Modulated Signal Source

 A short project for a modulated, portable signal source, ideal as the "other end" of an antenna test range.

Anyone who has tried to measure the performance of an antenna knows there are 2 halves to the problem ... detecting the signal and sending the signal.  Detecting and accurately measuring the signal is a well solved problem, the venerable HP415E is the weapon of choice.  The '415 is self-contained, battery powered and very accurate. With a simple diode detector it makes an ideal portable measuring device and has been the backbone of many professional and amateur test ranges for many decades. 

The problem with the '415 is it needs a modulated signal source, 1kHz of AM is needed on a carrier of the desired frequency. There is little point having a portable detector if you have to lug 20 kilos of bench equipment out into the field and provide it with mains power to generate the test signal.

To solve this, I used a cheap synthesised signal ADF4351 signal source from eBay, driven by an Arduino to set the control registers.  A cheap 20dB gain stage boosts the output to a level more than sufficient to be detected by the '415 ... and the Arduino drives a P channel MOSFET with a spare pin switched at 1kHz to modulate the power to the gain stage.  Finally a 7.2 LiPo battery pack intended for model aircraft is used as a power source and a 5V buck convertor efficiciently takes the battery power and provides a steady 5V for the electronics.  A diecast box houses the project.

Power consumption is around 120mA, which gives me 12 hours of use. A hex switch allows selection of amateur bands from 50MHz though to 3.4GHz  All in all a simple project that is very useful and saves a lot of frustration when measuring antennas in the field.



Arduino code: https://github.com/rszemeti/ADF4351_Loader/tree/CalSource

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