Hello everyone,

In this circuit, what is the importance of a logarithmic amplifier other than achieving a linear dB output? In all the papers, it is stated that it extends the dynamic range. How is that possible? Isn't the dynamic range already defined by the RMS power detector?

 

What papers might those be?

 

You can take them as an example.
You can also take a look here:https://www.analog.com/en/product-category/rf-power-detectors.html

 

A log amp gives you an output that is linear expressed in Db.

 

Yes, but it is also said in the papers that they extend the dynamic range

 

It seems that "extends" is a relative term. Extend compared to what? One paper mentioned superior dynamic range to a diode detector.

Here is an example. the ADL5513 has a 2140 MHz in input range of 68 dB with an error of +/-1 dB. Its slope is approximately 21 mV/dB.

ADL5513S https://www.analog.com/en/products/adl5513s.html

datasheet https://landandmaritimeapps.dla.mil/Downloads/MilSpec/Smd/11203.pdf

 

In this link, it is said that the logarithmic detector and rms log detector extend the dynamic range, it is compared to other topologies like rms and envelop detectors.
Link: https://www.analog.com/en/product-category/rf-power-detectors.html

 

Yes, but it is also said in the papers that they extend the dynamic range

Imagine this. You want to measure power over a range of 1uW / m^2 to 1W / m^2 (not sure these are the right units, but it is only the ratio that matters).

So you create a circuit that converts it to a voltage from 0 to 10V

So, 1uV = 1 uW, 2uV = 2uW

So for the level of uW, the next value would be 2uW. You would need 1uV accuracy at that level to get an error up to 100%. Meanwhile, at the top of to range, there is a difference of 1V is goes from .5W to 1W. So the resolution is very good on the high end, very bad in the low end.

Now change it to the log base 10 of power. So 0V is 1uW, 1V is 10uW, 2V is 100uW, 3V is 1mW, 4 is 10mV, 5V is 100mW and 6V is 1W.

That gives you a consistent ratio for each unit of output. This allows for similar accuracy across a large range using the same precision. In this case, 1mV resolution can give you precise results across the entire range.