Vermes
Advanced Member level 4
The instrument is suitable for measuring small power local generators and high power transmitters (with additional dampers), and after connecting appropriate antennas a a field strength meter, to check the antennas and their effectiveness and characteristics.
The meter is calibrated in dBm and its measurement range is about -80dBm to +17dBm. Highest linearity of +-0,4dB is provided in the range 0,1-100MHz. The main element of the device is a logarithmic amplifier AD8310 which has no limit of the lower frequency, which makes it suitable for measuring audio signals. Max frequency is 440MHz.
The picture above shows the main element of the device with the AD8310 system.
What makes presented device special:
There are several different solutions with the AD8310 system, but not displaying the result on ICL 7106 in the scale dBm. A simple layout on ICL7106 was made with two precise mounting potentiometers used to set the zero point and the slope (about 24mV/dB). AD8310 amplifier in a standard minimal configuration has a slope of about 24mV/dB and ICL7106 should be set similarly, but it is recommended to calibrate it on a measuring generator.
The instrument is powered by two 9V batteries 6F22. It can be also powered by 9V accumulators or the AC adapter. Just remember that AD8310 has to be powered by voltage less than 5V, so that there is 78l05 stabilizer on the board. Also power supply ICL7106 minus is separate from the rest, so that the AD8310 minus is not connected with the ICL7106 minus. Hence, there are two power links, what must be remembered when powering by the AC adapter to assure full galvanic separation of the two voltages. All information on the AD8310 can be found at Analog Devices. AD8310 with its voltage stabilizer and a few other SMD parts are placed on a board made in thermal transfer method showed on one of the pictures below. On the board there is a place for an additional resistor which enables the change of the characteristic AD8310 slope.
The resistor 52,3ohm is very important. Along with the internal resistance of the AD8310 it makes a 50ohm load at the meter input.
The board was drawn in ExpressPCB.
The part with the display and ICL7106 was made temporarily on a universal board.
Link to original thread (useful attachment) – Miernik mocy wcz na układzie AD8310 z wyświetlaczem LCD
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