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Use your PC’s “bi-directional” printer port to monitor

frequencies up to 32kHz.

This project is the latest in

our occasional series of PC-

based test gear projects. Like the

others, this Audio Frequency Me-

ter is based on a design that was

previously featured in an

Inter-

face

article (Oct ‘97)

(This refers

to the printed version of EPE,

Ed.)

The hardware and software

of the present design are not

identical to those described in the

original article, but they are es-

sentially the same.

This article takes things a

step further by providing full

constructional information, in-

cluding a custom printed circuit

board (PCB) design. Like the

original, it operates up to a

maximum frequency of

32 767kHz and has 0 5Hz reso-

lution. The unit therefore covers

slightly more than the full 20Hz

to 20kHz of the audio range.

SYSTEM OPERATION

In common with most digital

frequency meters, this system

determines the input frequency

by counting the number of cy-

cles in a given period of time. In

order to obtain reasonable reso-

lution at audio frequencies, it is

essential to use a fairly long

counting period. In this case a

gate period of two seconds is

used, giving 0 5 Hertz resolu-

tion. The general arrangement

used in the interface is shown in

Fig.1.

large input signals. Input volt-

ages of up to at least 50V peak-

to-peak can be handled safely.

The interface should work

with any reasonably modern

PC, but it does require a spare

bi-directional printer port. If your

PC is not suitably equipped, or

its printer port is already occu-

pied, adding an inexpensive

printer port card will enable it to

operate with this interface.

There is a slight advantage

in using a fast PC, but it should

be possible to obtain good re-

sults with anything from an old

80386 machine to the latest

super-fast Pentium marvel. The

MS-DOS software for the inter-

face runs under QuickBASIC,

QBasic, or GW-BASIC, and one

of these interpreted languages

is needed in order to run

the software.

A simple limiter stage at the

input clips the input signal and

prevents any damage from oc-

curring to the subsequent cir-

cuitry. The next stage is a buffer

amplifier that provides the unit

with an input impedance of a

few hundred kilohms, which is

high enough to ensure that

there is minimal loading on the

circuit under test.

The next stage is a Schmitt

ACCURACY

Although the accuracy of

the unit does not compare to a

high-quality digital frequency

meter, it still provides more

than adequate results for many

purposes. It is certainly easier

to use and more accurate than

an analog frequency meter.

The input impedance is a few

hundred kilohms, for input levels

of up to about 1 3V peak-to-peak.

Above this level the input

impedance falls due to the action

of a limiter circuit that protects

the unit against damage from

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Fig.1. Block diagram for the PC Audio Frequency Meter.

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trigger, and this performs two

functions. It provides a certain

amount of amplification that

boosts the input sensitivity of

the circuit so that an input level

of only about 100 millivolts

peak-to-peak is needed to drive

the unit properly. It also ensures

that input levels that are slightly

inadequate fail to produce any

output from the trigger circuit,

and do not provide a reading.

This avoids a situation in which

borderline signal levels produce

misleading results.

The output from the Schmitt

trigger is at normal logic levels,

and it is fed to a 2-input NOR

gate that acts as a signal gate.

This is controlled from the

Strobe handshake output of the

printer port, and the two-second

gate pulse is generated by soft-

ware and some of the internal

timing hardware of the PC.

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BINARY COUNTER

The output of the NOR gate

feeds into a 17-stage binary

counter, but prior to starting a

new count this must be reset to

zero. The reset pulse is gener-

ated in software and delivered

to the counter via another hand-

shake output (Init) of the printer

port. At the end of the gate pe-

riod the value held in the

counter is double the input fre-

quency. In order to obtain a

reading in Hertz it is merely

necessary to read the counter

and divide the returned value

by two.

There is a slight complica-

tion in that the value must be

read from the first 16 outputs of

the counter, but the printer port

has only eight data inputs. This

is overcome by using a simple

method of multiplexing.

The 16 outputs are read via

two 8-bit 3-state buffers that are

controlled via a third handshake

Maxfield & Montrose Interactive Inc

Fig.2. Circuit diagram of the PC Audio Frequency Meter input

stages.

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Fig.3. Circuit diagram for the logic stages of the PC Audio Fre-

quency Meter.

output of the printer port (the

ALF output). One buffer is con-

trolled directly from this output,

but the other is driven via an

inverter, and consequently only

one buffer or the other will be

active at any one time,

depending on the state of the

handshake line.

