Re: urgent
If the log amplifier is a 'true IF' log amp ie RF signal out = log ( RF signal in ) then you can use it to amplify a constant amplitude signal and still recover the frequency or phase modulation. If the signal has amplitude variations as a part of the modulation then these will be severely distorted by the amplifier.
IF the log amplifier you are referring to is is a successive detection logarithmic detector similar to the Analog Devices AD83xx series then you can use one only if you are trying to demodulate on/off keying. If you are trying to demodulate AM again you will get severe distortion, and you can not unless you are using the limited RF O/P ( if it has one ) demodulate phase or frequency modulation
Go to the analog Devices website for application notes on log detectors, they can be tricky things to use with lots of traps for the unwary.
Peter