This is only a preview of the October 2006 issue of Silicon Chip. You can view 40 of the 112 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Articles in this series:
Items relevant to "LED Tachometer With Dual Displays, Pt.1":
Items relevant to "UHF Prescaler For Frequency Counters":
Items relevant to "Infrared Remote Control Extender":
Articles in this series:
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1000:1 UHF Prescaler
for Frequency Counters
By JIM ROWE
Here’s a high speed prescaler which can
extend the range of virtually any frequency
counter to over 2.8GHz. It divides frequencies
by exactly 1000, so gigahertz can be read
directly in megahertz.
N
OT ALL THAT long ago, almost
the only items of domestic equipment operating on a frequency above
1GHz were microwave ovens, all of
which use a magnetron operating at
2.45GHz (the frequency which causes
maximum heating of water molecules).
But nowadays all kinds of equipment
transmits and/or receives at frequencies above 1GHz. For example many
cordless phones operate at frequencies
around 2.4GHz, sharing these frequencies with wireless CCTV cameras, AV
transmitters and receivers, security
systems, remote access locking systems and baby monitors.
Other items using frequencies in
the 2.4GHz region include “WiFi”
(802.11b & 802.11g) computer networking gear and “Bluetooth” wire36 Silicon Chip
less links for computer peripherals
(802.11a wireless networking equipment operates on even higher frequencies, at about 5GHz).
Then there are wireless internet
service providers, which mainly use
frequencies around 1.9GHz or 2.6GHz
and there are “3G” digital mobile
phones which operate on frequencies
of around 2.1GHz in metropolitan areas. We mustn’t forget GPS receivers
either. These operate on frequencies of
1.57542GHz and 1.2276GHz.
So how can you check the operating frequency of any of these devices,
when the range of most reasonablypriced frequency counters only extends up to 1GHz? Well, you can either
fork out the dough to buy another
counter that is capable of measuring
up to 3GHz or so, or you can build
yourself the UHF Prescaler described
here. This simply connects “in front”
of your existing counter and divides
the frequency of the signals you want
to measure by exactly 1000. So 1.5GHz
becomes 1.5MHz, 2.45GHz becomes
2.45MHz and so on, allowing you to
read the incoming frequency directly
and without any mental arithmetic.
The Prescaler uses some special
high speed ECL (emitter-coupled
logic) ICs to perform the 1000:1 frequency division and these are able to
operate at input frequencies up to at
least 2.8GHz. And because the output
frequency of the Prescaler is still only
2.8MHz for an input of 2.8GHz, this
means that it should be suitable for
extending the range of just about any
counter. In fact, it would be a good
companion for the 50MHz Frequency
Counter described in the October 2003
issue of SILICON CHIP.
So if you want to be able to measure frequencies up to at least 2.8GHz
with your trusty old lower frequency
counter, this project is for you. All of
the components and circuitry are on
siliconchip.com.au
a single PC board and although there
are quite a few very small surface
mount parts to fit on the board, this
isn’t unduly difficult providing you
take it slowly and carefully. You will
need a soldering iron with a very fine
chisel-shaped bit, plus steady
hands and an illuminated magnifier to help in seeing what
you’re doing.
We’ll also give you a few tips
on manual soldering of SMDs
(surface mount devices) in the
accompanying panel.
Circuit description
In terms of its basic operation the Prescaler is pretty
straightforward, as you can see from
the block diagram of Fig.1. The incoming UHF signals are first passed
through wideband input amplifier
IC1, to make the Prescaler reasonably
sensitive. The boosted signals then
pass through a high-speed divide-byfour stage using IC2, which is basically a pair of very fast ECL flipflops
in cascade.
The output of IC2 then passes to
IC3, which is another very fast ECL
counter programmed to divide by 125.
So the output from IC3 is a signal with
a frequency 1/500th that of the UHF
input signal.
Because the output of IC3 is in the
form of very narrow pulses, we then
pass them to IC4. This is an ECL JK
flipflop, connected here not only to
divide the frequency by a further factor
of two but also to provide square-wave
outputs so they’re more suitable for
triggering low-frequency counter input circuitry. Then to make the outputs
even more compatible with virtually
any common frequency counter or
scope, we finally pass them through a
simple logic level interface stage using
transistors Q1 and Q2.
