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FBEQUENCY METER
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Note pads of eacir
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tosether MOUNTI NG AND
on underside of
board
ACCOMPANY YOUR
DICK SMITH DELUXE
DIGITAL FREOUENCY METER
KIT - complete with pre-punched
chassis -and marvi-plate top.
L
kit -is completely operational to 20
MHz. lf you require your counter to
This
Apply a small amount of metho to a cloth
and rub area of chassis close to each
corner. Then remove the silicon backing
from each foot and press into place.
DISPLAY BOARD MOUNTING
boards are at right angles
to
MOUNTING UHF SOCKET
When this wiring is used, the LM340T-5
or 7805 lC has to be isolated from the
chassis. This is simply done by putting
a mica washer between the device & the
chassis. Secure in position with a nylon
screw and nut.
MOUNTING IC POWER REGULATOR
Leave the mounting of this device until
last. Take particular notice of the
mounting arrangement. l1 is soldered
in place before the PCB assembly is
pushed onto the PCB mounts.
Push the 3 pins into the appropriate holes
from the copper side of the board so that
they only protrude half way through.
While holding the lC in this position,
solder in place.
The total PC board can now be located
in position on the stand-offs.
Now bend the lC in position so it lies flat
on the chassis and over the mounting hole.
Secure in place with the screrfu & nut
supplied. NOTE: lf you intend to add the
optional 12V wiring, this lC will have to
be isolated with a mica washer, nylon
screw and nut.
MOUNTING RUBBER FEET
Your kit comes complete with adhesive
rubber feet. Before these can be applied
the chassis has to be cleaned.
board.
V
77Fla BOARD
CHASSIS
each other
before soldering all components.
not excessively.
Note 7805 pins
half vrav thru i
Note carefully the diagram and text on
how to mount the 77f 1b display board
to the 77f1a board. Make sure these 2
operate to.200MHz, simply purchase the
optional 95H90 lC and solder into place.
This lC is available fr6m all our stores.
The mounting sequence is as shown in
the diagram. Before it is tightened, make
sure the deluxe Polaroid f ilter is centred
on the front panel. Tighten securely but
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METHOD OF INSULATING
U.H.F. PANEL SOCKET
POSTTlON
OF P.C. BOARDS
ADDITIONAL NOTES TO
*ASHE B
'PRTNG
7805 REGULATOR
MOUNTING SCREW & NUT
OPTIONAL EXTERNAL 12V
CONNECTIONS
7805 REGULATOR MOUNTING
lnsert diode in position shown (note
polarity). Cut hookup wire to length.
Mount socket on rear panel and
connect to relevant position on PCB
with the hookup wire.
OPTIQNAL PARTS REOUIRED
FOR 12V EXTERNAL SUPPLY.
1
1
1
1
x mica washer
x nylon 684 screw
x nylon 684 nut
x EM401 or similar diode
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SUITABLE SOCKETS FOR
USE ON REAR PANEL FOR
CONNECTION TO 12V.
WOULD BE A 3.5mm. JACK
(P 1221 ) or 2.5mm. D.C.
socKET(P1665)
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opTloNAL COMPONENTS ANDWIRlNG FOR EXTERNAI12V. SUPPLY
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2 x lengths of hookup wire
1 x 3.5mm panel socket (P-1221],
or 2.5 DC socket (P-1 665)
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12V. suppty.
A low-Gost
digital frequency meter
. . . easily
built unit is ideal for workshop and laboratory
This article is reprinted with the permission
of Electronics Australia.
text refers to the 200MHz counter construction. The 2OMHz counter construction
is the same except for the inclusion of the optional 95H90 lC.
