A power meter is almost a necessity when building your RF equipment. Since I did not have one I decided to build one myself.
Off course the most important part is the sensor itself, I came across a ZX47-60 power detector from Mini-Circuits. This is wide band ( 10 MHz – 8 GHz ) power detector with an input range of appr. 70 dB. In my design I only used it in the -50 to 0 dBm range, this is more then enough for my purposes. After this the analog signal is multiplied by 2 to have a full range on the LTC1298 A2D converter. The ATMEGA328 processes the digital signal to have a correct analog and digital readout.
The RF Power Meter has a dual display : analog and digital. The analog is for ‘easy’ (distance) readings and the digital for more accurate readings.
Interesting detail is the scaling, normally the dB scale is logarithmic and Watt readings are linear but with my power meter this is reversed. This is due the fact that the ZX47 has ‘linear’ dB output, I did not want to do a lot of math on the original signal and found it easier to do it this way. Year mileage may very … 🙂
Since the analog display range is limited to 10 dB, the user can ‘offset’ the analog display by means of the up and down arrow keys in steps of 10 dB. The display shows if the actual reading is below, within or above the selected range. This could be done automatically but I decided not to implement it this way, so manual only, no auto-range function. The digital reading of the power has no range button, it displays a value from -50 to 0 dBm (or equivalent in Watt). If an external attenuator is used, this can be selected in the menu and will be added in both analog and digital readings.
Highlights of the RF Power Meter :
- Analog and digital display
- Capable of adding offset to accommodate external attenuators (max 60 db)
- Calibration at many HAM frequencies (ie 50, 144, 432 1250, 2350 and several others)
- Digital readout in Watt or dBm (menu selectable)
- Internal calibrator by W1GHz
The Power Meter is powered by a 9V wall adapter which is mounted inside the unit. All the programming is done in basic using BASCOM. Calibration is done by using a calibrated signal generator, the menu will lead you through the calibration procedure.