Fullscreen HTML5Med Res (??MB)HTML4

AMATEUR RADIO BY GARRY CRATT, VK2YBX Wideband preamplifier has response to 950MHz You can build this versatile amplifier for quite a range of amateur applications. It uses one tiny surface mount device on a PC board only 30mm square and will produce a gain of up to 18.5dB at 500MHz. A wideband amplifier can be a useful tool in any amateur shack. Apart from the obvious applications of improving receiver sensitivity and compensating for coaxial line losses, there are also instrumentation applications where this handy little device can improve the sensitivity of frequency counters and field strength meters. In fact, it could even be used as a masthead preamp for television use. We last described a wideband preamplifier in March 1991. Since then, the price of monolithic amplifiers has dropped dramatically. One leading supplier worldwide is Mini-Circuits, located in Brooklyn, USA. And fortunately for amateurs in Austra­lia, they have a new agent, the well known component supplier, Clarke and Severn Electronics, located in Sydney. Mini Circuits has an extensive 400page product catalog, of which 50 pages are dedicated to their range of amplifiers. For this project, we selected the MAR-6, a device with useable gain, adequate noise figure, 50Ω input and output impedances, and cap­able of being cascaded easily with a minimum of external compon­ ents. For such a reasonable price, these devices certainly do a great job! Even though there are a minimum of components used in this design, there are a number of important The prototype preamplifier was mounted in a small diecast box & the BNC input & output connectors are soldered directly to the PC board. construction techniques which must be observed if the amplifier is to live up to expecta­tions. Firstly, transmission lines must run flush to the IC package. This means that a 2.5mm hole must be drilled in the PC board to accept the plastic body of the amplifier, allowing the connection leads to be soldered directly to the transmission lines for both input and output connections. In addition, to minimise what is called the “step discon­tinuity” (the impedance mismatch) which is typically the equival­ent of adding 0.2nH of series inductance, the transmission lines feeding the amplifier should be tapered. Also, corners of trans­mission lines should be minimised and where bends are necessary, the corners should be chamfered to minimise extra shunt ca­pacitance. The standard rule of keeping lead lengths as short as possible also applies and this is why the design uses chip ca­pacitors. Ground planes should be kept as large and solid as possible to ensure a low impedance ground return. Gain, compres­sion and high frequency rolloff will all be degraded if proper grounding techniques are not used. If the amplifier is to be used in a 75Ω situation, the input and output SWR will increase from an ideal 1:1 in a perfect 50Ω system to 1.5:1 , a mismatch loss of 0.18dB per port. Internal circuit details Fig.1 shows the internal circuit of the MAR-6 amplifier. The internal resistive networks determine the individual transistor operating points and all we need to do is supply the correct voltage to the DC input terminal. Rc is an external bias resis­tor. This resistor 82  Silicon Chip Rbias V+ RF RF IN 11 IC1 MAR6 RC Q1 Q2 3 RF OUT INPUT Cblock 11 Rb Cblock OUTPUT 4 2 RS RFC OPTIONAL 3 Fig.2: the MAR-6 must be used with input & output coupling capacitors & an output inductor to isolate the DC supply. If the (optional) inductor is omitted, the available gain will be re­duced. V+ 4 RE 1 DOT OR TRIANGLE INDICATES PIN 1 2,4 GND Fig.1: the internal circuit of the MAR-6 monolithic amplifier, shown with its collector biasing resistor, Rc. compensates for increases in device Beta with temperature, by dropping the collector voltage as the amplifier attempts to draw more current. Mini-Circuits recommend the use of resistors with a positive temperature coefficient, such as carbon composite types. For bias stabilisation over the temperature range of -10°C to +100°C, a drop of at least 1.5V is necessary. The larger the voltage drop, the more stable the bias voltage will be, the optimum being about 2V. As the optimum DC condition for the device is achieved at 16mA at 3.5V, we used 100Ω from a 5V source. Other voltage /resistor combinations are 9V/344Ω, 12V/531Ω, and 15V/719Ω. Fig.2 shows more connection details for the MAR-6. An RF choke is used in series with the bias resistor, to ensure that the resistance does not appear in parallel with the load, and hence degrade the output match. At HF the value of this choke can be 10µH or so, but at high frequencies several turns of wire on a high permeability ferrite bead should be used. If the choke is omitted, a gain loss of several dB could be expected. 