Vintage hp instruments: hp 8406A frequency comb generator circa 1967
From: hpmemoryproject.org/wb_pages/w...
The HP 8406A Comb Generator
Even if it is not obvious to place the 8406A in the pulse generator category of instruments, it is still mainly a generator of pulses. It appeared for the first time in the 1967 catalog, in which it was presented as a convenient and accurate frequency calibration accessory for the HP Model 851A/8551A (broadband spectrum analyzer).
The 8406A comb generator has three internal crystal-controlled oscillators at frequencies of 1, 10, and 100 MHz, and a step recovery diode that shapes the oscillator outputs into extremely narrow pulses less than 100 picoseconds wide. The result is the generation of a comb of marker frequencies with useful amplitude from 1 MHz to beyond 5 GHz.
Although not really useful with a synthesized spectrum analyzer like the one shown below, the purpose of this picture is to show the comb of pulses output from a 8406A in a single 10 GHz span. Precisely separated markers at 1, 10, or 100 MHz was the only way to verify the horizontal frequency calibration of an analog spectrum analyzer. The HP 8406A was a precious, low-cost tool, for the engineer using an 8551 in the 1960s, or a 8558 in the 1970s. And even more precious to the service technicians who have to adjust the sawtooth shaping circuit of such an analyzer.