To start, the SWR meter mentioned in this article isn’t completely homemade because I used the circuit from the Arduino Projects of Amateur Radio book as a reference. Even though I purchased the book and acknowledge the authors’ copyright, I will still share my diagram here. What is the reason? Initially, the design showcased in the book is not unique and draws inspiration from other similar designs. An instance is provided. Furthermore, I made alterations, deleted certain items, and included new elements based on my preferences, resulting in a different design.
The design utilizes two AD8307 log amplifiers, linked to the forward and reflected ports of a directional coupler. The opamp (LM324) amplifies the input signal with the AD8307 amplifiers to produce a DC voltage of approximately 25mV per decibel. The opamp also supplies the reference voltage for the Arduino (AREF) so that the A/D converters can utilize their full ranges.
I utilized the fourth opamp in the quad LM324 for input voltage measurement. This is the reason why there are two power jacks on the PCB shown in the picture. One input and one outcome. In this manner, I am aware of the radio’s drive voltage. My main intention is to utilize this device for my Softrock.
I designed the schematic and layout in Eagle, then sent the gerber files to Elecrow. I got 10 PCBs in around 25 days.
The board is not much larger than a standard 20×4 LCD.
I opt for using an Arduino Nano over inserting an AVR328 onto the board and dealing with USB converters. I am happy that I did so. I did make a mistake on the board, though. The Arduino did not receive power from the 5V rail. The issue was caused by a mistake in the Eagle footprint for the Arduino Nano. I simply downloaded the footprint file without questioning it, straight from the Internet. Apart from that, everything was okay.
I encountered oscillations with the LM324 opamps when connected to the AD8307, which was another issue. The oscillations were approximately 40KHz and around 400mVpp on top of the DC signal. Therefore, calibrating the device was not feasible. The answer was to remove the output capacitors from the LM324 by desoldering them. I believe they are not necessary for a DC layout.
