The 351D Autohet™ Microwave Frequency Counter utilizes the heterodyne technique, offers a microwave instrument with both sensitivity of -30 dBm and wide band FM tolerance of greater than 40MHz peak-to-peak. Using improved solid state YIG and thin film technology, EIP has developed an innovative frequency measuring technique yielding the FM tolerance advantage of heterodyning and the sensitivity previously associated with transfer oscillator approaches. This reliable frequency counter covers the frequency range from 20Hz to 18GHz with -20 to -25 dBm sensitivity. The input impedance is 50 ohms and great resolution is guaranteed with an 11-digit readout.