From TekWiki
(Redirected from 180A)
Jump to: navigation, search
Tektronix 180A Time-mark generator
The Tektronix 180A was introduced in 1958 and 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 signal path of the 180A starts with the OCXO, which produces 1 MHz. That waveform is divided down using synchronized monostable multivibrators. Switches on the front panel select which of the divided-down pulse signals will be added together to form the output waveform. This scheme does not require precision in the time constant of the monostable multivibrators. The only precision component in the 180A is the OCXO (part number 158-007).

In addition to the time mark generator functionality, the 180A also produces sine waveforms at 5 MHz, 10 MHz, and 50 MHz. These frequencies are produced by feeding the 1 MHz OCXO output to a series of three frequency multipliers. First the 1 MHz is quintupled to get the 5 MHz signal. Then the 5 MHz signal is doubled to the 10 MHz signal. Then the 10 MHz signal is quintupled, yielding the 50 MHz signal. Each of the three frequency multipliers is composed of a 6DK6 pentode driving an LC resonator tuned to 5 MHZ, 10 MHZ, and 50 MHz.

The 180A 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.

The method is to zero-beat the WWV standard frequency broadcast with the internal crystal oscillator to align the OCXO frequency. First tune ham receiver to WWV broadcast - say 10MHz - and watch the audio subtone using an oscilloscope connected to the audio out of the receiver. Then wait for specific minutes of the hour where there is no audio subtone in WWV. Then select 1μs output on 180A, which will have 10MHz as the harmonic. Connect a wire from the output of 180A as an antenna and couple it to the antenna input of the receiver. This way the 10MHz WWV and 10MHz harmonic from the 180A, will beat in the receiver input. The difference will be visible as a wave in the oscilloscope. Then adjust C105 to align crystal frequency as close to the reference WWV 10MHz, that the frequency difference becomes zero or the waveform on the scope becomes flat.

You could also use the S-Meter in the receiver instead of the oscilloscope to perform the same.

The Tektronix 180 has toggle switches. The 180A has pushbutton switches.

The 180A, serial number 5599 and up, uses the 120-119 power transformer.


See Also