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Logarithmic_Amplifier_AD8307.php   5984 Bytes    18-11-2015 17:01:19


Logarithmic Amplifier with AD8307



•  few kHz to more than 400 MHz
•  low parts count
•  easy adjustement
•  -60 dBm ... +10 dBm >> +3V ... +10V, 1V/10dB




Logarithmic Amplifier with AD8307





CIRCUIT DESCRIPTION




Logarithmic Amplifier with AD8307


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.




CALIBRATION




With a Signalgenerator (e.g. HMC-T2100) apply a Signal of 10 MHz (100MHz) and 0 dBm. Adjust R4 and R7 to have a voltage of 9.0 V. (At the output) Add a attenuator of 20dB. The voltage should now drop to 7.0 V. In order to adjust the difference, use R4. In order to adjust the absolute value, use R7. It is necessary to repeat the measurement. (And to remove the attenuator several times).

In case you have a AM-capable Signalgenerator you may use the method described in the Datasheet of the AD8307




FREQUENCY RESPONSE




Logarithmic Amplifier with AD8307


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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t1 = 3943 d

t2 = 319 ms

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