The handshake line is set

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high to read one byte and then

low so that the other one can be

read. A simple software routine

then combines the two bytes to

reconstitute the 16-bit value.

It is important that any over-

flow of the counter is detected,

as erroneous results will other-

wise be obtained. Having

reached its maximum value the

counter will simply cycle back to

zero and start counting once

again. Slightly excessive input

frequencies will therefore pro-

vide what appear to be valid

readings while they are in fact

very much lower than the true

input frequency.

This problem is overcome

by monitoring the 17th output of

the counter using the Busy

handshake input of the printer

port. In normal operation the

17th output will always be low,

but this output goes high if the

counter overflows.

If the input frequency is well

above the maximum that can be

handled by the counter, the out-

put of the 17th stage could go low

again by the end of the gate pe-

riod. It therefore drives a simple

latch circuit, which ensures that a

valid overflow signal is always

present at the end of the gate pe-

riod. The state of the overflow

output is detected via the Busy

handshake input, and a suitable

on-screen warning is displayed if

this output is high. A fresh count

cycle is then commenced.

than about

0 65V, and D2 be-

comes forward biased when volt-

age is more than about +0 65V.

Transistor TR1 operates as a

simple emitter follower stage.

This does not provide any voltage

gain, but it provides the unit with

a high input impedance. Capaci-

tor C5 couples the output of TR1

to a conventional Schmitt trigger

circuit based on TR2 and TR3.

Component IC7c is the signal

gate, and this is one of the four

CMOS NOR gates in a 4001BE

device. Taking the control input at

pin 1 high forces the output to the

low state, but when this input is

low the output assumes the oppo-

site state to the other input. This

gives the required gating action,

and the inversion through IC7c is

of no practical consequence.

The circuit diagram for the

logic stages of the unit is given in

Fig.3. IC2 and IC3 are the 8-bit

buffers, and IC1 is the inverter

that ensures they are controlled in

anti-phase fashion. There are ac-

tually six inverting trigger stages

in IC1, but in this circuit only one

is actually utilized, and no con-

nections are made to the other

five sections of this component

(being an LS-type IC, the unused

inputs do not need to be tied

to a power line, unlike CMOS

devices).

The outputs of the buffers

drive the bi-directional data lines

of the printer port, but there is a

minor complication here, because

these lines default to the output

mode when the PC is first

switched on. They only operate

as inputs when the frequency me-

ter software is run. This means

that initially the outputs of the

buffers will be connected to the

outputs of the printer port.

Resistors R1 to R8 provide

current limiting that protects both

sets of outputs until the printer

port is set to the input mode. The

value of these resistors is made

low enough to prevent them

from hindering normal operation

of the interface, but high

enough to ensure that no

damage occurs to either the

interface or the printer port

hardware.

The counter consists of

three 4024BE 7-stage binary

dividers (IC4 to IC6) connected

in series. This obviously pro-

vides a 21-stage divider, but the

outputs of the final four stages

are left unused. The output of

the 17th stage drives a simple

latch circuit based on IC7a and

IC7b. The latch is reset by the

same signal that is used to reset

the counter.

Note that one section of IC7

is not used in this circuit, and so

its inputs (pins 5 and 6) are sim-

ply tied to the 0V line, as rec-

ommended for unused CMOS

inputs.

A +5 volt supply is required,

and the current consumption of

the circuit is about 70 milliamps.

Obviously the circuit could be

powered from its own power

supply unit, but it is cheaper and

more convenient if power is ob-

tained from the PC. Unfortu-

nately, there is no supply output

of any kind on a PC printer port,

and power must therefore be

obtained from one of the other

ports. Suitable outputs are

available from the keyboard and

game ports.

CIRCUIT DESCRIP-

TION

The circuit diagram for the

input and gate stages of the PC

Audio Frequency Meter is

shown in Fig.2. The limiter cir-

cuit uses resistor R9 and diodes

D1 and D2 to clip the signal at

approximately

0 65V. Diode

D1 becomes forward biased

when the input voltage is more

CONSTRUCTION

The topside component lay-

out and hard wiring for the printed

circuit board are shown in Fig.4,

together with an (approximately)

actual-size copper track master

pattern. This board is available

from the

EPE Online Store

(code

7000232) at

www.epemag.com

Bare in mind that IC4 to IC7

are CMOS devices and that

Maxfield & Montrose Interactive Inc

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&RQVWUXFWLRQDO 3URMHFW

they require the usual anti-static

handling precautions. The most

important of these is that you

ensure that you discharge static

electricity from your body before

handling them. Touching the

bare metal of a nearby water

or radiator pipe is usually satis-

factory.

