STEP ATTENUATOR

(2014-03-08)

Preface

Attenuators are very useful devices at our RF laboratory. If we have a signal generator of a fixed level, we can get different levels using different attenuator values. Attenuators are also useful for improving input or output return loss of devices. For example, if a device presents an input of SWR=3:1, that means that the return loss is -6 dB. If we place for example, a 6 dB attenuator in front of the device, the return loss will be improved by twice the attenuator value, such is -18 dB.-18 dB return loss is the same as an SWR ratio of 1.29:1. Not too bad, is not it?

Here I will describe how to build a simple unit that works well for HF work.

The schematic

Attenuator design formulas are very easy to manage, but even easier is using QUCS attenuator calculator. QUCS is the Quite Universal Circuit Simulator, is free software and works very well. I used QUCS to calculate the following step attenuations:

1,2,3,6,10,20,20,20,20,20 dB

Here is the schematic of the step attenuator (KICAD free software):

FIGURE1: STEP ATTENUATOR SCHEMATIC

Building the attenuator

Attenuator construction is a bit tricky and you will need some patience and skills. As pictures speak for themselves, I will not describe it further. Watch the pictures and that is all. I have used cheap DPDT switches, single layer PCB board and 1/4 W resistors

FIGURE1: MARKING SWITCHES POSITION

FIGURE2: MARKING POSITION OF SLIDING BUTTONS

FIGURE3: SOLDERING THE SWITCHES TO THE PCB BOARD

FIGURE4:SOLDERING BNC CONNECTORS

FIGURE5:SOLDERING BNC CENTRAL PINS

FIGURE6: SOLDERING THE WIRES

FIGURE7:SOLDERING ATTENUATOR RESITORS

FIGURE8: SOLDERING THE BY-PASS CABLES

FIGURE9: FRONT PANEL MARKING

FIGURE10: BEER CAN FOIL FOR MAKING SHIELDS BETWEEN STEPS

FIGURE11: FINISHED UNIT READY TO TEST

Attenuator testing

I tested the attenuator the hard way, that is, using a signal generator and a 50 ohm termination at oscilloscope input. It was a tedious and time consuming task. Now I own a network analizer, hope to measure it again some day.

Here we have the list of measurement data:

FIGURE12: MEASURED DATA USING SIGNAL GENERATOR AND 50 OHM LOAD AT OSCILOSCOPE INPUT

Plotting the results

Graphical results of measured data:

FIGURE13: ATTENUATIONS AT DIFFERENT FREQUENCIES

Attenuator errors:

FIGURE14: ATTENUATOR ERRORS

We can see that attenuator errors are bigger at higher frequencies. Also oscilloscope input capacitance could be worsening the situation. Another errors are caused because I could not find the exact required resistor values for each step.

Another way of testing the attenuator

Using a signal generator and an SDR receiver with Linrad software is an useful way of testing the attenuator. Let's see several screenshots on 14 MHz band using my home made Reinventing The Wheel SDR:

FIGURE15: (1+2)-3-(1+2+3)-(6)-(1+2+3+6)dB ATTENUATOR STEPS

FIGURE16: 0-1-2-3-(3+1)-(3+2)-6-(6+1)-(6+2)-(6+3)-(6+3+1)dB ATTENUATOR STEPS

FIGURE17: 10-20-(10+20)-(10+20+20)dB ATTENUATOR STEPS

Improving the attenuator

The attenuator could be improved somewhat getting resistors closer to the calculated values. Also adding very small capacitances to the wires could extend the high frequency range. I am lazy now to try that..... I hope you will enjoy building this useful piece of equipment as I did!

Bibliography

Experimental Methods In RF Design. ARRL. Wes Hayward, W7ZOI, Rick Campbell, KK7B, and Bob Larkin, W7PUA 

  Copyright (c)  2013 Ramiro Aceves .
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