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Monday, May 21, 2018

Field Strength Meter Monday: Say Hello To A Smart F.S Meter Part 1

 As part of the continuing "Field Strength Meter Monday" series, here is part of the schematic of a finished product to be released later this summer.

Detecting RF

Use of 1N34 germanium diodes have been the standby detector for a long time, but does not offer linear detection across a wider frequency range or sensitivity.   PIN diodes with a voltage bias increase sensitivity, but still do not have regulated output across a wide frequency range. This is why a new and low cost modern "smart" field strength meter is needed.

The HVDN designed Smart F.S Meter has the Analog Devices LT5507 as its RF detector.  The range is 100 kHz to 1 GHz, so this will find a lot of use for the amateur radio community.  Its also possible to use the LT5534 as a direct replacement to offer 50 mHz to 3 GHz range if desired for those interested in things like detecting 2.4 GHz Wi-Fi, bluetooth and more.

The LT5507 is small, in case you did not know. 
Moving to the LT5507 provides additional benefit as well compared to other older detector solutions. When RF signals are received by the LT5507 (or LT5534) the output signal is a calibrated voltage from -0.3 to +6.5 volts which  is easy to translate into RSSI values. This is important since it can easily be read by most analog to digital converters that operate up to about 5V, such as the embedded capability of an ESP8266 or Arduino.

Here are the datasheets for the LT5507 and LT5534 power detectors.

Basic schematic for LT5507 based RF power detector.
This uniform voltage compared to frequency received is what makes this a building block of  a smart field strength meter.

However, the LT5507 and LT5534 are not very sensitive for this purpose, so adding an operational amplifier (OpAmp) is needed.  The common LM324 is used for this purpose as illustrated in below schematic.

OpAmp expanded LT5507 RF FS Meter

In the "OpAmp expanded LT5507 RS FS meter" schematic, the "signal output" can be fed to a simple analog 150 mA meter, A/D converter as part of a ESP8266 or even a LM319 bargraph add on circuit for more visual readouts.

The 47K potentiometer can either be a regular analog one or a digital potentiometer which can then be manipulated by interfacing it to GPIO on an Arduino or ESP8266.    The function that the 47K potentiometer controls is sensitivity of the OpAmp up to 2x or to even reduce sensitivity by 1x.

Adding the "Smart" to the FS Meter

Now that there is a sensitive, broad banded RF detector that can output a signal that is easy to interface to, now we can start to look at integrating this with the "smart" aspects of the project.

Stay tuned.....

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