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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.