interface.pdf

By Allen R. Wooten, WD4EUI

106 Belair Road, S.E.

Huntsville, Alabama 35802

INTERFACE YOUR

COMPUTER TO THE

"POOR MAN'S

SPECTRUM ANALYZER"

Add features found in expensive

commercial units to your "Poor

Man's Spectrum Analyzer l l

S ince the publication of the "Low-Cost

so I interfaced the "Poor Man's Spectrum

Analyzer" to my IBM AT compatible com-

puter and wrote a control program in

Microsoft" Quick Basic! The interface

requires a digitizer board that's mounted

inside the computer, a frequency control

board mounted with the spectrum analyzer,

and an interconnecting cable.

The computer controls user-selected start

and stop frequencies through the parallel

printer port. After the amplitude versus fre-

quency data has been acquired, it's plotted

on the screen. The "print screen" command

then plots the data on paper via the printer.

Figure 1 shows my spectrum analyzer and

the computer interface.

The program lets you select the start fre-

quency in two bands (l to 500 MHz and 393

to 900 MHz) for use with the two tuners

available from WA2PZO. You choose the

start frequency, and the program establishes

it using a 12-bit D/A converter (DAC). The

frequency sweep uses an 8-bit DAC to obtain

high resolution over a limited frequency

range above the start frequency. The maxi-

mum permissible stop frequency is calcu-

lated based on the start frequency, and then

displayed. After you select the desired stop

frequency, the program makes one sweep

and displays the plot on the monitor screen.

Spectrum Analyzer with Kilobuck Fea-

tures" by Robert Richardson, W4UCH,

in the September 1986 issue of HAM

RADIO, 1 there have been several follow-up

articles about modifying and improving this

interesting project.13 2 ,3,4 I purchased the

tuners and receiver board from WA2PZO*

and built my own version. I combined the

VHF "cable ready" tuner and the VHF/UHF

tuner (the VHF section isn't used) in a

single enclosure and now have a spectrum

analyzer that covers from I to 900 MHz in

two ranges.

Background

I added a sawtooth generator to produce

the frequency sweep described by Joe Carr

in the September 1987 issue of HAM

RADIO} This provided a good oscilloscope

display of the frequency spectrum. A turns-

counting dial, attached to the multi-turn

frequency-control potentiometer, let me set

the center frequency accurately after

calibrating the dial. The more expensive

spectrum analyzers offer additional features

like calibrated display (amplitude and fre-

quency), programmable start and stop fre-

quencies, and the capability to plot the

spectrum display.

I wanted to add these features to my unit,

• IBM AT is a registered trademark of IBM Corporation. Quiek BASIC

and GW-BASIC are registered trademarks of Microsoft Corporation.

• WA2PZO, Science Workshop, Box 393, Bethpage, New York 1\714.

Communications Quarterly

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Figure 1. Spectrum analyzer.

Digitizer board

The schematic for the digitizer board is

shown in Figure 4. Integrated circuit V3 is

an 8-bit A ID converter that requ ires no spe-

cial logic for interfacing with the computer

data bus. The output latches of the A ID

converter have high-impedance outputs that

connect directly to the computer's data bus.

The A ID converter only outputs data when

addressed by the computer. Potentiometer

Rl sets the reference vo ltage for the con-

verter a nd thu s determines the converter's

full-scale sett ing. VI and V2 are address

decoders wired so an address of 22F hex-

ade cimal (559 decimal) will start the conver-

sion .

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Figure 2. Spectru m plot using the VHF tuner.

DAC operation

While in operation, the computer treats

the digitizer board as just another memory

location. The board only digitizes the ana-

log inpu t when memory location 22F is

addressed. V4 is used as a switch to select

th e appropriate DAC on the frequen cy con-

trol board when the computer sends fre-

quenc y control da ta. Wh en the computer

add resses memory location 22B hexadecimal

(555 decimal), the DAC-select line is a logic

high. When the computer addres ses mem-

ory location 220 hexadecimal (557 deci-

mal) , th e DAC-select line is a logic low.

Mor e ab out the DAC-select line later.

Digitizer construction

The digitizer board is mounted inside the

computer in one of the expansio n slots.

Each expansion slot has two receptacles for

the edge co nnectors on the expansio n

boards. The digitizer board requi res j ust on e

edge connector and uses only th e receptacle

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Figure 3. Spectrum plot using the UHF tuner.

Figure 2 shows a typical plot from the

spectrum analyzer program using the VHF

tuner. This sample shows the FM broadcast

band with a part of the aircraft band as

received in the Huntsville, Alabama area.

Figure 3 is a typical plot using the VHF

tuner. In th is plot , you can clearl y see the

video, colo r, and audio carriers of television

channel 19.

November 1990

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f igure 4. Schema tic for the digitizer board .

PERFORATED

IBM AT-COMPATIBLE

PROTOTYPE BOARD

~~fERENCE

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OUTPUT ( l/S"

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CONNECTOR )

I ( SIDE SHOWN) A31

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UNUSED COMPUTER

MOTHERBOARD

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EDGE CONNECTOR

(DOUBLE

SIDED )

Figure 5. Layoul of the digilizer board .

