Output CLOCK signal to GPIO pin of CYCLONE IV E

Question

I am new to FPGA and I am trying to send a CLOCK signal as an output of a GPIO pin of an Altera Cyclone IV E. I first made a program:

module CLOCK_out (input CLOCK_50, output CLOCK_pin);
      assign CLOCK_pin = CLOCK_50;

endmodule

It just sends out the default 50 MHz clock signal of the DE0-Nano Board to a GPIO pin. This is a success. However, when I viewed the signal with an oscilloscope, what I have is a 50MHz sine wave (see attached picture). I feel like I produced the wrong signal because I am expecting a square wave instead . I don't know where I am wrong. I am using 3.3V LVTTL standard for the IO. Is there something wrong with what I have done or is the output really a sine wave after all?

I need help. Thanks in Advance

Answer 1

What's the bandwidth on your scope? If it's not significantly greater than 50 MHz (say, 500 MHz or more), the scope's bandwidth limit will turn your square wave into a sine wave. This is because a square wave consists of a series of odd frequency harmonics...in this case 50 Mhz, 150 Mhz, 250 MHz, etc. So if you have, say, a 100 MHz scope, you will miss all of the frequency components except the fundamental at 50 MHz, and you'll get a very nice sine wave even though you actually have a square wave with very sharp edges and lots of high frequency harmonics. If you use a 500 MHz scope, then you'll get 50, 150, 250, 350, and 450 MHz which will result in a nice square wave. And using mile long jumper wires like that will certainly not help with signal integrity.