRACE CLAMP

FREO. SPAN ADJUST

SWEEP TO SCOPE

+ ( X AXIS)

3vP-P

10-12H2

POWER SUPPLY

CENTER FREO ADJUST

Fig. 2. Block diagram.

TO ANTENNA

DUMMY LOAD

TO ATTENUATOR

AND

HF SPECTRUM

ANALYZER

TO TRANSMIT

OR LINEAR

TER

Low-Pass Filter,

Mixer, and Vco

Fig. 5 shows the details of

these circuits. The low-pass

filter consists of three pi-

sections, separated by

shielding. The cutoff fre-

quency of the filter is about

60 MHz. Three sections are

used to give a high attenua-

tion at the 90-MHz i-f fre-

quency and above.

Each port of the double-

balanced mixer is padded

with 50-Ohm attenuators to

100-1000 WATT SAMPLER

.25-1 WATT SERIES ELEMENT

1-10 WATT SERIES ELEMENT

10-100 WATT SERIES ELEMENT

Note 1. Carbon composition (noninductive) resistors.

Note 2. "Fuse" is single, hair-thin copper strand from ac "zip" cord.

Note 3. Connect SO-239 connectors with RG-8 center conductor wire.

Note 4. Test-run sampler before connecting to attenuator.

Note 5. Keep BNC connector 3" away from SO-239s;space resistor sets318" minimum; "fuse" is 112" to

314" long.

Fig. 3. L-pad power samplers.

73 Magazine

August, 1982

encourage good mixer per-

formance (low mixer spurs)

at the expense of extra con-

version loss. Mini-Circuits

SRA-I and SBL-I are good

commercial mixers. It is

quite possible to build a

suitable double-balanced

mixer from small ferrite tor-

oids and hot carrier diodes,

if you have trouble finding

these commercial units.

(Consult Reference 1 for de-

tails.)

2N5379 vco amplifier. The

output of this amplifier

drives the local oscillator

port of the mixer. A diode-

capacitor rf detector pro-

vides a dc output for check-

ing amplifier output power.

The wideband amplifier de-

sign is based on data from

Reference 1. The oscillator

design is based on third-

attempt desperation! Note

the use of the feedthrough

. ..

capacitors and shielding.

These are as much a part of

Photo E. VCO layout. Osci//ator is near the feedthroughs.

The vco consists of an

MRF901 Colpitts oscillator

coupled to a wideband

2N5179 amplifier. The

MRF901 was eventually

chosen for the oscillator

transistor because of its

well-behaved phase-shift

characteristics between 90

MHz and 150 MHz. The two

MV109 hyper-abrupt Epi-

cap diodes act as tuning ca-

pacitors and account for

the oscillator's wide tuning

range. A small pick-up loop

near the oscillator coil pro-

vides an output for check-

ing frequency and doing

other tests. The oscillator is

also lightly coupled to the

the circuit as the MRF901.

ture-coupled to each other.

resonators. The input and

Bandpass Filter

output resonators are tap-

The two center resonators

are slightly stagger-tuned to

The bandpass filter is de-

coupled to the input and

tailed in Fig. 6. It consists of

output connectors. The

give the filter bandpass a

four relatively small helical

four resonators are aper-

sharp "nose."

The 3-dB

UG-625BIU

R.58 COAX TO

EQUIPMENT

UNDER TEST

(t20dbm MAX0

RG~58COAX TO

LOW PASS FILTER

(6dbmTO IOdbmI

1 300

Note 1. DPDT toggle switch-Radio Shack 275-1546 or equivalent.

Note 2. BNC receptacle-Radio Shack 278-105 or Amphenol 31-236.

Note 3. Resistors 112 or 114 W, 5% noninductive.

Note 4. Attenuator box made from single- and double-sidedG-10 circuit board plus copper shim stock.

Fig. 4. 0-59-dB step attenuator

RG-58 COAX TO

BAND PASS FILl

ATTENUATOR

1-6 TO -1Odbml

+I2VDC FROM

POWER SUPPLY

VCO TUNING VOLTAGE

FROM SWEEP CIRCUIT

2-I8V. 10-12 Hi

AND ACCESSORY

314 TURN *I4

1/2" INSIDE DlA

I I

VCO OUTPUT

LEVEL ( D C 1

Note I. Resistors are 114 W, 5%; unspecified capacitors are 50-V ceramic.

Note 2. Capacitors marked "SM" are A 5% silver mica.

Note 3. 1000-pFfeedthrough capacitors available from Alaska Microwave.

Note 4. MV-209sor MV-309smay be substituted for MV-109s (contact Motorola distributor).

Note 5. Box built from single- and double-sided G-10 circuit board plus copper shim stock.

Fig. 5. Low-pass filter, mixer, and vco.

73 Magazine

August, 1982

will begin contributing to

the output. As the output is

made still larger, the 5th

stage will saturate or limit.

From this point it will con-

tribute no additional volt-

age across the Ik output re-

sistor. At about this same

signal level, the 3rd log amp

stage will begin to contrib-

ute some output, and so on.