It is recommended that you

mount the devices in sockets

rather than soldering them di-

rect to the board. Do not fit the

ICs into their sockets until

the board is completed, and

try to touch the pins as little as

possible.

There are number of a link-

wires on the board, and these

are made from about 0 56mm

(24 s.w.g.) tinned copper wire.

In some places there are sev-

eral wires running side-by-side

and very close together. These

wires can be insulated with

pieces of PVC sleeving to en-

sure that there is no risk of any

accidental short-circuits.

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The connections are

densely packed in some parts of

the board, and many of the cop-

per pads are quite small. In or-

der to avoid numerous short-

circuits, it is necessary to use a

soldering iron having a fine bit

and to take great care when

making the connections. Check

the finished board for short-

circuits using a magnifier, and

make sure that you completely

remove any that are found.

In other respects, construc-

tion of the board is largely

straightforward. The layout is

designed for polyester capaci-

tors having 7 5mm (or 0 3 inch)

lead spacing, and it could be

difficult to use components hav-

ing a different lead spacing. At

this stage only fit single-sided

solder pins at the points where

connections to SK1, SK2, and

PL1 will eventually be made.

2 32

Fig.4. Printed circuit board component layout, wiring details,

and (approximately) full-size copper foil master pattern for the

PC Audio Frequency Meter.

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COMPONENTS

Resistors

R1 to R8 270 ohms (8 off)

R9 10k

R10 470k

R11 680k

R12, R15 3k9 (2 off)

R13 56k

R14 27k

R16 470 ohms

R17 15k

R18 8k2

R19 2k2

All 0.25W 5% carbon film or better

Capacitors

C1 100n ceramic

C2 100u radial electrolytic, 10V

C3, C4 470n polyester, 7.5mm

lead spacing (2 off)

C5 10u radial electrolytic, 25V

C6 150p ceramic plate

Semiconductors

D1, D2 1N4148 signal diodes (2 off)

TR1 BC550 low-noise

npn

transistor

TR2, TR3 BC549

npn

transistors

(2 off)

IC1 74LS14 hex inverter

IC2, IC3 74LS541 octal tri-state

non-inverting buffers (2 off)

IC4 to IC6 4024BE 7-stage binary

ripple counters (3 off)

IC7 4001BE quad 2-input NOR gate

Internal layout of the completed PC Audio Frequency Meter

showing positioning of the PCB and the ribbon

cable exit cutout.

standard oscilloscope test leads a

BNC socket should be used

instead, as this matches the

BNC plugs normally fitted to

these leads.

A flat file is used to make a

broad notch in the rear panel for

the ribbon cable that connects

the unit to the printer port of the

PC. Socket SK1 can be a 2mm

or 4mm type mounted on the

rear panel, or the power lead

can simply pass through the

same notch as the data cable.

The printed circuit board is

mounted on the base of the

case using either plastic stand-

offs or metric M3 screws, with

spacers about 6mm to 12mm

long to keep the underside

of the board well clear of the

metal base panel.

connector (a 25-way male D-

type connector). It is unlikely

that 13-way cable will be avail-

able, but a 20-way cable is eas-

ily pruned down to size. There

should be no difficulty in tearing

away a seven-way strip to leave

the required 13-way cable.

Both ends of the cable are

prepared in the same way, and

this starts by separating the ca-

ble into individual leads over a

length of about 20mm to 30mm.

Next, a few millimeters of insu-

lation are removed from the end

of each lead and the bare wires

are then tinned with solder.

The connector details are

shown in Fig.5.

Provided the pins on the

circuit board and the appropri-

ate terminals of the printer port

plug are also tinned with solder,

it should then be easy to make

the connections. However, the

connections are tightly packed

together and it is necessary to

take due care in order to avoid

short-circuits.

Miscellaneous

SK1 4mm socket

SK2 phono socket

PL1 25-way male D-type connector

PCB available from the

EPE Online

Store

www.epemag.com

(code

7000232); metal instrument case,

about 150mm x 100mm x 50mm;

supply adapter cable (see text);

14-pin DIL socket (5 off); 20-pin DIL

socket (2 off); connecting wire;

solder pins; solder, etc.

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