A true logarithmic amplifier for radar IF applications

There are certain radar receivers where the settling time of an AGC loop is unacceptable and an amplifier is required which will compress the dynamic range instantaneously. A common technique for accomplishing this is to use a logarithmic amplifier. This has other advantages in radar applications in that a logarithmic amplifier can assist in separating wanted signals from unwanted signals known as `clutter´ caused by unwanted targets such as raindrops. In systems such as MTI radar systems, where it is required to detect moving targets, the phase information is important hence the logarithmic output must be at the IF frequency. In order to preserve the phase information the phase shift or delay through the log amplifier should not vary with input signal level. This type of amplifier is known as a true logarithmic amplifier. The device described in this paper is capable of producing a true logarithmic amplifier with phase matching of /spl plusmn/4/spl deg/ over an 80 dB input dynamic range at 70 MHz.