For a more detailed understanding
of the Prescaler, let’s refer now to the
main circuit diagram – see Fig.2.
The UHF signal to be measured enters via CON1 and first passes through
an input termination and overload
protection circuit formed by two 100W
resistors and diodes D1 & D2. The two
resistors are in parallel to provide an
input termination of 50W, while D1 &
D2 are 1PS70SB82 very low capacitance Schottky barrier diodes, having a very low forward voltage drop.
Because they’re connected in inverse
parallel, they limit the input signal
siliconchip.com.au
The UHF Prescaler circuit is housed inside a standard diecast aluminium
instrument case which provides the necessary shielding from stray signals.
level to no more than 2V peak-peak.
The signal is then coupled to the
input of IC1 via a 10nF capacitor. IC1 is
a Mini-Circuits ERA-2SM monolithic
broadband amplifier device, with
about 12dB of gain up to over 5GHz.
IC1 is fed with DC power via its output
(pin 3), with the 47W resistor chosen
to set the correct operating current.
As the power feed is effectively in
parallel with the output of IC1, choke
RFC3 is used to provide a reasonable
load. This choke is a Mini-Circuits
ADCH-80A, a special very wideband
device chosen because it has a very low
parasitic capacitance and is therefore
not self-resonant at frequencies below
about 8GHz.
From the output of IC1 the boosted
signal is fed to the clock input of IC2
via another 10nF capacitor. By the way,
it’s the value of the coupling capacitors
at the input and output of IC1 which
determine the lowest frequency that
the Prescaler will work at. The 10nF
capacitors as shown allow it to work
down to below 50MHz. The reason
why we don’t use larger values to
extend the range even lower down
is that larger value capacitors tend
to self-resonate at frequencies below
4GHz – which we don’t want because it
would lower the maximum frequency
of operation.
IC2 is our first and most critical frequency divider and it’s an MC10EL33
device from On Semiconductor. This
is an ECL divide-by-4 device with very
impressive specifications. It can operate at input frequencies up to at least
3.8GHz and has a propagation delay of
less than 800ps (picoseconds!). It even
includes its own bias voltage source
(Vbb, pin 4) which is used to provide
the correct ECL bias for its two inputs
(via the 2.2kW resistors).
IC2 has complementary outputs
(pins 7 & 6) which both need to be tied
Fig.2: the block diagram for the UHF Prescaler. The incoming signal is first
amplified and then divided by 1000 using IC2, IC3 & IC4. It is then fed to
two separate output sockets via transistors Q1 & Q2.
October 2006 37
Parts List
1 double-sided PC board, code
04110061, 81 x 111mm
1 diecast aluminium box, 119 x
93.5 x 34mm
1 reverse polarity PC-mount
SMA socket (CON1)
2 PC-mount BNC sockets
(CON2, CON3)
1 PC-mount 2.5mm concentric
DC connector (CON4)
1 PC-mount DPDT toggle switch
(S1)
2 10mH RF chokes (RFC1, RFC2)
1 ADCH-80A UHF wideband RF
choke, SMD (RFC3)
1 TO-220 heatsink, 6073 type
(19 x 19 x 9.5mm)
1 12 x 12mm aluminium sheet
(1mm thick)
1 small quantity of thermal grease
1 M3 x 6mm round-head machine screw
6 M3 x 15mm countersunk machine screws
6 6mm-long untapped metal
spacers
7 M3 nuts & star lockwashers
Semiconductors
1 ERA-2SM UHF monolithic
amplifier (IC1)
1 MC10EL33 high speed divideby-4 ECL divider (IC2)
1 MC10E016 high speed ECL
programmable counter (IC3)
1 MC10EL35 high speed ECL
JK flipflop (IC4)
1 7805 +5V 3-terminal regulator
(REG1)
2 PN200 PNP transistors
(Q1,Q2)
1 3.3V 1W zener diode (ZD1)
1 3mm green LED (LED1)
2 1PS70SB82 UHF Schottky
diode (D1,D2)
1 1N4004 1A diode (D3)
Capacitors
1 2200mF 16V RB electrolytic
1 10mF 16V RB electrolytic
1 4.7mF 16V tantalum
3 100nF multilayer monolithic
ceramic (leaded)
6 100nF X7R dielectric 1206
SMD chip
8 10nF X7R dielectric 1206
SMD chip
Resistors (0.25W 1%)
2 2.2kW 0805 SMD chip
1 430W
1 330W
2 300W
1 120W
2 100W 0805 SMD chip
2 100W
1 75W
2 56W 0805 SMD chip
3 51W
1 47W 0805 SMD chip
Specifications
This UHF Prescaler is a high-speed frequency divider designed to extend
the range of low-frequency counters to at least 2.8GHz. It divides the input
frequency by a factor of 1000, so GHz (gigahertz) may be read directly in
megahertz. There are two independent outputs, both compatible with the
input of virtually any frequency counter or oscilloscope.
