The 54XA series counters are microprocessor-based heterodyne instruments. The 545A and 548A span the frequency range from 10 Hz to 18 GHz and 10 Hz to 26.5 GHz. respectively. The model 548A, when equipped with frequency extension capability (Option 06), is used in conjunction with a remote sensor (See Model 590) to measure up to 110 GHz. Using keyboard control, the 54XA series counters provide frequency offsets and frequency selectivity. Options include Power Measurement, full Systems capability via GPIB or BCD/Remote Programming and D/A Converter output. Full frequency range is covered in three bands. Band 1 is a high impedance input (1 M ohm/20 pF), and spans a 10 Hz to 100 MHz range, with a sensitivity of 25 mV RMS. Band 2 has an input impedance of 50 ohms, a 10 MHz to 1 GHz range, with a sensitivity of —20 dBm. Band 3 has an input impedance of 50 ohms nominal over a range of 1 GHz to 18 (or 26.5) GHz, and a sensitivity to -30 dBm. For frequencies above 26.5 GHz a remote sensor, with an appropriate waveguide input, is called Band 4. Measurements are presented on a 12 digit LED display that is sectionalized to read GHz, MHz, kHz, and Hz. When the optional power measurement function is activated, the digits on the far right display power in dBm with 0.1 dB resolution, and frequency resolution is limited to 100 kHz.