The
t
desiriblefeature, it was not possible to
incorporate it in the present design.
a
No provision has been made for
period counting or event counting under
manual control. Nor is there any sensitivity adjustment. Measurements are restricted to a single low rate of one every
two seconds, to keep the timebase circuitry simple and to eliminate timebase
switching.
lnput impedance of the meter on the
"x1" and "x10"
ranges
is 1
megohm
shunted by about 50pF. Sensitivity
q
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with the recent
release of several new cMoS integrated circuits in Australia, we have been able to produce a new high performance frequency
meter with greatly simplified circuitry. Our latest design has seven digits
and can measure up to 2OOMHz. lt employs wvelve integrated circuits
and a handful of discrete components.
by LEO SIMPSON
Yet again, rapidly improving technology has' enabled us to present a
project which is considerably simpler
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is
about 50mV RMS or better from l0Hz t<r
about 10MHz. On the
range, the
input impedance is 75 ohms or more and
input sensitivity is 20OmV RMS up. ro
about 180MHz. A signal of B@mV p-p
is required to guarantee operation to
and cheaper in real dollar terms than its
predecessor. ln 1970, we published a
design for a 3'22-dig;1 70MHz counter
which employed a total of 43 lC's. That
was superseded in December 1973 by a
4th-digit 200MHz design employing 24
lC s. And now with haff that number of
IC s, we present a 200MHz frequency
meter with a seven-digit readout.
Apart from the seven-digit readout,
almost every aspect of the new design
is simpler than its 1973 predecessor.
There are less controls, and assembly is
more straightforward.
We hope the new styling will be
popular too. The unit is housed in a low
profile case measuring approximately
230 x 68 x 21Omm (W x H x D). The front
panel is made cif red polaroid film which
we assume suppliers will have screen-
expected
to be within a few
parts in
100,000. Resolution is 1Hz on the "x1"
range, 10Hz on the "x10" range and
100H2 on the "x100" range.
printed in white. When the unit is off; the
front panel is dark and inscrutable, with
no digits showing. When power
220MHz and above. Minimum input
frequency on the "x100" range is 1MHz.
Accuracy of the frequency meter will
depend on the accuracy and stability of
the crystal timebase. This can be
is
applied all digits are alight.
Brilliance of the readouts is quite high
and adequate even in sunlit rooms,
although readers might gain a different
impression from the lead photo. Perhaps
we should have had the photo retouched by an artist!
Some of the features of the new
frequency meter are as follows: The
range switch has only three positions-x1,
x10 and x100. On the first range it will
measure to above 3MHz. On the "x10"
range measurements can be made to
between 25 and 30MHz, depending on
the input signal. And on the "x'100" range
measurements can be made to above
20OMHz, again depending on the input
signal amplitude.
The new design does not have leading
zero blanking. While we regard LZB as
Power consumption ls fairly modest.
Current drain is about 500 milliamps from
the regulated 5V supply. The unit is normally powered from the 240VAC mains,
but provision has been made on the PCB
to power the unit from a 12Y car battery
via an isolating diode.
Our estimated cost for the unit is less
than $120. When you consider that the
1973 design was worth about $140 and
the extent of inflation since then, the
projected cost for a kit of parts is a bargain in real dollar terms. And the nearest
comparable frequency meter retails for
many hundreds of dollars more!
Heart of the design is a pair of 74C926
CMOS 4-decade counters. These have
been recently released by NS Electronics
Pty Ltd.
Like many multi-function integrated
circuits these days, the internal complexity is so great that the manufacturers do
not bother to publish a schematic. The
74C926 contains virtually all the circuitry
needed, apart from the timebase and
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display drivers, to make a four-digit
frequency counter.
Contained in the chip are four decade
counters (same f unction as the 7490 Tf L
decade counter), four 4-bit latches
(equivalent to 16 flip-flops), BCD to
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seven segment decoders and drivers,
plus an oscillator and multiplexing circUitry for the four digit driver transistors.
Thus, even if the multiplexed output ieature is nottaken into account,the 74C926
is equivalent to eight lC s as used in our
previous design.
