What is the meaning of Vtt
Vbb - reference voltage for ECL used in single-ended input mode or for some types of termination. Usually you bypass to Vcc and tie the differential input that's not being used to Vbb, or tie inputs through back-to-back Schottky diodes to Vbb. Some devices have a Vbb output or it can be generated with an ECL gate. 

Vtt - termination voltage- usually a separate supply at Vcc - 2.0V that allows termination with a single resistor rather than a divider. 

Much more on ECL termination here: 

Edit: From your information regarding the counter part number and related datasheet: 



Vtt should be Vcc - 2.0V. 

It might be easier to terminate with a Thevenin-equivalent resistor pair. 

R1 || R2 = 50\$\Omega\$ 

\$\frac { R1}{(R1 + R2)} = \frac {V_{CC}-2}{V_{CC}}\$

(you can do the sums depending on your choice of Vcc)