RF Field Strength Meter (FSM) - Spectron.us

<strong>RF</strong> <strong>Field</strong> <strong>Strength</strong> <strong>Meter</strong> (<strong>FSM</strong>)<br />

This is a description of a <strong>RF</strong> <strong>Field</strong> <strong>Strength</strong> <strong>Meter</strong> or <strong>RF</strong> “sniffer” if you prefer that name. The goal<br />

is to sensitively detect low level of high frequency <strong>RF</strong> fields. In this design the foc<strong>us</strong> is on the 2.5<br />

GHz band. However the described instrument is so sensitive that it will detect low level fields<br />

below 1000 MHz.<br />

The block diagram of the <strong>FSM</strong> is shown in the figure below.<br />

The <strong>RF</strong> sensor is 1 turn loop of 3 mm copper<br />

connected to a variable ceramic capacitor. The<br />

circuit is turned to the frequency range you want to<br />

measure, in our case 2.5 GHz. The signal from the<br />

tuned circuit is rectified by Scotty diode and<br />

filtered and then wired to a differential amplifier.<br />

The differential amplifier has a high common mode<br />

rejection (CMRR) of about 120 dB. This is done to<br />

minimize hum, and other unwanted signals.<br />

After this is a high gain OP amplifier where the<br />

gain and the output offset can be set by<br />

potentiometers. The signal is wired to a micro<br />

ampere panel meter and also to a discriminator.<br />

The trip point for the comparator (discriminator) can be set by the level potentiometer.<br />

If a <strong>RF</strong> field is detected the comparator will trip and in turn activate the LED (lamp) and also give<br />

an audible ton in the buzzer. This is hand beca<strong>us</strong>e you don’t have to watch the instrument all the<br />

time; the operator will be alerted by the audio ton.<br />

The relative strength of the <strong>RF</strong> filed will be<br />

displayed on the panel meter.<br />

For measurement purposes there are outputs for<br />

the relative voltage of the <strong>RF</strong> field as well as an<br />

output after the comparator. The later can be <strong>us</strong>ed<br />

to trigger an oscilloscope etc.<br />

The <strong>RF</strong> sensor is completely shielded. Very short<br />

wires are <strong>us</strong>ed and SMD components are <strong>us</strong>ed.<br />

SM6FIE, Bo Gärdmark, Gothenburg Sweden<br />

Email: bag@agnitumit.se, web: www.spectron.<strong>us</strong>/SM6FIE<br />

Copyright 2009, all rights reserved, Read the EULA and safety warnings on web site<br />

<strong>RF</strong> <strong>Field</strong> <strong>Strength</strong> <strong>Meter</strong>.doc, 2009-03-08 17:33, page: 1 (3)

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The <strong>RF</strong> sensor is connected to the measurement amplifier detector circuit via a two wire shielded<br />

cable. Some pictures of the <strong>RF</strong> sensor can be seen to the right. I did place a boom stick to make<br />

it easy to place and <strong>us</strong>e the sensor or <strong>RF</strong> pickup coil, if you want, to exact spots of my choice.<br />

The instrument is built into a small plastic box and<br />

is powered by a 9-volt battery. The intention is that<br />

the instrument shall easily be portable.<br />

The complete schematic for the instrument can be<br />

found via a link below in the reference section. The<br />

only comment I have is a note on the novel way to<br />

get the pl<strong>us</strong> and min<strong>us</strong> supply over a common<br />

ground for the operational amplifiers. Fancy!<br />

The circuit was built dead bug, New York style,<br />

follow this link for more information. A picture of the<br />

assembly can be found to the right. For more information about the “Dead Bug Manhattan Style”<br />

see the references below. For prototyping this technique is very good, fast, simple and good for<br />

up to above 100 MHz if you do it correctly. All is done on a circuit board that acts as a ground<br />

plane.<br />

During some test I found that the<br />

instrument easily could detect a<br />

DECT phone as well as my mobile<br />

phone. My WiFi wireless device<br />

gave a very strong indication. There<br />

was no problem to detect weaker<br />

signal like T<strong>RF</strong>-2.4G Transceiver<br />

from Spark Fun (based on the<br />

Nordic n<strong>RF</strong>2491 chip). This device<br />

has an output of 0 dBm +/- 3 dB or<br />

about one mille watt. In the picture<br />

“<strong>FSM</strong> in Action” you can clearly see<br />

how the <strong>FSM</strong> detects the 2.4 GHz<br />

signal from the T<strong>RF</strong>2.4G that is run<br />

in continues mode in this case.<br />

When doing software development for example for the T<strong>RF</strong>2.4G it has big values to actually see<br />

when the device radiates <strong>RF</strong> energy. When looking at the oscilloscope at the same time as you<br />

trace the program, row by row, I have also found that the audible ton from the <strong>FSM</strong>, when it<br />

detect <strong>RF</strong>, is very helpful.<br />

The electronic schematic, as said earlier, is included as a link at the reference section below.<br />

They schematics are more or less self-explanatory. I have also included, in PDF format the layout<br />

of the front panel for the instrument. It was done by Microsoft Visio. If someone want to have the<br />

original file please drop me a line and I will send it via email.<br />

Good <strong>RF</strong> hunt!<br />

SM6FIE, Bo Gärdmark<br />

You can also download this article in PDF format, see j<strong>us</strong>t click here.<br />

(www.spectron.<strong>us</strong>/SM6FIE/Electronics/<strong>FSM</strong>/<strong>RF</strong>_<strong>Field</strong><strong>Strength</strong><strong>Meter</strong>.pdf)<br />

SM6FIE, Bo Gärdmark, Gothenburg Sweden<br />

Email: bag@agnitumit.se, web: www.spectron.<strong>us</strong>/SM6FIE<br />

Copyright 2009, all rights reserved, Read the EULA and safety warnings on web site<br />

<strong>RF</strong> <strong>Field</strong> <strong>Strength</strong> <strong>Meter</strong>.doc, 2009-03-08 17:33, page: 2 (3)

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References:<br />

Electronic schematics: www.spectron.<strong>us</strong>/SM6FIE/Electronics/<strong>FSM</strong>/<strong>FSM</strong>_Schematics.pdf<br />

Panel layout: www.spectron.<strong>us</strong>/SM6FIE/Electronics/<strong>FSM</strong>/ Panel_<strong>FSM</strong>_V2.pdf<br />

Prototyping: http://www.dartmouth.edu/~sullivan/prototyping.pdf<br />

Manhattan Building Techniques: http://www.k7qo.net/manart.pdf<br />

Manhattan Style: http://radio.thulesi<strong>us</strong>.se/div_mtrl/manhattan.htm<br />

SM6FIE, Bo Gärdmark, Gothenburg Sweden<br />

Email: bag@agnitumit.se, web: www.spectron.<strong>us</strong>/SM6FIE<br />

Copyright 2009, all rights reserved, Read the EULA and safety warnings on web site<br />

<strong>RF</strong> <strong>Field</strong> <strong>Strength</strong> <strong>Meter</strong>.doc, 2009-03-08 17:33, page: 3 (3)