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AMATEUR RADIO
BY GARRY CRATT, VK2YBX
A universal wideband preamp
Ever needed a wideband preamp to boost
signals into a frequency counter? How about
compensating for the long cable run from the
TV antenna, or a preamp for a shortwave
receiver? This simple, wideband amplifier
offers usable gain, respectable noise figure and
absolute simplicity.
After carefully considering the
OM350/360, the uPC1651 and some
of the newer HP amplifiers, we selected the MWA120, one of a series of
amplifier modules available from
Motorola, for our RF preamplifier. Our
selection was based on ease of mounting , durability and, primarily, noise
figure. It would be pointless using an
amplifier with a higher noise figure
than the receiver with which it is to
he used!
Table 1 shows a comparison of the
relevant parameters of the d evices
evaluated for this proj ect. No doubt
there exist devices with a low er nois e
figure than th e MWA-120 , but ease of
mounting precludes their use, as some
require surface mount techniques.
Fig.1 (below): use this circuit to
power the preamp if you want to use
it as a masthead amplifier. The lOµH
choke stops the output of the 78Ll2
regulator from loading the signal
output from the preamp.
The MWA-120 contains a single
transistor and three resistors. The
construction of the device ensures
good bandwidth (up to 400MHz) and
stability. All that is needed externally
to complete the amplifier are bias
resistors, plus bypass and coupling
capacitors. We did consider making a
discrete amplifier, similar to the one
we described in the June 1988 issue.
However the temptation of using a
" prebuilt" amplifier was too appealing, and considering the trouble required to build the unit from discrete
components and the inevitable larger
physical size of a discrete unit, we
decided against it.
The amplifier we built uses two
MWA-120s in cascade and a minimum of parts, and is hous ed in a
diecast box fitted with male and female BNC connectors . This arrangement allows the unit to be mount ed
on the front of a frequency counter,
directly on top of a scanning receiver,
TABLE 1
TO
RECEIVER
uPc1651
OM350
OM360
MWA120
HPMA2135
GND
70
SILICON CHIP
or using the power feed system shown
in Fig.1, at the antenna end of a long
run of coax cable.
In fact, the unit we built was
mounted directly at a discone antenna, 10 metres up . Fig.2 shows the
comp lete ci rcuit. As usual, component leads should be kept as short as
possible and components should be
physically small. Monolithic capacitors must be used. Each amplifier is
housed in a TO-39 metal can which
must be soldered at several poin ts to
the ground plane of the board. We used
a double sided PC board, the comp onent side being the groundplane.
Fig.3 shows the PC board layout for
both component side and solder si de.
The top and bottom sides of the board
are tied together with tinned copper
wire pin-throughs in several places.
If the preamplifier is to be used
with a scanning receiver, a "gain"
control could easily he added in series with the main supply. This would
allow adjustment for optimum performance by enabling the user to reduce the gain in the event of signal
overload .
Remember that the amplifier has
gain from 100kHz to 400MHz and is
wideband in every sense of the word
- no filters have been included in the
unit to roll off the response to interfering signals, particularly VHF television.
The preamplifier is powered via a
Gain
Vdd
Noise
Bandwidth
19dB
18d8
23dB
14dB
23dB
6V
12V
12V
5V
5V
5.5dB
6.0dB
7.0dB
6.0dB
4.0dB
10-1200MHz
40-860MHz
40-860MHz
0.1-400MHz
0-600MHz
PARTS LIST
1 double-sided PC board, code
SC06103911, 76 x 19mm
2 F29 ferrite beads
1 diecast aluminium case , 93 x
39 x 31mm
1 female BNC chassis mount
connecter
1 male BNC chassis mount
connector
1 3.5mm jack socket
1 5mm LED bezel
Semiconductors
2 MWA 120 hybrid amplifiers
(IC1 ,IC2)
1 4.7V 400mW or 1W zener
diode (ZD 1)
1 1N4001 silicon diode (01)
1 red LED (LED1)
View inside the completed preamplifier. Keep all leads as short as possible &
don't forget to solder the metal cans of the MWA120s to the groundplane. The
PC board is suspended by the wires connecting it to the BNC connectors at
either end of the case.
01
1N40D1
_lJ
Capacitors
5 0.1µF monolothic capacitors
2 .001 µF monolithic capacitors
L3
LED1
INPUTr
~
0.1
0.1
~
Resistors (0.25W, 5%)
1 560Q
1 82Q
.,.
~
OUTPUT
OUT
Miscellaneous
1 metre of 0.125mm enamelled
copper wire, hookup wire, 50Q
coax cable.
INOGND
L1, L2 : 3T, 25B&S ECU ON F29 BEAD
L2: 12T, 0.125mm ECU ON 100n RESISTOR
VIEWED FROM
BELOW
WIDEBAND PREAMPLIFIER
Fig.2: the circuit uses two MWA120 ICs in cascade to give 28dB of gain. It can be
powered directly from a 12V DC plugpack or from the circuit shown in Fig.I.