Each log amp stage pro-

vides a gain of about 12 dB

until it saturates. The gain

of the i-f strip, from the Ik

resistor's point of view, then

drops 12 dB. It is this suc-

cessive limiting and drop-

ping off of i-f stages that

creates the logarithmic vid-

eo output characteristic.

Note that when the 1st log

amp stage saturates, the log

amplifier reaches its full-

scale output.

I was surwrised how accu-

rately the 'logarithmic am-

plifier does track a logarith-

mic curve. Using my com-

mercial ster, attenuator as a

reference, the calibration

of my logarithmic amplifier

was within 1 dB. The sensi-

tive i-f system must be

shielded to prevent interfer-

ence from commercial FM

stations

picked up by the 12-V-ac

power leads.

The heart of the sweep

generator is the 555 IC

timer. The two 2N2907s act

as current sources. Each

generates linear ramp volt-

ages across 10-uF tantalum

capacitors. The 555 syn-

chronizes the ramps. The

ramps are set at a 10-Hz-to-

12-Hz repetition rate. One

ramp is fed through a dc-re-

storing capacitor-diode

clamp to the output con-

nector for the oscilloscope

horizontal (X) axis. The sec-

ond ramp is fed to the 5k

frequency-span potentiom-

eter through an inverting

operational amplifier buf-

fer The output from the fre-

quency-span pot IS summed

with the output of the 5k

center-frequency pot in the

vco-tuning voltage amplifi-

er. The output of this ampli-

fier is fed to the vco-tuning

voltage input.

When the ramps are reset

by the 555, pin 3 of the 555

also trim the retrace VMOS

clamp transistor through

the retrace comparator am-

plifier. This shorts the loga-

rithmic amolifier video out-

put to ground during re-

trace. Otherwise, the video

is fed to the outout connec-

tor for the oscilioscope ver-

tical (Y) axis. The 4th ampli-

fier in the TL084C quad-op-

erational-am~lifier IC is

used simply a; a 6-V-dc ref-

erence by the other three

amplifiers.

Photo F. Bandpass filter layout

bandwidth of the filter is

about 220 kHz. Insertion

loss is somewhat high, but is

acceptable for this applica-

tion.

Notice that each stage in

the log amplifier has an rf

detector across its output

consisting of a 50-pF capac-

itor, a IN914 diode, and a

10k resistor. The rf detector

on the buffer stage is just a

tuning aid. The outputs of

the rf detectors on the 1st

through 5th log amp stages

are tied to a common I k re-

sistor (in parallel with a

150-pF capacitor). Because

of its relatively low value,

the detector outputs are

more or less summed

across the Ik resistor.

Preamplifier

and Log Amplifier

The schematics of the

preamplifier and log ampli-

fier are shown in Fig. 7. The

preamplifier consists of two

wideband 2N5179 amplifi-

ers. The log amplifier con-

sists of six tuned 90-MHz i-f.

stages. Each stage uses the

friendly 40673 dual-gate

FET. The input stage acts as

a buffer amplifier. The next

five stages form the loga-

rithmic signal-strength vid-

eo detector. The log ampli-

fier may remind you of an

i-f strip in an FM receiver. In

fact, it uses the limiter prin-

ciple in its operation.

Power Supply and Sweep

Generator Circuits

These circuits are shown

in Fig. 8. The power supply

is straightforward, provid-

ing +I2 V dc, +24 V dc,

and -6 V dc. Note the

feedthrough capacitors

used to filter out any rf

A small input signal is

amplified by all five log

amp stages. Only the 5th

stage will develop enough

signal to provide an output

from its detector. As the in-

put signal is made larger,

the 4th stage detector also

Shielded Enclosure

Construction

All circuits in the high

frequency spectrum ana-

lyzer except the sweep gen-

erator and the power sup-

ply must be installed in

shielded enclosures. I built

each enclosure for my ana-

lyzer using 1116-inch, C-10

epoxy circuit board stock.

Enclosure base ~latesare

made from singl'e-sided or

double-sided stock. Dou-

ble-sided stock must be

used for the enclosure

sides, ends, and partitions.

(See Fig. 9 for construction

details.)

STOCK

DETAIL

SIDE VIEW

FRONT VIEW

Note 1. Coils are 6 turns of #12, 112" inside diameter, 518" long, taps at 114 turn.

Note 2. 10-pFpiston trimmer, Sprague-Goodman GGP8R500 or equivalent; alternate, air-variable, John-

son 189-564-1.

Note 3. Filter box made from single- and double-sided G-10 circuit board plus copper shim stock.

Note 4. Filter box is 1-118" deep.

Note 5. Mount BNC connectors near front side.

Note 6. Coupling apertures are 318" x 3116". Drill 318"-diameter holes in compartment wall pieces and

then solder copper shim strips across tops and bottoms to narrow apertures.

Fig. 6. Bandpass filter.

73 Magazine

August, 1982

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