Maximum input frequency................................................. 2.8GHz minimum
Minimum input frequency.................................................. 50MHz maximum
Input sensitivity................................................. less than 250mV peak-peak
Input impedance..................................................................................... 50W
Output level......................................................................875mV peak-peak
Output impedance.................................................................................. 75W
Power requirement............................................................................. 9V DC
Current drain......................................................................................190mA
Power dissipation..................................................................................1.7W
38 Silicon Chip
to ECL low logic level via termination
resistors of close to 50W. Here we use
56W chip resistors, because this value
is more readily available than 51W.
From pin 7 of IC2 the signal (now
1/4 the input frequency) passes directly to the clock input of IC3, an
MC10E016 ECL 8-bit programmable
synchronous binary counter able to
count/divide input frequencies up to
at least 700MHz.
We have programmed it to divide by
125, by tying its parallel load inputs
(P0-P7, pins 3-7 and 21-23) to the appropriate ECL logic levels. For division
by 125, we set the parallel inputs to
the binary code for 256 - 125, or 131:
ie, 10000011. Note that the ECL high
or “1” level is established by the 75W
and 430W resistors, forming a voltage
divider across the 5V supply rails.
The output signal from IC3 (1/500
of the input frequency) appears at the
terminal count or TC-bar pin (19),
which again must be tied to the ECL
logic low level via a terminating resistor (here 51W, because it’s a standard
leaded part). The ECL logic low level
is established by ZD1, a 3.3V zener
diode.
By the way if you’re wondering
where the current for ZD1 comes from,
to establish the nominal 3V level, it’s
sourced from the various ECL outputs
tied to it via the termination resistors,
plus the inputs of IC3 that are connected directly.
As mentioned earlier, the output
signal from IC3 is low in frequency (below 8MHz) but it’s in the form of very
narrow pulses which would probably
pose problems for the input circuitry of
many low-frequency counters. That’s
why we don’t program IC3 to divide
by 250 (which is easily done).
Instead, we program it to divide by
125 and feed its output to a third ECL
device, IC4. This is an MC10EL35, a
very fast JK flipflop with its J and K
inputs tied to ECL logic high level so
it operates in toggle mode as a divideby-two counter.
So at the complementary outputs
(pins 7 and 6) of IC4 we finally get
output signals of exactly 1/1000th
the input frequency and, just as importantly, in the form of symmetrical
square waves which are much more
compatible with typical counter input
circuits. The outputs of IC4 are again
tied to ECL logic low level via 51W
terminating resistors.
Since the outputs from IC4 are still
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Construction
As you can see from the photos, all
the Prescaler circuitry is on a doublesided PC board measuring 111 x 81mm
and coded 04110061. This board has
rounded cutouts in each corner so that
it fits snugly inside a standard diecast
aluminium instrument case, measuring 119 x 93.5 x 34mm. It’s actually
mounted on the box lid, which forms
the Prescaler’s base.
All the connectors, power switch
S1 and the power indicator LED
(LED1) are mounted on the top of the
board, along with the regulator (on its
heatsink), transistors Q1 and Q2 and
the other leaded components. The
surface-mount ICs and other components are mounted on the underside
of the board.