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It is appropriate to note at this stage
that there are a number of competitive
counter lC s, notably from Mostek, Ferranti and lntersil. Some of these may be
better in some respects than the 74C926
but we regarded the National device as
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the best all round choice, especially
when price, availability and compatibility
is considered.
More about thd74C926 device later in
the article.
Most of the timebase circuitry is contained within a single MOS integrated circuit, the MM5369N. This is described by
National Semiconductors as a 17-stage
programmable oscillator/divider. lt can
be programmed during manufacture 1o.divide by a selected number between
10,000 and 98,000.
The particular device used by us
is
programmed to work with an American
standard colour TV subcarrier crystal
operating at 3.579545MH2. Output from
the 5369 is 60H2. This is a very
economical method of obtaining a
timebase since both the 5369 and the
3.58MHz crystal are quite cheap.
The 60Hz output from the 5369 is fed
to a BC5lt! transistor to make it compatible with the following TTL stages. A7473
dual .lK flip-flop is interconnected to
ciivide by three, to give 20H2. This signal
is then further divided by a7490 decade
divider to give a 2 Hertz square wave.
fhe2Hz output from the timebase cir-
cuitry is interfaced with the
74C.926
counters in what logic designers quaintly
term a "housekeeping" circult. The circuit consists ol a 7473 dual JK flip-flop,
a7410 triple 3-input NAND gate and 3
Schmitt triggers in a 74LS'14 hex tr:igger
which are employed as inverters. The
housekeeping circuit configuration
NOTEi lC sockets shown in photograph
are optional. 7805 or LM340T-5 regutator
is mounted on the underside of boald.
At top is the component layout tor the
main pC
board. Above shows the completed
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board assembly with the display board soldered in position and wired.
ers. The latch information is used td drive
the LED displays. Early frequency counters did not have latches and so the
display was rapidly cycled during each
count period. With latch circuitry the
readout
is constant if the input frequency
is constant-there is no blinking or flick-
erin8.
So the "latch enable" pulse is the command to lhe74C926 to transfer the BCD
count from the decade counters to the
4-bit latches.
pulse is delivered to pins 5 of both
74C926s to transfer the BCD count into
the latches. Then in the last half-second
another 250mS pulse is fed to pins 13 of
the 74C926s to reset the counters to
zero.
So the display is updated once every
two seconds.
Notice that the second 74C926 only
drives 3 digits instead of 4. We have omitted the fourth digit, because the 74C926
similar to that in our previous design.
Three different pulse trains are derived
from the 2Hz timebase by the house-
The basic measurement cycle takes
two seconds. ln the first second, gate 1
of the 7410 is turned on by the second
keeping circuitry, to control the
flip-flop in lhe 7473 to allow a one
typical maximum count rate of between
3 and 4MHz plus the one second gating
time mean the maximum count cannot
go beyond 4,000,000-well within the
74C926s: One second pulses for gating
and 250mS pulses for "reset" and "latch
second burst of input signal to be fed to
the clock input (pin 12) of the first
capacity of seven display digits.
The front end of the new design con-
74C926. The first 74C926 counts the first
enable".
As noted above, each74C926 has four
4-bit latches. These are equivalent to a
chain of flip-flops which are used to store
the BCD count of the four decade count-
four decades (to 9999) and generates
"carry oul" pulses to allow the second
sists o{ a FET, an NPN emitter-follower,
a CA302B RF amplifier and one section
of the 74LS1 4 hex Schmitt trig. The FET
is
enable". Let us explain the term "latch
74C926 to count the following decades.
During the third half-second of the 2second measurement period a 250mS
provides
an input
impedance
of
1
megohm. The NPN emitter-follower
combines with the
FET
to provide a very
low source impedance to drive the
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any special precautions when soldering
the CMOS lCs except to use a small low
voltage iron. If you are worried, connect
the soldering iron to the PCB earth pattern with a jumper lead and then solder
the supply and earth pins of the CMOS
Hii,.#"
lCs first.
