Here is the circuit diagram of the decade frequency divider and high frequency (HF) harmonic generator used for precision frequency measurements. The reference signal should be from a 1 MHz frequency standard with TTL signal level output in order to provide a 1 kHz calibrator ("frequency comb") signal. If a different reference frequency is used, for example 100 kHz or 10 MHz, fewer or more 7490 divider stages will be required. There is nothing special about the decade frequency divider itself which can be built using 74LS90 or similar decade divider chips, and I claim no credit for any originality. The harmonic generator circuit is adapted from several circuits given in the article by Hans Evers, PA0CX, "A frequency and level standard", published in Ham Radio magazine, January 1986 (see reference 16 in the measurement bibliography). A +5V supply is required to power the circuitry.