DC socket, allowing it to be fed from
an external power supply, such as a.
DC plugpack adaptor. The circuit has
an onboard zener regulator which ensures +4. 7V is always supplied to both
amplifiers, regardless of input voltage.
Dl is a blocking diode which prevents any accidental damage to the
circuit should reverse polarity be in-
advertently applied. Quite a few capacitors are connected to the +4. 7V
supply to provide plenty of bypassing
and thereby ensure circuit stability.
We did consider using a 9 volt battery inside the unit as the power
source, but the current drain is too
high (75mA) to allow the long term
use of a battery.
The circuit board can either be
etched from the PC artwork provided
or made by carefully scribing the outline on one side of the board, and
then cutting away the copper foil with
'8
I
0
0
0
0
0
0
0
Fig.3: the preamp is mounted on a double-sided PC board with one
side operating as a ground plane. Both sides of the PC board are tied
together at several points using tinned copper wire pin throughs.
L
0
0
0
,
0
0
0
0
0
0
0
O')
O')
M
0
M
0
T""
c.c
T""
c.c
0
u
0
u(/)
0
0
0
0
T""
T""
T""
T""
.J
(/)
X
Fig.4: here are the full-size patterns
for the PC board.
MARCH 1991
71
March bargains
will save you $
from Sheridan's
1.44 Mb 3.5"
Disk Drives
NEW Teac
1 Year Warranty
ONLY $135.00
IDE & Floppy
Controller Card
This card controls 2
Floppy drives plus 2
Hard disk Drives .
Includes Cables .
1/0 PC Card
* 1 Serial Port
* 1 Parallel Port
* 1 Games Port
$29.95
The preamplifier was built into a metal diecast case to
shield it from interference & to ensure stability.
IE
: I -------:
1 ------,...........
i---.--
1/0 PC Card
* 2 Serial Ports
* 1 Parallel Port
* 1 Games Port
ONLY $35.00
76
DIMENSIONS IN MILLI METRES
Fig.5: you can make the PC board without etching by using
this plan to scribe and cut the copper pattern (note: black
areas to be removed).
NOW ONLY $65.00
12 Volt Fans
80mm x 80mm
Mini Hobby Motor
Great for those
broken remote
control vehicles
9 VDC 25mm x 18mm
These are quality
new fans and
$2.95ea
$27.00 in
lots of 10
only
$9.95
LCD Display
This un it is advertised elsewhere for $65.00
Contains 160 alphanumeric characters and you can program your
own symbols or characters. Great for many projects and can be
used with or without microprocessor.
Includes data sheet
OUR PRICE $9.95
Mini PCB Momentary
push to make switch.
As used in popu lar
alarm re mote keys.
Has hundreds of other
uses .
MOLEX 10 way PCB
plug & socket.
Great fo r use in alarms . This
one can be used as a replacement for many commecial models .
a scalpel. Fig.5 shows the required pattern (not to scale).
The unit is mounted in a diecast box, fitted with the
two BNC connectors. Th e PC board can eith er be suspen ded by pieces of tinned copper wire from the centre
pins of the BNC sockets, or affixed to the bottom of the
diecast box using double sided foam tape . Ensure that a
connection is made from each socket to the groundplane
(component side) of the PC board, to ensure stability.
Because the amplifier mu st remain stable over a wide
range of frequencies, the bypassing components have
been selected to operate at differe nt fre quencies . This is
w hy the supply line to each am plifier is bypassed with
both .001µF and 0. lµF capacitors .
The input to the pteamp uses the fe male BNC connector wh ile the output uses the male BNC plug.
Indu ctor L2 is made by w inding 12 turns of 0.125mm
enamelled copper wire onto a 100Q resistor. This forms
an RF choke. isolating the two amplifi er stages and ensuring stabilit y over the wide operating bandwidth.
Each MWA-120 must be bonded to the top foil of the
PC board as shown in the layout. Prior to appl ying the
soldering iron to the can of the MWA-12 0, scrape some of
the n ickel p lating from the can with either a scalpel or a
small file . This will ensure a good connection . We soldered the prototype in four places on each can , ensuring
a good, low impedance earth.
SC
Where to get the parts
$0.S0ea
$9.00 in lots of 20
SHERIDAN ELECTRONICS, I
328 ELIZABETH ST,
.
(Cnr Kippax St. opp. Central
Station & Dental Hospita l)
VISA
SHOP
SURRY HILLS, NSW, 201 O MON· FRI
TELEPHONE: (02) 281 7727 .._s.AT_ _ _ _ _ _ __ .
72
SILICON CHIP
The diecast box is available from Geoff Wood Electronics . Phone (02) 427 1676.
The BNC connectors are available from Farnell
Electronic Components, phone (02) 645 8888, part
No.GE35086H (male) and GE35007H (female).
The MWA-120 should be available from VSI Electronics (Australia) Pty Ltd. Phone (02) 439_8622.
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