The 535B/538B series counters are microprocessor-based heterodyne instruments. The 535 and 538 span the frequency range from 10 Hz to 20 GHz and 10 Hz to 26.5 GHz respectively. Through keyboard control, the 53XB series counters provide frequency offsets and frequency multiplication. Options include full systems capability via GPIB, high stability time bases, and frequency extension to 20 GHz for 535B. Measurements are presented on a 12 digit LED display that is sectionalized to read GHz, MHz, kHz, and Hz. 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, a sensitivity of -20 dBm. Band 3 has an input impedance of 50 ohm nominal over a range of 1 GHz to 20 GHz (or 26.5 GHz), and a sensitivity to -30 dBm (typical).