Summary of DIY SWR and Power Meter
Summary: The article describes building a modified SWR and power meter based on the AD8307 log amplifiers and an LM324 quad op-amp with an Arduino Nano. The author redesigned and PCB-mounted the circuit, fixed LM324 oscillation by removing output capacitors, used the op-amp to provide Arduino AREF and monitor input voltage, and produced compact SMT PCBs. Minor issues included an incorrect Arduino Nano footprint (missing 5V) and one schematic error. The device displays accurate SWR and power, uses a larger LCD, and intentionally keeps features minimal.
Parts used in the SWR and Power Meter:
- AD8307 log amplifier (two units)
- LM324 quad op-amp
- Arduino Nano
- 20x4 LCD (larger screen)
- Directional coupler
- PCB (custom SMT board from Elecrow)
- Power jacks (two)
- Various passive components (resistors, capacitors)
- Soldered connectors and wiring
Homebrew SWR and power meter
To begin with, this SWR meter featured in this article is not completely homemade, as I used the circuit design from the schematic in the highly regarded book Arduino Projects of Amateur Radio. Despite purchasing the book and honoring the authors’ copyright, I will still provide my own diagram here. What is the reason? Initially, the design featured in the book is not entirely unique and is inspired by designs found in other sources. An illustration is provided here. Secondly, I made modifications, eliminated some elements, and incorporated new features according to my preferences, resulting in a design that is no longer identical.
he setup utilizes two AD8307 log amplifiers, connected to the directional coupler’s forward and reflected ports. The AD8307 amplifier provides a voltage of approximately 25mV per decibel of the incoming signal, and this signal is then amplified using an LM324 op-amp. The opamp plays a role in supplying the reference voltage for the Arduino’s AREF, ensuring the A/D converters utilize their full range.
I utilized the fourth opamp within the quad LM324 for input voltage monitoring. This is the reason why the PCB in the image is equipped with two power jacks. One input leads to one output. In that manner, I am aware of the voltage required for the radio’s operation. My main intention is to utilize this device for my Softrock.
I designed the schematic and the board in Eagle, then sent the gerbers to Elecrow. I got 10 PCBs in approximately 25 days.
The board is not much larger than a standard 20×4 LCD.
I opted for using an Arduino nano instead of installing an AVR328 on the circuit and dealing with USB converters. I am happy that I did it. I made a single error on the board though. The Arduino did not receive power from the 5V line. The issue occurred due to a mistake in the Eagle footprint for the Arduino Nano. I recently downloaded the footprint file without verifying its validity. Besides that, it was okay.
Another issue was the occurrence of oscillations in the LM324 opamps that were linked to the AD8307. The frequency of the oscillations was approximately 40KHz, with an amplitude of around 400mVpp superimposed on the DC signal. Therefore, calibrating the device was not possible. Desoldering the output capacitors on the LM324 was the solution. I believe they are not necessary for a DC design.
This was my initial SMT design, and I am quite satisfied with how it turned out. The SWR and power readings appear to be precise. I employed the Arduino program in the book and made changes to incorporate a bigger screen and additional components. You may observe that the device does not have any buttons. At first, my intention was to utilize a rotary encoder and multiple menus, but as mentioned on the Soldersmoke podcast, menus are designed for eateries. I completely agree. Moreover, it is completely crazy to add various features to the device. This is merely a basic SWR meter.
The files are available here, should you be interested. Note that there is an error in the schematic. The Arduino is missing +5V, but it should be easy to fix.
Source : DIY SWR and Power Meter
- What log amplifier is used for forward and reflected ports?
The AD8307 log amplifiers are used for the forward and reflected ports. - How is the Arduino AREF supplied?
The LM324 op-amp is used to supply the reference voltage for the Arduino AREF. - Can the device monitor input voltage?
Yes; the fourth op-amp in the LM324 is used for input voltage monitoring. - Why are there two power jacks on the PCB?
Two power jacks are used because one input leads to one output and the author monitors the radio supply voltage separately. - What resolved the LM324 oscillation issue?
Desoldering the output capacitors on the LM324 eliminated the approximately 40 kHz oscillations. - Why was an Arduino Nano chosen?
The Arduino Nano was chosen to avoid installing an AVR328 and handling USB converters, simplifying development. - Was there an error with the Arduino on the PCB?
Yes; the Arduino Nano footprint omitted the +5V connection, causing it not to receive power on the board. - Are the SWR and power readings accurate?
Yes; the author reports the SWR and power readings appear to be precise. - Where were the PCBs manufactured?
The gerber files were sent to Elecrow, which produced 10 PCBs in about 25 days. - Is the schematic exactly the same as the book design?
No; the author modified the design, removed some elements, and added new features, and notes one schematic error regarding Arduino +5V.