Notes about some of the compondnts
are appropriate here. Polarity of the 1uF
input tantalum capacitor is unimportant.
The NPN emitter-follower must be
a
transistor with a very high value of gainbandwidth product (Ft), otherwise the
output impedance will inirease at high
frequencies and thus reduce the gain of
the CA302B.
The CA3028 may be supplied in a circular can or an 8-lead minidip package.
lf a circular type, the leads will have to
be bent to suit the copper pattern.
(Experimenters who may wish to use the
CA302B as a preamp in other circuits
because
of its very wide
bandwidth
should note that because of the low sup:fr1:, i
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ply voltage and cascode connection it
does not have a linear transfer characteristic. That is immaterial in this circuit.)
Two of the Schmitt trigs. in the 74LS14
are unused. Their inputs (pins 3 and 13)
are grounded and outputs (pins 4 and 12)
have no connection. The 12k resistor
from pin 9 of the 74L514 may have to be
changed to another value to bring the
input voltage down to around l.3volts
which is between the positive and
negative thresholds of the device. This
maximises the sensitivity of the
circu it.
Logic diagrams for the MM74C926 four decade counters.
CA3028 RF amplifier. The latter makes up
terminal plastic pack regulator then
for the signal loss in the FET source-
derives the 5V rail.
follower and NPN emitter-follower and
provides the extra gain to allow the signal
to toggle the Schmitt trigger.
On the "x10" range the signal from the
7414 Schmitt trigger (pin B) is fed via a
high speed 7490 or 741590 decade
divider. These decade dividers have a
maximum toggle rate of 40MHz so the
maximum frequency limitation on the
"x10"
range is set by the 74LS14.
On the "x100" range the input signal
fed to the 95H90 prescaler and thence
7490A /74L5% decade divider to
give a resultant division of one hundred.
The 95H90 prescaler is very similarto that
in the previous 1973 design except that
is
to the
the bias circuitry has been slightly
modified to improve the sensitivity.
The power supply is simple. A 15V
centre tapped secondary transformer
drives a
full wave rectifier and two
2500uF capacitors
in parallel. A three-
All of the circuitry, apart from the input
switching and the seven segment
readouts, is mounted on a single PC
board measuring 178 x 127mm and
coded 77F1a. The seven segment
readouts are accommodated on a
separate PC board measuring 1'18 x
55mm and coded 77F1b. This board is
mounted and secured at right angles to
the main board. i
Take care when making solder ]oints
to avoid damaging the PCB. Use a low
wattage soldering iron with a small
chisel-shaped bit. Mount all the wire
finks first. Then mount the discrete coinponents and PC pins. Leave the 5369 and
74C926 tCs until last. Ncte that all the lCs
with the exception of the 74C926s are
oriented in one direction-with notched
ends towards the rear of the PCB.
We have not found it necessary to take
Low power Schottky lCs may be substituted for TTL lCs if available. This has
the advantage of lowering the power
consumption. The devices in question
ar e 7 4LS1O, 7 4LS1 4, 7 4L573 and 741590.
To achieve the high speed capability of
the pulse shaping circuit following the
CA3028 it has been found a 74L514
is necessary for maximum performance.
We have specified 8C338 (or 8C337)
for the digit drivers because of their low
collector saturation voltage. lt is possible
tosubstitute 8C548 but the readouts will
not be as bright and there is also the
likelihood that they will not all have the
same brightness.
Pin 7 of the MM5369N is a buffered
output for the 3.5MHz oscillator.
However it is of no real use in calibration.
It could be handy for troubleshooting to
check that the oscillator is actually running.
The 2N4258 transistor following the
95H90 is a very high speed switching
type. Do not subtitute types other than
those suggested, unless they are known
to be suitable for high speed operation.
The readout PCB is easy to assemble.
