Odd level translator output waveform from 3MHz
I am feeding 1.8Vpp from signal generator


Your oscilloscope traces show that the input signal to the SN74LVC1T45 is invalid, and therefore you should not expect normal behaviour at the IC's output. This is due to the incorrect setup of (or lack of ability to control) the signal generator which is providing that input signal to the IC, as its output is a bipolar signal (i.e. it swings both positive and negative with respect to 0 V).

Summary: Make the signal generator output a unipolar (positive only) 0 V to 1.8 V waveform, and try again.



Details:

Both of the oscilloscope input traces (at 1 MHz and at 4 MHz) show the problem, but the extra overlay of information on the 4 MHz input trace shows the details more clearly. Therefore I'll explain the problem using your original 4 MHz input scope trace, with some added markings:



Meaning of numbers added to the above image:


This shows the Channel 1 zero volt (sometimes called ground) marker. (The Channel 1 trigger marker is nearly on top of the Channel 1 zero volt marker, since the trigger voltage was set at just 40.0 mV. That is why the black "1" in the yellow left-pointing triangle is not quite so obvious as on the output trace images.)

Notice how the IC's input signal (i.e. the signal generator's output signal) is bipolar i.e. it swings positive and negative with respect to 0 V. This is common for signal generator outputs which have not be setup to produce a unipolar digital signal.
This icon shows that Channel 1 is DC-coupled, so the voltage measurements shown below are referenced to zero volts.
This block of data again confirms point 1 above. I'm not using Vpp, Vmax and Vmin since those values include the overshoot &amp; undershoot. Instead, look at Vamp, Vtop and Vbase.


Vamp shows that, ignoring over/undershoot, the signal has an amplitude of 1.78 V. That fits with the stated 1.8 Vpp amplitude of the signal generator output. However...
Vtop shows that the maximum signal amplitude is (+) 920.0 mV
Vbase shows that the minimum signal amplitude is minus 860.0 mV


That all confirms that the signal generator's output signal (i.e. the input signal to this IC) is bipolar, which is invalid for the input to this IC and therefore does not guarantee normal behaviour.

The input signal should not drop below 0 V (standard logic level requirement, stated as "VI Input Voltage" on page 5 of the SN74LVC1T45 datasheet, under Recommended Operating Conditions)). Even the absolute maximum rating for minimum input voltage is -0.5 V (although that can be exceeded if the clamp current rating is not exceeded). But there is no guarantee of correct behaviour once the input signal drops below 0 V.

When the signal swings positive then, after the overshoot, the SN74LVC1T45 IC's input voltage sits at 920.0 mV (Vtop). However according to the SN74LVC1T45 datasheet, when the power supply is 1.8V, the minimum voltage guaranteed to be recognised as a logic high (VIH) is VCCI x 0.65. However this means (1.8 V x 0.65 =) 1.17 V is the minimum valid logic high input voltage, so 920 mV (0.92 V) is not a valid input voltage, even when the bipolar input signal is positive instead of negative. This is another way in which the IC is being operated outside its specification shown in the datasheet.


Summary: Make the signal generator output a unipolar (positive only) 0 V to 1.8 V waveform, and try again.