let say i give input signal of 1V pk-pk from frequency generator then i have to sweep the frequency everytime i want to see the response at all frequency?
I am assuming you are using a simple one pole resistor/capacitor filter, [ not terminated, except for the high impedance scope connection and the signal generator set on the 50R output]
You still have not posted a simple diagram of you LPF.
If you have designed the LPF correctly for a cut off frequency of 1MHz, its just a matter of determining the Bode plot.
Connect the scope Y1 channel to the input of the filter, connect the Y2 channel to the output of the filter.
Set the Y1 and Y2 to the same sensitivity, say 0.1V/division.
Starting at 500K [ sine wave input] the output should be the same amplitude as the input.
Slowly increase the frequency of the generator [
ensure that the Y1 is kept at the original amplitude] , as the frequency increases the output amplitude will start to fall.
When the output is 50% of the input, thats the -6dB point on the plot.
As you increase the freq, when the out is 33% of the input thats the-10dB point,, when its 10% thats the -20dB point.
As a check,
reduce the frequency until the output is 70%, thats the -3dB point.
Now plot your recorded Freq and dB values on
log/log graph paper.
Once plotted you should see that output falls at about -20db/decade, ie: slope
Do you follow that OK.?