A while ago I decided to put together a small, fixed-frequency QRP CW tranceiver kit. At first I settled on the Pixie II, a small, fixed-frequency (around 7.023MHz) transceiver that outputs a Continuous-Wave carrier (CW, used for Morse Code) of around 2 Watts. The one I got was a cheap clone, readily available from eBay for about $6 (yes, six! dollars). The kit worked, but I found the clicking noises in the headphones when sending morse code to be very annoying, waay to loud for my ears. I researched a bit and came across a similar kit, but with better reviews, called the Forty-Niner. (http://www.norcalqrp.org/files/49er.pdf). Slightly larger parts count, single frequency, low power CW (about 2 Watts) and at $12 (whoa, double the price!) still cheap as chips. Sending morse with this kit was a lot kinder to my ears. It was with this kit that I made my first on-air morse code contacts. Then, in August 2017, Hans Summers from QRP-Labs finished the design of his QCX; a 5-Watt, single BAND, CW transceiver kit, on sale for just $49! (given the many more features this kit has, the price is still absurdly low).
One thing I have always been wondering since my early days of playing around with the Pixie, is how much power is actually coming out the radio? Often, analogue power meters for HF are poor at showing accurate measurements when the needle is barely moving up from the resting position.
Then I remembered, I once ordered samples of an Analog Devices chip that featured in some articles on (very) low-signal power measurements.
Meet the AD8307 Log Amplifier:
- DC output voltage of 25mV per dB
- 92dB dynamic range
- operating range of up to 500MHz
The earliest articles (around 2001) used the AD8307 with additional opamps and driving a panel meter or digital multimeter to show the measurement values. Then, over time, came articles where the chip provided a signal to a microcontroller. Some used Microchip PICs, some used Atmels (many Arduinos have the Atmel ATMega328). Since an Arduino Nano clone can be purchased for about $5 on eBay I decided to make my own Low Power RF power meter with a 2x16 backlit LCD.
The design is based around the AD8307 Log Amplifier, but the amplifier is housed in a separate box from the microcontroller and LCD to prevent it from picking up noise. In a later design I may try to incorporate it all in one box.
For showing the measurements I chose to use a backlit 2x16 LCD. The specific one I had only had the ringle-row 24-pin data and power connector, but there exists a small add-on board that adds I2C functionality to the display.
Although the 5V linear voltage regulator on the Arduino Nano is the main power supply for the ATMega chip (and a reference voltage for the internal ADC), I deciced to make use of a very stable external voltage reference in the form of the MCP1541; a TO-92 package, 4.096V voltage reference. This assures that the voltage coming out of the Log Amp is always referenced against a known, stable voltage, ensuring that the dBm measurement is as accurate as possible.
For more information on the details of this project:
Original Article by Wes Hayward (W7ZOI) Bob Larkin (W7PUA) as published in QEX magazine, June 2001, pages 38-43