There are quite a few connections
between the two copper layers of the
board but these aren’t likely to pose a
problem even if you don’t get a board
with plated-though holes. Some of
the connections are achieved simply
by soldering the leaded component
leads on both top and bottom, while
the others are mostly “vertical links”
between the upper and lower groundplane copper areas. These links are
easy to make using short lengths of
tinned copper wire (eg, resistor and
diode lead offcuts).
The location and orientation of all
the parts on both sides of the board
are shown clearly in the two PC board
overlay diagrams of Fig.3, so you
siliconchip.com.au
Fig.3: this is the full circuit diagram. IC1 is the input amplifier and this provides about 12dB of gain. The boosted signal is then divided by four
in IC2, by 125 in IC3 and by two in IC4. Q1 & Q2 buffer the complementary outputs from IC4 and drive the output sockets.
switching between ECL levels (nominally +3V and +4V), the remaining
step is to pass them through a level
translation and output buffer/interface
circuit, to provide them as buffered
low-impedance signals referenced
to ground. This job is performed by
transistors Q1 and Q2, connected as a
differential switch. This has the advantage that it allows us to easily provide
the Prescaler with two independent
outputs, so that it can drive either two
different counters or perhaps a counter
and an oscilloscope.
Because all the Prescaler circuitry
operates from a single 5V DC supply,
the power supply is very straightforward and involves only a 7805 regulator (REG1), driven from an external
9V DC plugpack. Although the total
current drain is about 190mA, giving a
regulator dissipation of about 800mW,
the regulator is provided with a small
heatsink so it keeps reasonably cool.
October 2006 39
Fig.3: install the parts as shown in these two diagrams. The red dots show where you have to solder to both sides of
the board and where to install vertical wire links (but only if your board isn’t supplied with plated-through vias).
shouldn’t have any problems if you
use these and the photos as a guide.
Since there are quite a few surfacemount parts (SMDs) to fit to the board
as well as the leaded parts, we recommend that you assemble everything in
the order set out below.
First, fit the various connectors to
the top of the board, beginning with
CON1, which is a reverse polarity
SMA socket. Follow this with CON2
and CON3 (the BNC sockets) and
finally the DC power input socket
(CON4). That done, fit the DPDT power
switch (S1).
Fitting the SMDs
Next, turn the board over and lay
it “bottom copper up” on your workbench, using a small block of wood
or plastic if necessary to support it.
40 Silicon Chip
This will then allow you to fit all of
the surface-mount devices, with a
minimum of obstruction. Fit the chip
resistors first, then the chip capacitors
and finally the input protection diodes
(D1 & D2), the ICs and RFC3.
We have prepared an accompanying
2-page panel with some diagrams to
guide you in manual assembly of the
various SMD parts. There’s also a photo of a small rotary “SMD work table”
which you might like to duplicate. We
also recommend the use of a magnifier
lamp – ie, the type that’s fitted to an
articulated, spring-loaded arm.
After you’ve fitted all of the SMD
parts, the board can be turned over
again and the smaller leaded parts
fitted, including the resistors, RFC1
and RFC2 and the small capacitors. As
mentioned earlier, some of the leads
of these parts are used to make connections between the top and bottom
copper – so remember to solder the
leads concerned on both sides. They’re
identified with a red dot on the PC
board overlay diagrams of Fig.3.
If your PC board is not provided
with plated-through hole vias, there
will also be quite a few “vertical links”
to fit, to provide low impedance links
between the top and bottom copper.
These are also identified on the overlay
diagrams with a red dot, so don’t forget
them. They can be made using resistor or diode lead offcuts – just don’t
overheat or dislodge any of the SMD
parts nearby when you’re soldering
them in place.
Next fit LED1, the Prescaler’s power
indicator. This mounts in the front
centre of the board, with its leads bent
siliconchip.com.au
Above: the top of the PC board carries all the leaded components,
along with the sockets, the power switch, the indicator LED and the
regulator and its heatsink. Keep all leads as short as possible.
Right: the surface-mount devices all go on the reverse side of the
board. Refer to Fig.3 and to the 2-page panel in this article for the
details on mounting these.
forwards by 90° so that it lines up with
CON1 and switch S1. Position it so
that it will later protrude through its
mating hole in the front panel.
