The RF is directly coupled to the AD8307 via R1 and C1. The output of the AD8307, usually 25mV/dB is pulled down to 20mV/dB with R4 and R5. The signal is then amplified by an OP07 with an amplification factor of 5. Therefore you'll get an output of 100 mV/dB or 1 V/10 dB of RF-Power. The slope of the log-amp is adjusted by R7.

"The AD8307 is the first logarithmic amplifier made available in an 8-lead SOIC_N package. It is a complete 500 MHz monolithic demodulating logarithmic amplifier based on the progressive compression (successive detection) technique, providing a dynamic range of 92 dB to ±3 dB law conformance and 88 dB to a tight ±1 dB error bound at all frequencies up to 100 MHz. The AD8307 is extremely stable and easy to use, requiring no significant external components. A single-supply voltage of 2.7 V to 5.5 V at 7.5 mA is needed. A fast acting CMOScompatible control pin can disable the AD8307 to a standby current of 150 µA." says the datasheet.

In order to calibrate this thing, we suggest the following setup. It does produce a squarewave (am) modulated signal with -30 dBm / -70 dBm.




Use R4 (50kΩ) to adjust for 4 Vpp, representing the 40 dB difference. Then adjust for the absolute value with R7 (50kΩ).

Knowing the Frequency Response enhances the range of usability.
Vertical : Volt, Horizontal: MHz. Blue: 0 dBm, Red: -20dBm. Yes, we calibrated at 100 MHz.

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