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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
1MHz 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]].
An 181 with the OCXO mod is a "181 S1".

Internally, the 181 is similar to the [[180A|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,
these pulse width ratio of one stage to the next is 1:10.  
The result is that when a stage triggers on a pulse at its input, it propagates
that event to its output, but inhibits the next nine pulses.  It does this by time,
not by counting.  After a stage returns to its resting state, it is triggerable again.
So it triggers once every ten input pulses.

The 181 also provides a 10MHz output.  This is generated a 10MHz resonator that
is driven by an amplified and waveshaped copy of the 1MHz crystal oscillator output.
Since it is a harmonic, the 10MHz output has same frequency percentage accuracy is 
the reference oscillator. 

The power supply of the 181 produces +300V and -150V regulated voltages and 
+400V, -8V, and -25V 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]].

* [http://w140.com/mmm/tek-181.pdf Tektronix 181 Manual (PDF)]

<gallery>
Image:Tek 181.jpg|181
Image:Tek rm181.jpg|RM181
Image:Tek co181.jpg|CO181 oven-stabilized oscillator
Image:Tek 181 block.png|Block diagram
File:Tek 181 oscillator and multiplier.png|oscillator and multiplier
File:Tek 181 frequency dividers.png|frequency dividers
File:Tek 181-switching detail.png|switching detail
File:Tek 181 power supply.png|power supply
</gallery>