Commun ications Quarterly

D-TYPE CONNECTOR ON SPECTRUM ANALVlER ENCLOSURE

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Figure 6. Schematic for the frequency control board.

through B31 refer to edge-connector pins on

the solder side. I made a cutout in the board

to clear the unused expansion slot recepta-

cle. Rl is mounted near the top of the

board. With the computer cover removed,

you can adjust Rl while the board is

mounted inside the case.

at the rear of the computer.

I built the board on a perforated proto-

type board made especially for the com-

puter. These boards are available from most

electronics hobby stores and mail-order sup-

pliers for about $25. I used point-to-point

wiring. All ICs are mounted in sockets to

facilitate repair.

The component layout isn't critical. Fig-

ure 5 shows the layout and edge-connector

pinouts of the digitizer board. Note that the

edge connector is double sided. In the

schematic, pin numbers Al through A31

refer to edge-connector pins on the compo-

nent side of the board. Pin numbers Bl

Installing the digitizer board

The analog input and DAC-select line are

the only connections external to the com-

puter. I attached the slot cover from the slot

in which the digitizer board is installed to

the rear end of the board (see Figure 5).

November 1990

Two small brackets stand the cover off from

the board so the cover fits properly against

the computer rear frame when the digitizer

board is in the machine. I mounted a BNC

connector on the slot cover for the analog

input and a lI8-inch phone jack on the slot

cover for the OAC-select signal.

Frequency control board

The schematic for the frequency control

board is shown in Figure 6. The circuit uses

two OACs. A 12-bit OAC decodes the start

frequency; an 8-bit OAC sweeps the fre-

quency from the start to the stop frequency.

System logic

Both OACs are controlled through the

computer's parallel-printer port. The DAC-

select signal generated on the digitizer board

chooses the OAC to be addressed by the

printer port. When the OAC-select line is a

logic high, the 12-bit OAC is selected. Con-

versely, when the OAC-select line is a logic

low, the 8-bit OAC is selected.

The parallel-printer port is an 8-bit port.

To control the 12-bit OAC, two 8-bit words

- Word I and Word 2 - are sent from the

computer. Each word contains six bits of

data. The remaining two bits identify

whether Word 1 or Word 2 is being sent.

Bits 00 through 05 from the parallel-

printer port contain the data. Bits 06 and

07 are the word identifiers. The identifier

for Word 1 is a logic 0,1 for bits 06 and 07,

respectively; the identifier for Word 2 is a

logic 1,0 for bits 06 and 07, respectively.

The 6-bit data of Word 1 from the paral-

lel printer port are temporarily stored in

latches Ul and U2. The data are latched

under command of the Word I latch signal

generated by V3 and VII. The 6-bit data

stored in Ul and U2 are then stored in

latches U5 and U6. The 6-bit data of Word

2 from the parallel-printer port are stored in

latches U6 and U7 under command of the

Word 2 latch signal generated by U3 and

Ull. Thus the 12-bit data required by the

12-bit DAC are stored in latches U5, U6,

and U7, following two consecutive outputs

from the parallel-printer port. The 12-bit

OAC (U9) will then output a current

proportional to the 12-bit word present at

its inputs. Ul3 converts U9's output current

to an output voltage.

The OAC-select signal is inverted and

applied to one input of NAND gates U4

and U8. When the OAC-select signal is low,

these gates are activated and the 8-bit data

from the parallel-printer port are applied to

8-bit OAC UW's inputs. The computer

generates a series of 8-bit words and outputs

them to UIO to generate a ramp at UIO's

output. Ul3 converts UIO's output current

to an output voltage.

The voltage-converted outputs of the two

OACs are summed by non-inverting ampli-

fier U12. (U12 is an audio power driver

intended for use in high-fidelity audio

amplifier designs.) Because it can operate

with high power supply voltages, I used it in

this application as a high-voltage op amp.

The tuners used in the spectrum analyzer

require tuning voltages ranging from zero to

about 30 volts to cover their wide frequency

range. U12's output can produce the required

tuning voltages and drive the tuners directly.

The op amp doesn't need to deliver a large

current because tuning is accomplished

inside the tuners with reverse-biased

variable-capacitance diodes. Potentiometer

R7 is used to calibrate the start frequency.

Control board construction

I built the frequency control board on a

perfboard using point-to-point wiring. All

integrated circuits are mounted in sockets to

facilitate repair. Once again, the board lay-

out isn't critical. The board is mounted

inside the spectrum analyzer enclosure. I

added switch S2 in series with the tuning

inputs of the two tuners so I could select

manual (internal) or computer (external)

control. See Figure 1 for details.

Interconnecting cable

The computer and the frequency control

board in the spectrum analyzer are con-

nected via a lO-conductor cable. I mounted

a 25-pin Ootype connector on the spectrum

analyzer enclosure for connection to the

cable. One end of the cable has a 25-pin 0-

type connector compatible with the one on

the spectrum analyzer. The computer end of

the cable has two connectors. One is a 25-

pin Ootype compatible with the computer's

parallel-printer port. The other connector is

a lI8-inch phone plug. This plug carries the

OAC-select signal and fits into jack 12 on

the computer slot cover mounted on the

digitizer board. The pinouts for the Ootype

connectors on the spectrum analyzer and

computer end of the cable are shown in

Figure 7.

Computer program

I originally wrote the computer program

in Microsoft Quick BASIC for use with an

EGA monitor so I could tap into the high-

resolution graphics capability. I've included

line numbers on all lines of the code (Quick

BASIC doesn't require line numbers for all

lines) so the program can run with the more

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