180
The Tektronix 180A is an all tube time mark generator with an oven-controlled crystal oscillator (OCXO). Time mark generators put out pulses at user-selectable intervals. They are primarily used for calibrating the time-base of oscilloscopes. The 180A divides down the clock signal using monostable multivibrators. This scheme does not require precision in the time constant of the monostable multivibrators. The only precision component in the 180A is the OCXO. The manual suggests an interesting procedure for calibrating the OCXO, by attaching a small antenna to deliberately interfere with the national time standard broadcast, and observing the interference with a communication receiver.