The Model 527A FDM is used to determine the fractional frequency difference between two stable oscillators, to adjust two oscillators to the same frequency, to offset one oscillator from another by a specified frequency, and to analyze short and long term frequency stability at nominal frequencies of 100 kHz, 1 mHz, 2.5 mHz, and 5 mHz. 1-5. DESCRIPTION OF EQUIPMENT. 1-6. The FDM is 3.5 inches high, 12.75 inches deep, 16.875 inches wide, and weighs approximately 15 pounds. The rack mounted model is 19 inches wide. It contains 14 printed circuit plug-in boards: a power transistor board, a flip-flop board, 4 error multiplier boards, a reference input/9 mHz amplifier board, a 5 mHz/1 mHz divider board, a buffer amplifier board, a power supply board, a differentiator/integrator board, a single shot/phase comparator board, a crystal filter board and a scope driver board. A PCB extender board is provided to faciliate trouble shooting and maintenance. Refer to table 1-1 for a functional description of the FDM controls, connectors, and indicators.