08 March 2014

AD8307 Power Meter. The Analog Meter Calibration

In this procedure the aim was to adjust the current to voltage converter that drives the analog meter so that the meter would read about 75% of full scale deflection (FSD) at the maximum rated input of the power meter which is +15dBm. 

Since the meter has a FSD of 1mA the 75% point would require a drive current of 750uA. I started out using 2 X 2.2K resistors which resulted in a reading of 517uA for +14dBm. 

Subsequent calculations and tests (shown in the lab notes below) resulted in the use of 2 X 1.5K resistors which gave a reading of 835uA at +14dBm. The input voltage to convert to 835uA with this resistor of 3K was 2.51volts. This was deemed to be satisfactory and would give enough safety to ensure that the meter would not be damaged if excess input is applied in error. 

For these measurements I used my K2 Elecraft as the RF source since that is what I had. I had previously calibrated my 1N4148 based RF power meter using DC voltages. Thus this was the starting point. Using this meter I set the K2 power output to 1 watt (+30dBm). I then inserted fixed attenuators (16dB) to bring the level down to +14dBm. This level was then used to set the meter deflection.

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