PI & T Attenuators. Version This document is for printed circuit board version 0.0a for both the PI and T attenuators.

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1 PI & T Attenuators Version This document is for printed circuit board version 0.0a for both the PI and T attenuators. Overview & Features: The PI and T attenuators use 0805 resistors and for most values of attenuation will be accurate to within 0.25dB from DC to 3GHz. Note that resistor values above about 100 ohms will adversely impact high frequency response. For more detail see the attenuation graphs in the performance section. The resistor table below provides the resistor values, actual attenuation and expected return loss. In general the T attenuator configuration will provide better frequency response for high attenuator values because of the lower resistor values used. Attenuators are symmetrical and the input and output impedance is 50 ohms 1% resistor values are used and result in ideal accuracy typically better than 0.1dB Maximum input power is +23dBm Circuit Description 0805 size surface mount resistors R1 to R3 form the attenuator. The attenuation table below gives resistor values for different values of attenuation. J1 is a male SMA edge mount connector and J2 a female edge mount connector. The PCB is 20 x 10mm. Schematics & PCB PI Attenuator Schematics & PCB T Attenuator VE3CZ0 PI & T Attenuators 1/6

2 Resistor Table The following table shows the 1% resistor values used for various PI and T attenuators along with their ideal attenuation and return loss. Attenuator Resistor Values PI Attenuator T Attenuator Atten R1 R2 & R3 Atten RL R1 & R3 R2 Atten RL db Ohms Ohms db db Ohms Ohms db db Assembly Equipment you ll need Surface Mount Device (SMD) soldering kit o Temperature controlled soldering iron with fine tip. Insure the tip is grounded! o Small vise to hold the PCB o Liquid no clean solder flux o Two pair of SMD Tweezers o Isopropanol 99% o Cotton swabs Needle nose pliers Heat gun for heat shrink Scissors VE3CZ0 PI & T Attenuators 2/6

3 Before You Begin Check the contents of the kit to make sure that none of the parts are missing. Print out this document to help locate components on the printed circuit board. Assembly Tips 1. Use two pair of tweezers, one for pulling up the hold down tape on the parts sheet and opening the SMD part carrier, and the other for mounting components on the PCB. This prevents the tape glue residue from getting on the tweezers used for SMD mounting work. 2. If adhesive does get onto the tweezers tips, parts will stick to them. If this happens wet a cotton swab with isopropyl alcohol. Spread the tweezers tips to clamp around the swab and draw them across the swab to clean the tips. 3. Orient all resistors that have values marked on them in the same direction so that they can more easily be read and identified after installation. Use the same orientation as is used on the PCB silk screen. 4. As you go through each assembly section check off components in the square box beside the component identifier as you install devices. Assembly 1. Install all top side PCB components In the next steps the attenuator resistors are soldered to the PCB. Typically there is a PCB silk screen printed on the green solder mask that indicates the position of the resistors. However in this case the solder mask alters the impedance of the coplanar waveguide tracks so both the solder mask and the silk screen have been removed over the coplanar waveguide. This unfortunately also removes the aids that help position the resistors. Look at the PCB layouts above. Note the locations where a resistor is to be soldered to the ground plane. In these areas additional solder mask has been removed so that the solder area juts out further into the ground plane. The resistors to ground should be placed in the center of these areas. R1 Per table of Attenuator Resistor Values R2 Per table of Attenuator Resistor Values R3 Per table of Attenuator Resistor Values In the next step SMA PCB edge mount connectors are installed. Fit each connector into the PCB and align it so that the center pin is in the middle of the PCB s coplanar waveguide track. Also make sure the SMA connector is square to the end of the PCB. Solder the center conductor first then the two top ground plane connectors. Then turn the PCB over and solder the bottom side ground plane leads. J1 SMA edge mount male J2 SMA edge mount female Functional Verification Test 1 A quick DC test will confirm that the attenuator resistor values are correct and reasonably soldered. Measure the DC resistance from the SMA center pin to ground on both ends of the attenuator. The value measured will be the same at both ends of the attenuator. For the T attenuator the resistance must be the sum of the input and shunt resistors (R1+R2 or R3+R2). For the PI attenuator the resistance will be the input shunt resistor in parallel with the sum of the series resistor and output shunt resistor (R2//(R1+R3) or R3//(R1+R2). A spreadsheet Attenuator DC Resistance Calculator can be used to provide a value for the resistance from the SMA center pin to ground by entering attenuator resistor values. Physical Assembly Use the scissors to cut the attenuation value out of the label that held the SMA parts. Cut along the cell guidelines and place the attenuation value on the PCB top side nesting it between the SMA connectors. Place the heat shrink over the assembly then apply heat to VE3CZ0 PI & T Attenuators 3/6

4 the heat shrink. Carefully manage the position of the heat shrink as the heat gun may blow it about. You may want to hold the heat shrink in place by using tweezers to hold it against the PCB. It should be positioned so that it will not interfere with the male SMA nut, and stretches out over the female connector covering a portion of the threads. Electrical Performance The following graphs show typical attenuation versus frequency performance for the 3 and 10 db PI and T attenuators. Note that the vertical scale is 0.5dB per division for the 3db attenuators and 2dB per division for the 10dB attenuators. VE3CZ0 PI & T Attenuators 4/6

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