Here is second version of my RF power meter device. It has improved RF shielding and noise immunity. A little bit more expensive to make but quality pays.
C14 is also added for improving noise immunity from supply line.
C14 added and other SMA connector removed. It’s replaced with T splitter externally.
Casing
Case is bought from Digi-Key. It’s part number is PIP-11766-C. DC jack outer diameter is 5.5mm and inner 2.5mm.
This meter measures the transmitting power of the radio transmitter and the voltage of the battery used as a power source. Power is displayed in watts and dBm. The device has a three-digit LCD display. The functions are controlled by a push button. The unit will automatically turn off after five minutes if the button is not pressed. By pressing once device wakes from sleep mode and show measured power in wats. By pressing again device switches to dBm mode and show power in dBm. Pressing third time device show measured supply voltage.
Comes as a bag of
parts kit and is easily assembled if customer can follow the
silkscreen indicators and have beginning experience with a soldering
iron. Customer will need to read the resistor bands or use a
multimeter to determine the resistor sizes.
Works with single
7…14 volts DC power supply connected with DC barrel plug.
Internally 5.0V voltage is used through regulator 7805. ADC reference
voltage is taken from supply voltage with voltage divider. Its
voltage is 2.5V. One analog input is connected with supply voltage
also with voltage divider. its voltage is adjusted to a max. of 2.5V
at 14V so that accurate voltage measurement is made possible.
The heart of the
device is AtMega328P-PU at 16MHz clock frequency. This makes it to be
Arduino™ compatible system.
For
measuring power, an AD8307 chip is used. It allows the RF power to be
converted to a voltage between 0 and 2.5V. The permissible power is
between 0 and 1kW. The
permissible frequency range is between 10MHz and 1GHz. It
is possible to calibrate the measuring range by ± 3dB with
on-board trimmer resistor.
The output is connected to the analog input of the microcontroller.
Measurement is made
with following equations:
dBm = 40(U-1) W = 10(dBm/10)/1000
The display is a three-digit LCD display. LCD display requires AC voltage. It can be simple square wave signal from DC 5V. Frequency is not critical. It can be between 30-200Hz. Phase difference lights the segment. This device uses regular CMOS series IC’s to control LCD segments. 4056 is used to control digits. Every digit needs own IC. One 4054 is used to control two decimal points and to invert common backplane signal. Frequency is made with Arduino™ PWM output. It is about 30Hz. Below is an self-explanatory illustration.
Current consumption is very low because of CMOS integrated circuits used and AtMega328P power saving mode which is programmatically activated. LCD display consumes very little current compared to regular LED display. Also power measurement circuit ’chip enable’ signal is controlled with AVR.
Physical size of device is with casing 13x13cm. PCB is 10x10cm. Dimensions excludes connectors and push button.
Usage examples
Amateur radio station power meter
Power measurement
Circuit diagram
Development stage
One prototype has been ordered so far with double sided printed circuit board and with through hole components. UNTESTED at this point. Work is in progress. It is designed with KiCad.
Target MSRP
BOM (bill-of-material) cost with casing and SMA connectors is about 50 euros. It does not include any basic components with standard values (I use my own stock).
10 pieces small batch BOM cost is $64.90 + PCB $3.02. PCB will cost $1.33 in order quantity of 1000 pieces. PCB’s are bought from Itead Studio from China. The components are bought from Digi-Key from USA. The same components can also be purchased from Mouser. The price in larger quantities are much less. If order quantity is 1000 pieces then it costs $40.04 + PCB $1.33. Manufacturing costs should also be taken into account. (In this case packing.) So total cost for parts is roughly $43.00.
Typical distribution margin is 5-7% and retail margin is 15-25% with this type of products. So the MSRP would be between $51.93 and $57.51. Realistic actual selling price would be $79.00.
Advantages
Ease of use. The
device has only one button with three functions. ’Plug-and-play’.
Safe operating voltage. Safe to use. Hackable design.
Other similar
devices
Surecom SW-102 Digital Antenna Power & SWR Meter VHF/UHF 125-525MHz.
Soldering
The project requires
no special soldering skills. All components are their through hole
versions.
Programming
All parts are open
source. The program is solely written in C++. The program is very
simple and with its 200 lines of code it uses only 4.6kB of program
memory. AtMega328P-PU chip is used.
The bootloader must
first be programmed using external programmer. I personally like to
use Arduino™ as ISP and Arduino™ IDE. Be sure to choose the
Arduino™ Nano from the Tools→Board menu. After installing the
bootloader, the program can be uploaded using same setup.
Program does not use
any external libraries. Only standard built-in power saving related
libraries are used.
Approvals
Arduino is a registered trademark. It is still ok to build a commercial product based on Arduino. Only the name may not be the same. https://www.arduino.cc/en/Main/FAQ#toc10