The final parts to fit are power diode
D3, the two electrolytic capacitors and
regulator REG1. As shown on Fig.3 and
in the photos, the regulator mounts
flat against a small 6073 type TO-220
heatsink and this assembly is secured
to the board using an M3 x 6mm screw
and nut. Tighten the screw before sol-
dering the regulator’s leads, to avoid
stressing the solder joints.
Functional checkout
At this stage your Prescaler should
continued on page 44
Table 1: Resistor Colour Codes
o
o
o
o
o
o
o
o
o
siliconchip.com.au
No.
1
1
2
1
2
1
2
3
Value
430W
330W
300W
120W
100W
75W
56W
51W
4-Band Code (1%)
yellow orange brown brown
orange orange brown brown
orange black brown brown
brown red brown brown
brown black brown brown
violet green black brown
green blue black brown
green brown black brown
5-Band Code (1%)
yellow orange black black brown
orange orange black black brown
orange black black black brown
brown red black black brown
brown black black black brown
violet green black gold brown
green blue black gold brown
green brown black gold brown
October 2006 41
How to manually solder SMD parts
Many surface-mount components or
SMDs are very small – the 0805 size
chip resistors are only 2 x 1.3mm, while
1206 size chip capacitors are only slightly
larger at 3 x 1.5mm. Many SMD IC packages have leads spaced only 1.27mm
apart. SMDs are not really designed for
manual assembly but it’s quite feasible
to fit many of the more common types
by hand if you take care and use the
right tools.
For a start, your soldering iron should
be fitted with a fine chisel-point tip, which
should be well tinned and kept as clean
as possible. Ideally it should be of the
low-power temperature-regulated type
as well. You also need to use fine-gauge
resin cored solder, ideally no more than
0.8mm in diameter.
Fig.7: 0805 and 1206 size SMD
chips can be soldered into position
with the aid of a toothpick (to
hold the device in position) and a
soldering iron with a fine tip.
42 Silicon Chip
It helps a great deal if your PC board
has the copper pads solder-plated, as
this makes it much easier to fit the SMD
parts.
Manual assembly of SMDs is also a lot
easier if the board is held horizontal and
level, as they’re less likely to move out
of position while you’re soldering them.
In many cases, you can simply place
the board flat on your workbench copper
side up, although if there are leaded parts
already mounted on the other side of the
board you may need to support it using
small blocks of wood, plastic or metal.
Because it often helps to be able to
rotate the board for easier soldering at
each end or side of an SMD, I made up
a small rotary work table by adapting a
ball-bearing swivel base from an industrial
castor wheel assembly. By removing the
wheel and axle and then bending the
upper ends of the fork sides outwards
at 90°, I made a fairly sturdy rotating
bracket (it even has a brake lever, which
can be used to lock the table and prevent
it from rotating).
The swivel flange was then attached
to a block of aluminium to serve as a
base, while a 120mm square of 4mm
aluminium sheet was fashioned into an
octagonal plate with a 6mm centre hole
and 3/16-inch holes tapped in each “corner” for fastening board clamp screws.
Two further holes were also drilled in the
plate to line up with the former axle holes
in the bent-over fork ends, so the plate
could be bolted to the top of the fork to
form the actual operating table, with its
centre hole directly over the centre axis
of the base swivel.
You can see the basic construction in
the photos, which also show three of the
support blocks and clamp brackets I fashioned to hold boards in place. Also visible
is a pair of modified crossover tweezers
mounted on a pivoting arm arrangement,
which can be used to hold some SMDs
in place while they are soldered – a kind
of “third hand”.
Such a work table is not necessary
for all SMD work but it might be worth
considering if you’re likely to be building
up quite a few projects.
Another useful accessory for manual
SMD work is an illuminated magnifier – a magnifying glass about 120mm
in diameter surrounded by a circular
fluorescent lamp in a metal hood that’s
mounted on an articulated, spring-loaded
arm attached to a swivel base (so you can
position it easily just above the operating
table). They’re not cheap but if you’re likely
to be doing a fair bit of manual SMD or
just fine PC board assembly, they are a
good investment.
One at a time
Before we go any further, here’s an
important tip: when you have quite a few
SMDs to solder to a board, handle them
one at a time. If you try to tackle more
than one at a time, it’s all too easy to
accidentally send one or more flying off
while you’re concentrating on soldering
the first one in position.
