The Tektronix Type 181 is a time mark generator introduced in 1955. There is a benchtop version, the 181, and a rackmount version, the RM181. MOD 110 substitutes a oven-stabilized 1 MHz crystal oscillator (OCXO), the CO181 (Tek part number 158-007), thereby achieving 2 parts per million frequency stability in a 24 hour period, the same stability as the 180A. A 181 with the OCXO mod is a "181 S1".
Internally, the 181 is similar to the 180. The oscillator drives a chain of synchronized monostable multivibrators. Each multivibrator is triggered by the output of the previous stage, and cannot be triggered again for the duration of its pulse width. In the 181, the pulse width ratio of one stage to the next is 1:10, i.e. when a stage triggers on a pulse at its input, it propagates that event to its output, but inhibits the next nine pulses (by time, not by counting).
The 181 also provides a 10 MHz output. This is generated by a 10 MHz resonator that is driven by an amplified and waveshaped copy of the 1 MHz crystal oscillator output. Since it is a harmonic, the 10 MHz output has same relative frequency accuracy as the reference oscillator.
The power supply of the 181 produces +300 V and -150 V regulated voltages and +400 V, -8 V, and -25 V unregulated. The voltage reference is a 5651 tube. The output tubes for both regulators are 12B4 and the feedback amplifier tubes are 6AU6. The 181 does not contain a fan or a thermal cutoff.