The EIP 371 Source Locking Autohet Microwave Counter automatically measures the frequency of any CW signal within the range of 20 Hz to 18.0 GHz. This frequency range is covered in three bands: 20 Hz to 300 MHz. 100 MHz to 850 MHz. and 825 MHz to 18 GHz. Measurements in Band I (20 Hz to 300 MHz) are made with a 300 MHz direct electronic counter. Band II (100 MHz to 850 MHz) uses a prescaler to divide the input signal by a factor of four into the frequency range of the 30D MHz direct counter. Band III (850 MHz to 18.0 GHz) measurements are made by heterodyning the input frequency with an automatically selected harmonic of an internal 200 MHz comb generator, producing a difference frequency which falls within the range of the 300 MHz direct counfer. The inaccuracy of the indicated reading by the counter, is directly related to the quality of the time base oscillator over the entire operating range of the counter