Solder in all the wire links first. This is
necessary because some run upderneath
and between the LED displays. Any common cathode LED display such as Monsanto MAN-7 or Litronix DL-7O4 mav be
used.
The readout PCB is attached to the
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Bend leads as shown
and insert in 8 PIN
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ROUND
position
51b/ 749OA/
CHOKE
Wind on 6 turns of the f ine
hookup wire supplied.
)
main PCB by soldering the line of pads
together on both. There is a line o{ holes
along the bottom edge of the readoul
PCB. These should be lined up with the
top surface of the main PCB by means
of a couple o{ wire links runnint between
the boards. Set the boards at right angles,
and run a fillet o{ solder between each
set of matching pads on the boards. One
oI the photographs illustrates
the
method.
This makes the majority of connections to the display PCB. There are five
wires lefi to be run to the dlsplay PCB
from the main PC8. This completes the
PCB assembly.
Now assemble the chassis hardware.
Solder the mains leads to lhe power
transformer, tape them and bolt the
PARTS LIST
MAIN PCB ASSEMBLY
1 PCB,77F1a, 178 x'l27mm
1 3.58MHz crystal
11 PC pins
1 150uH RF choke
1 FX115 lerrite bead
SEMICONDUCTORS
2 74C926 4-digit counters
1 95H90 ECL prescaler ( optional )
1 CAi028 RF cascode/differential
amplifier
1 MM5369N oscillator / divider
1 741590 or 7490A high-speed
slbl
S1a/ 1uf
1
installation.
The screen printed panel should be
located on the front of the chassis
using the toggle switch, rotary switch
and UHF socket as location guides.
When the panel is lined up sr-1g6ys
with the top and sicles, the switches
and socket can be tightened firmly
but.not excessively. Remember the
UHF socket is insulated, make sure
it is in the centre of the mounting
hole.
The three-core mains cord should be
'l 2N5484 or similar VHF FET
4 1N914 (1N4148) signal diodes
2 1N4001, EM401,EM402, EM4o4,
silicon diodes.
CAPACITORS
2500uF/ electrolytic
100uF/ tantalum electrolytic
tantalum electrolytic
47uF/
'l 'luF /
tantalum electrolytic
ceramic
5 .M7uf
2
2
2
transistor
1 PN4258,2N4258,2N600't PNP high
speed switching transistor
1 2N356i,2N3662,8F780 NPN RF
51 SWITCH WIRING
( REAR VlEW
iear ol the
c-hassi-s and anchored with a
cord clamp. Terminate the earth wire to
a solder lug on the chassis. Terminate the
aclive and neutral conduclors plus the
wires to the transformer primary and tlte
rnains switch to a three-way insulated
terminal block.
With chassis assembly complete, fit
Richco supports to the PCB and mount
it in the chassis. Alternatively, use s€rews
and nuls lo support the PCB 1Omm off
the chassis. Attach lhe regulalor to its
heatsink and ruo _ttre wires to thcjnput
switch. Run lhe wires as shown in the
photo. Do not attempt to lace.lhe wiring
into a neat cable form or run the wires
close together as this will cause faulty
CHASSIS & HARDWABE
'l deluxe metal case
& marviplate
cover
1
fully
screened Polaroid red
7 SPSI miniature toggle switch
'l 4 pole three-position switch
'l power transformer, 75V centretapped at 1 amp DC; A&R 2155, DSE
21
55
2
4
4
100uf RB electrolytic
caps
RE5'STORS
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or
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ASSEMBLY
PC board,77Flb, 118 x 55mm
7 seven-segment common cathode
LED display, Monsanto MAN-7,
Litronix DL-704 or equivalent
filter
'l knob
1 ferrite balun former
ceramic
1 3jpf NPO ceramic
1 10-40pf ceramic trimmer
)
passed through a Brommetted hole in the
2.01uF/
1x 10M, 1x 1M, 1x39k, 1x 10k,
1 x 12k, 1 x 4.7k, 1 x 2.2k,4 x 1k,
2 7473 dual lK flip-flop
2 x 220 ohms, 1 x 120 ohms,
1 74L51+ hex schmitt trigger (LS only) 2 x 100 ohms, 14 x 47 ohms,
1 7410 3-input NAND gare
1 x 1k trimpot
1 7805 or LM340T/5 5V/'tA regulator
READOUT PCB
8C338,
7
BC337, NPN transistor
1 8C548, BCl08, BC318 NPN
7490A112
transformer to the chassis.