To handle tiny 0805 and 1206 size
SMD chips and bring them to the board,
use a small pair of stainless steel cross
over tweezers. They’re available in
almost any Asian bargain store, either
alone or in sets of tweezers for only $2.
Having brought each part to the board,
release it from the tweezers and carefully nudge it into position over its mating
copper pads, using either the tip of the
same tweezers or the point of a small
wooden toothpick.
That done, hold the part in position
using either the toothpick or a pair of
modified crossover tweezers as a clamp,
while you clean the soldering iron tip and
then melt a very small amount of solder
onto its end. The tip is then brought
up to one end of the
SMD, at a fairly low
angle so the tiny drop
of solder comes into
contact with both the
board copper and the
end of the SMD (see
Fig.7). The iron tip is only in contact
for about half a second – just long
enough to allow the drop of solder to
tack-bond the two together and hold
the SMD in place.
The toothpick or tweezers can
now be removed and you can
solder the other end of the SMD
in the more “normal” fashion before returning to the first end and
quickly re-soldering it properly
as well. The sequence is shown
in Fig.7.
The same basic approach can be used
siliconchip.com.au
with SMD diodes, transistors and ICs,
with slight variations to suit the various
packages. The idea is to hold the SMD
in position using a toothpick or crossover
tweezer clamp while you tack-solder one
of its leads to hold it in place. That done,
you can remove the clamp and solder
all of the remaining leads properly – and
finally, the first lead again. Doing this
is much the same whether the SMD
has flat horizontal leads emerging from
underneath, S-shaped leads that bend
outwards at the bottom or J-shaped leads
that bend inwards and underneath. Fig.8
shows the idea.
About the only kind of SMD package
you can’t solder in this way is the type
with no leads at all – just “solder bumps”
underneath. These really aren’t suitable
for manual soldering.
One last tip: whether you’re soldering
SMD chip resistors, capacitors or other
devices like diodes, transistors and ICs,
make all joints as quickly as you possibly
can while at the same time taking care to
make a good joint. The faster you make
the joint, the lower the risk of damaging
the SMD by overheating (which is very
easy to do, since they’re so tiny). Also use
the smallest amount of solder necessary
to make a good joint – the less solder
you use, the lower the risk of accidentally
bridging between device leads with a blob
of excess solder.
Fig.8: these two sequences show how to solder SOT, SOIC and PLCC devices
into position. Note that it’s important to use a soldering iron with a very fine
tip for this job, to prevent shorts between pins.
This rotary SMD work table was made up using a ball-bearing swivel
base, an aluminium plate, some support blocks fitted with clamp
brackets and a pivoting arm arrangement fitted with a pair of crossover
tweezers. A thick aluminium block forms the base.
siliconchip.com.au
October 2006 43
Fig.4: the mounting details for the
PC board. Note the aluminium heatsink under IC3.
The rear panel provides access to the two BNC output sockets and the DC
power socket.
carrier frequency or strictly, 1/1000
of its frequency. So if the camera or
AV transmitter module is operating
at say 2.432GHz, the counter will read
2.432MHz.
Final assembly
be electrically complete and ready for
a quick functional checkout before
it’s fitted into the box. To check it
out, place the PC board assembly on
a clean timber or plastic surface and
connect 9V DC supply (eg, from a 9V
250mA plugpack or similar) to CON4.
The positive input should connect to
the centre pin of CON4.
Now turn on power switch S1 and
you should see LED1 light up. This
will confirm that LED1 is fitted with
the correct polarity and also that REG1
is providing a +5V supply rail to the
Prescaler’s circuitry. To make sure that
the supply voltage is correct, you can
check it with a multimeter or DMM,
connected between the centre and
output pins of REG1.
You can also check the voltage
across zener diode ZD1 which should
measure about 3.1V if the ECL circuitry
is working correctly.
Self oscillation
If all seems well so far, try turning on
your frequency counter and connect-
ing its input to one of the Prescaler’s
outputs (ie, CON2 or CON3). You
may well find that the counter shows
a reading straight away, even with no
input signal applied to the Prescaler as
yet. That’s because IC2, the Prescaler’s
input divider, tends to self-oscillate
when there is no input signal. So if
you connect the second Prescaler
output to a scope, you’ll probably see
a squarewave of about 1.6MHz.
