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AMATEUR RADIO By GARRY CHATT, VK2YBX Designing and building attenuators Many people are haffled by the theory behind attenuator design hut once understood, attenuators can he easily made for most applications at minimal cost. This article outlines the theory behind attenuator design and provides practical guidelines for home construction. Basically, an attenuator is a network of resistors intended to produce a specific loss between a known source impedance and a kriown load impedance. Attenuation is normally expressed as a ratio in decibels, and is the same regardless of the direction of operation. Two basic forms of symmetrical resistive networks are available that can be used as attenuators. Although there are many other configurations that could be discussed here, this article restricts itself to easily constructed, symmetrical networks. These two configurations are called the "T"-section and the "1r" -section. Fig. l(a) shows the circuit for the T-section attenuator, while Fig. l(b) shows the 1r-attenuator. The value of both types Gan be calculated as shown. For the T-section attenuator with 50 ohms impedance: Attenuation in dB = 20 log (Rl + 5'0)/(Rl - 50) when R2 = (50 + R1)/2R1 T SECTION 7r SECTION (a) (b) Fig.1: circuit configurations for T·section (a) and 1r-section (b) attenuators. For the 1r-section attenuator with 50 ohms impedance: Attenuation in dB = 20 log (Rl + 50)/(Rl + 50) when R2 = 2 x 50R1/(R1 + 50) For those not mathematically minded, Fig.2 shows calculated resistance values for values of attenuation between 0dB and 40dB, for an impedance of 50 ohms. Multiply these figures by 1.5 to obtain 75-ohm attenuators and by 12 for 600-ohm attenuators. For fixed attenuators, intended for the lower frequencies (ie, from audio to 50MHz or so), the physical layout is of no real consequence. These " pads", as they are called, can be used between transmitter stages to stop interaction, to match levels, and to calibrate S-meters. Switched attenuators A more useful instrument is the switched or stepped attenuator, which can be used to insert preset amounts of attenuation while maintaining the correct impedance. Such a device is useful for evaluating the gain of antennas, preamplifiers and power amplifiers, for preventing receiver overload and for determining power amplifier compression. Once the required amount of attenuation is determined, a fixed value pad can be built into the equipment. At VHF and UHF, the techniques used in the construction of such attenuators become important. This is because the higher frequencies are prone to attenuation errors, due to stray coupling between stages. For this reason, it is better to cascade several stages having a lower value of attenuation, rather than use a single stage of high attenuation, as the coupling error will be a much smaller percentage of each stage. In practice, 20dB is the largest single step of attenuation achievable. At these higher frequencies , the OUTPUT INPUT SBG;fl 68Q ! 68i ? f Fig.3: practical design for a 50-ohm 8-step attenuator with an attenuation range from 1dB-80dB and an upper frequency limit of 450MHz. It should be built into a diecast metal case with shielding between stages to prevent RF leakage. 70 SILICON CHIP resistors must be non-inductive types such as carbon composition, or better still, cracked carbon. Fig.3 shows the design of a 50-ohm, 8-step attenuator having an attenuation range from ldB to 81dB and a useable upper frequency limit of 450MHz. Such a device can be used with a directional antenna to locate or track a hidden transmitter. The closer you get to the source, the more attenuation required to maintain the same signal level. ATTENUATION NETWORKS R1 I Attenuation dB 0 Construction The physical construction of a stepped attenuator for VHF and UHF use is important, and the following guidelines should be noted: (1). House the attenuator in either a diecast metal box, or fabricate the housing from double sided printed circuit board. Shields between the stages can be made from sections of double sided PCB, and can be soldered into place, or slid into the internal ribbing of the diecast box. (2). Use good quality connectors. Normally the most convenient type is BNC but ensure that it is the correct impedance and rated for RF operation. Avoid cheap video BNC connectors. (3). Use non inductive resistors. Remember that if you use quarterwatt types, the power handling ability of the attenuator without causing damage is one quarter of a watt! Parallel combinations of halfwatt resistors will give a higher power rating, but in all cases keep lead lengths to an absolute minimum. (4). It is preferable to use full size DPDT slide switches as they give greater isolation than smaller types. Subminiature toggle or slide types are not acceptable. (5). Ensure that your design is practical. It is extremely difficult to accurately provide more than 80 to 90dB of attenuation, as leakage around the outside of the unit will affect any measurements that are made. Design for the maximum practical attenuation you are likely to need. Use several smaller stages of attenuation rather than one large stage, and never attempt to exceed 20dB in one step. 11 11 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 11 .0 12.0 13.0 14.0 15.0 16.0 17.0 18.0 19.0 20.0 22.0 24.0 26.0 28.0 30.0 32.0 34.0 36.0 38.0 40 .0 R1 R1 IT 7r SECTION T SECTION 50 Ohms R1 0 0.298 0.568 0.86 1.15 1.43 1.74 2.02 2.29 2.58 2.87 4.32 5.73 7.16 8.56 9.93 11 .32 12.68 14.00 15.33 16.61 17.88 19.14 20.35 21.53 22.69 23.82 24.91 26.0 28.0 29.92 31.71 33.37 34.9 36.32 37.62 38.82 39.92 40.87 42.64 44 .07 45.22 46.17 46.92 47.55 48.04 48.42 48.76 49.01 50 Ohms R1 R2 00 4184 2190 1455 1089 872 720 619 545 482 434 287.7 215.2 171.1 i41.9 120.7 104.1 92.42 82.3 73.9 66.99 60.9 57.1 51 .3 47.3 43.75 40.6 37 .75 35 .13 30.62 26.81 23.57 20.78 18.37 16.26 14.41 12.71 11.37 10.1 7.99 6.3 5.02 3.98 3.17 2.51 1.99 1.58 1.26 1.00 0 0.6 1.14 1.71 2.29 2.87 3.47 4.04 4.59 5.19 5.72 8.69 11.62 14.62 17.71 21 .5 23.96 27.05 30.37 33.82 37.29 41.05 44.75 48.72 52.85 57.12 61 .57 66.2 71.17 81.65 93.25 106.1 120.2 136 153.9 173.6 195.3 222.5 247.5 312.7 394.7 498.75 629.2 791.7 994.2 1250 1580 1985 2500 R2 00 8375 4782 2908 2175 1743 1436 1240 1092 966.7 870 579.2 436 349.6 292.1 251.7 220.9 197.1 178.4 163 150.6 139.9 130.7 122.9 116.1 110.2 105 100.3 96.17 89.25 83.5 78.84 74.92 71.63 68.83 66.44 64.4 62.64 61 .11 58.63 56.73 55.28 54.14 53.27 52.57 52.03 51 .61 51.27 51.01 Fig.2: calculated resistance values for 50-ohm attenuators (0-40dB attenuation). The values are simply scaled for 75-ohm and 600-ohm attenuators. Component Suppliers (1). Cracked carbon resistors: Allen Bradley Pty Ltd, 22 Parramatta Rd, Lidcombe, NSW 2141. Telephone: (02) 648 2652. (2). DPDT slide switches: use 3PDT types from Dick Smith Electronics, Cat. No. S-1017, $2.99. it MAY1988 71