AnsweredAssumed Answered

Log Detector or RMS detector ?

Question asked by ZhengNice on Apr 14, 2017
Latest reply on Apr 25, 2017 by jdobler

Hi, enash

       Recently, I use the directional coupler and log detector to measure the forward and reflected power in a ultrasound system.  the block diagram  is shown as follows


signal generator input signal is sine wave, the frequency is between 1.30 MHz and 1.40 MHz (1.30 MHz~1.40MHz). The log detector is used the AD8310 evaluation board.


But when I do some experiment, the same value power (measured by the Power Meter, Bird 4421) is detected, the output voltage of AD8310 is different.


Bird 4421 Power Meter  measured Value

Output voltage of AD8310

Signal 1




Signal 2





So I used the oscilloscope to measure the waveform of the two signal, the result is as follows. 

Signal 1 is harmonic wave in the sine wave

Signal 2 is sine wave.

two signal have different crest factor.


as I look up for the technical document, the reason is that the intercept of log detector changed with the crest factor,  and recommend the rms detector.


So I have some question


1. In the my power measurement, I don't know the measurement signal's crest factor,  the rms detector is more suitalbe for me?  the signal may be sine wave or may have harmonic wave


2. the sigal frequency is between 1.30 MHz and 1.40 MHz (1.30 MHz~1.40MHz),  the frequency is a little low, I have search the rms detector product in your website 

there are two types

(1) the output voltage is linear in voltage :ADL5511

(2) the output voltage is linear in dBV: AD8362, AD8363

Could you recommend the most suitable rms detector for me? 



3. If I used the rms detector to measure power, how to convet the rms voltage value to the power value?

for example, the rms voltage value of signal is 0.8V, what is the power value?



waiting for your reply