Loosen the clickei plate of the input
switch before installing it so that it can
be easily operaled with a small knob,
Also cut the shaft to lenglh lrefore
decade counter
1 7490 decade counter
transistor
Sla/.O1uf
51cl741O/13
6
CA3028 ( bottom view
74141A
b/T3lc
for choke.
rubber teet
Richco 10mm high PCB supports
1 solder lug
1 three-pin mains plug and threecore flex
1 mains cord clamp and grommet
'l three-way insulated terminal block
Miscellaneousj screws, nuts, lockwdshers, shoTt length of shielded
cable, hook-up wire, tinned copper
wire, insulation tape, so/der, etc.
Note: Capacitors and resistors with
higher ratings may be used it
p hy si call y
com p atibl e. Other substitutions, unless mentioned in the text, are
not recommended.
close-up view of the wiring behind the tront paneL Note
that the short length of shielded cable trom the range switch
A
is earthed only'at the PC board, and not at the switch itselt.
Tape the mains switch to avoid the possibility of shock.
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Photograph at lett illustrates how the Cisplay PC board is soldEreil to-rhe fitain PC board. Above is the compo nent overlay
pattern for the display board.
operation. Note the short length of
shielded cable from the input switch,
which should not be omitted.
Check wiring and apply power, then
quickly check the 5V rail to make sure
that all is well. The LEDS should show
zeros, except for the least significant digit
which may show "1". The 12k resistor
associated with the 74L514 should be
adjusted as noted above. lf too low a
value is used it may prejudice the sensitivity and may result in random readings
on the least significant digit.
Now adjust the 1k pbt for optimum
sensitivity of the 95H90 prescaler. Feed
in a VHF signal at 100MHz or higher and
adjust the pot until a stable reading can
be obtained with the smallest input signal. With too small a signal the reading
.will drop from its correct value and vary
randomly-or it may drop to zero. With
optimum sensitivity the 95H90 becomes
sensitive to random noise and will show
a random reading on the lasl three digits
with no signal applied.
When the pot has been a<ljus{red for
maximum sensitivity the 95H90 will run
quite warm to the touch. This is sorrrial.
Calibration presents a problern.
However without calibration the unit will
have a typical accuracy of .O1%
t 1 count,
which should be adequate for most
applications. The easiest method is to
calibrate the meter by making comparison measurements with a frequency
counter of known accuracy. That is how
we did it.
There are two other methods, neither
of which we have tried. One is to use a
crystal set tuned to a strong broadcas.l
station and measure the signal when
there is no modulation. This may be dii-
ficult and Selectivity may present
problems, but it could be worth tr.ying.
The other method is to use a signal
generator and a broadcast or short-wave
receiver. Set the generator for zero beat
against a station of known frequency
(and precision). Then measure the output of the signal generator.
For the best result, the calibration
slrould be carriOd out only after the
couhter has been on for an hour or so,
to stablise its temperature. lf the receiver
is fitted with an "S" meter, this can be
used to get closet to the true zero beat
condition than i{ the audio output is used
alone. The idea is to try and get the needle of the meter swinging as slowly as
possible.
A brief note about using the frequency
meter. d signal too high in frequency or
Here is an actual size reproduction of the display board pattern.
insufficient in amplitude may result in a
false reading or zero. The tip off is to
watch the 2nd and 3rd most significant
digits. lf these vary, it is miscounting. e
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