2 us to use monolithic ceramic types in the prototype. Some designs use a combination of low and high pass filters on input and output ports, and this may be desirable if the amplifier is to be used on a dedicated band. However, the wideband version presented here offers greater versatility as a general purpose unit. Our proto­ type produced a high frequency 3dB point of 950MHz, sufficient for most amateur needs. a pair of tweezers is mandatory during construction, to hold the components as they are soldered. The PC board we used is single sided and the components are wired directly to the top of the PC board which in this case is the copper side. To assist in physically locating the components before soldering, we drilled the PC board, just as if the components were going to be inserted from the non-copper side. The component leads can be cut off flush with the underside of the PC board after soldering. Begin assembly by drilling the diecast box to take two BNC sockets. Use internal tooth lockwashers 5/16-inch) between the sockets and the box to ensure a good conductive bond. The input and output connections on the PC board must be soldered directly to the centre pin of the BNC sockets and at the same time the PC board ground connections must be able to be soldered to each side of each BNC socket. To ensure that the PC board mechanically fits, use a rat-tail file, to carefully file away the exposed fibreglass between the Construction The suggested PC board layout is shown in Fig.4. The entire unit can be wired into a diecast metal box, and fitted with BNC sockets (either male or female or a combination of both). The active device is mounted on top of the PC board (copper side up), and is located in a 2.5mm hole drilled in the centre of the PC board. Power for the unit should be supplied from an external source via a socket on the side of the amplifier enclosure. Because we have endeavoured to keep lead lengths as short as possible, 100  180  .01 L1 ZD1 5.1V 400mW 0.1 11 3 2 0.47 K 560 0.1 4 1 DOT OR TRIANGLE INDICATES PIN 1  LED1 IC1 MAR6 INPUT +12V A L1: 3T, 0.25mm DIA ENCU WOUND ON FERRITE BEAD Final circuit Fig.3 shows the complete circuit of the prototype. In addi­tion to the choke, a .01µF bypass capacitor has been used to ensure a low impedance path to ground for any signal that does get past the choke. The circuit is powered from 12V with a zener diode used to regulate down to +5.1V. In addition, a LED has been included as a power indicator. Surface mount DC blocking capacitors can be used to ensure the best possible impedance match. In practice, the difficulty in obtaining 0.1µF chip capacitors in small quantities forced 3 OUTPUT 4 3 A K 2 WIDEBAND PREAMPLIFIER Fig.3: the complete circuit of the prototype amplifier has a 5.1V zener diode regulator & LED power indicator. January 1995  83 GND +12V K INPUT SOCKET LED1 A 560  180  ZD1 .01 100 L1 0.47uF OUTPUT SOCKET 0.1 1 0.1 IC1 Fig.4: the component layout of the preamplifier. All components are mounted on the copper side & a 2.4mm hole must be drilled to allow the MAR-6 device to sit flush with the copper surface. Fig.5: actual size artwork for the PC board. turns of 0.25mm enam­ elled copper wire around a UHF ferrite bead. Allow about 3mm of connection wire either side of the bead and tin these leads prior to soldering. Before wiring the amplifer assembly into the metal box, connect a 12V DC power supply and check that the current consump­tion is about 30mA (15mA for the LED and 15mA for the IC). Once soldered into the box, it is quite difficult to remove the PC board cleanly, should there be a wiring error. Testing edge of the PC board and the input/ output pads. The result will be two “half moon” notches, adjacent to each connection pad. Once the mechanical considerations have been attended to, the circuit can be assembled. We found it easiest to fit the MAR-6 first, keeping the leads as short as possible. It is quite easy to hold the amplifer chip in place with a pair of tweezers with one hand, and solder one of the connection leads. After this, the other leads can be soldered without any need to hold the device. 84  Silicon Chip Preparation of each device is important prior to insertion to ensure a good clean bond. Due to the very short lead lengths, soldered joints must be made quickly to ensure that no damage occurs to the components, due to excessive heat. It may be necessary to scrape off some of the insulating material on resistor and capacitor leads to ensure the shortest leads possible and to produce good soldered joints. Pay particu­lar attention to the polarity of both the zener diode and the LED. The RF choke is made by winding three After final assembly, the unit is ready for testing. In our case, we connected a signal generator to the input and a spectrum analyser to the output. The prototype amplifier exhibited a flat response from 1MHz to about 850MHz. The only special components required are the diecast box, obtained from Farnell Electronic Components Pty Ltd (phone (02 745 8888) and the MAR-6 device from Clarke & Severn Electronics (phone 02 482 1944). All other components are commonly SC available.