The 8600A digital marker is part of the spectrum and network analyzer family of HP products from the 1960s and 70s. It was designed to be used with the 8601A generator/sweeper and is normally connected to the AUX output of the sweeper. The digital marker provides a precise readout of the frequency currently being analyzed, without the need to estimate the frequency using the sweep dial or graticule. It also generates marker tones that can then be displayed as sharp peaks on the analyzer's display. The 8600A contains some primitive digital logic combined with analog RF circuitry that is spread out across several cards installed in a backplane. It uses an internal oscillator and a series of dividers and counters to generate the marker tones, which essentially form an accurate ruler on the analyzer's display. The rotary encoders are used to adjust the marker spacing. Nixie tubes are used to display the numbers that make up the frequency readout.