There’s no cause for concern about
this self-oscillation because as soon
as you feed in a “real” UHF signal, it
stops. The Prescaler’s output changes
immediately to a square-wave with
a frequency 1/1000 that of the input
signal.
If you have a source of UHF signals
like a wireless CCTV camera or an AV
transmitter module, try connecting
its output to the Prescaler’s input via
a suitable SMA cable (note: you may
need an SMA/RP SMA adaptor at one
or both ends of the cable, depending
on its own connectors). The counter
should immediately begin reading its
If your Prescaler passes this quick
checkout with no evident problems,
you’ll now be ready to assemble it in
the box. This assumes that your box
and its lid have been prepared, with
of the holes shown in the diagram of
Fig.6 having been drilled. If the box
hasn’t been drilled yet, then now is
the time to do so.
Note that the holes for the BNC
connectors in the rear of the box are
extended to form slots, so the box can
be slipped down over the connectors.
As mentioned earlier, the PC board
assembly is mounted on the lid on
6mm-long untapped metal spacers.
It’s then secured using six M3 x 15mm
countersink-head machine screws, as
outlined below.
Before the board is fitted, attach the
small aluminium heatsink plate to IC3,
the PLCC28 device. This IC gets fairly
warm in operation and the plate helps
keep it cool by conducting heat away
to the box lid.
The plate is first prepared by smearing it thinly on both sides with heat-
Fig.5: these full-size artworks can be copied and attached to the front and rear panels of the case. Cover them with
wide, clear adhesive tape before attaching them, to protect them from damage.
44 Silicon Chip
siliconchip.com.au
sink compound. That done, press one
side to the top of IC3’s body, sliding
it around a bit so any air bubbles are
worked out. Then position it squarely
over the IC body, where it will tend
to stay put until you fit the board assembly to the box lid.
Attaching the board assembly to the
lid is straightforward if you first fit the
six countersink head screws through
the lid holes and then turn the lid over
and place it on the workbench. You
then fit one of the 6mm spacers on each
screw before lowering the inverted PC
board assembly into position. Be sure
to press the board down gently just
over the position for IC3 (see Fig.3),
so that the heatsink compound on the
lower surface of IC3’s heatsink plate
is partly transferred to the box lid
underneath, to form a good thermal
bond – see Fig.4.
After this, you can fit an M3 star
lockwasher on the top of each board
mounting screw, followed by an M3
nut. It’s then just a matter of carefully
tightening each mounting screw and
nut to secure the board and sandwich
the aluminium heatsink in position.
The final assembly step is to fit the
box over this assembly. To do this,
first remove the nuts and lockwashers
from BNC connectors CON2 and CON3
and also remove one nut, the keyed
flat washer and the lockwasher from
power switch S1. Thread the remaining nut right down to the switch body
and then refit the keyed flat washer
with its locating lug facing towards
the switch body. This washer should
also be down against the nut.
Now you should be able to bring
the inverted box down over the PC
board/lid assembly, at an angle so
CON1, LED1 and switch S1 can be
mated with the matching holes in the
front end of the box. The box can then
be lowered at the rear end and moved
back at the same time, until the slots in
its rear slip down around the threaded
ferrules of CON2 and CON3. The box/
cover will then be fully mated with
the lid, allowing you to invert the
complete “shebang” and fit the four
box assembly screws.
After this, all that remains is to fit
the front and back panel dress stickers
to the box (see Fig.5) and finally, refit
the remaining nut to power switch S1
and the nuts to CON2 and CON3 at the
back. Your UHF Prescaler should now
be finished and ready for use.
One final tip: when you’re screwing
siliconchip.com.au
Fig.6: the drilling details for the metal case. Drill pilot holes for the larger
holes first, then carefully enlarge them to size using a tapered reamer.
SMA cable connectors and adaptors to
the Prescaler’s own input connector, be
careful. These connectors are designed
for precise mating, so they can operate
reliably, with low losses up to about
8GHz. As a result they’re small and
have a fine thread, making them easily
SC
damaged by rough treatment.
October 2006 45
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