PLL output square wave distorted

Question

I am testing PLL output from an old Stratix board. The PLL

has an onboard 80MHz crystal as input clock.

However at > 60 MHz, the PLL output is not as square as I wish with ripple

and 50mv undershot. At 250MHz, it simply looks like sine wave. The output

is observed by a fast scope with 50 ohm termination.

Here is my question: is this expected, or something is wrong in my setup that

prevent me from geting perfect square at high freq?

altera_forum · Answer

you did not tell us which scope was used for measurement.

Your scope should have sample rates of at least 2.5 GS/s.

What kind of probes did you use ?

Be aware that the characteristic impedance of the transmission line used on your PCB is important and how it is connected to your chip.

altera_forum · Answer

My scope is tek TDS5034B, sampling rate is 5G/s.

My stratix board have an SMA connector for PLL output, from which I use a SMA-BNC adaptor + BNC cable to the scope. all cable and connector should have a bandwidth well beyond 2GHz.

Anybody have monitored your PLL output at high frequency?

altera_forum · Answer

what i/o standard and settings are you using?

altera_forum · Answer

checking the pin planner, the output pin is 3.3 v LVCMOS.

altera_forum · Answer

I suggest to think about some basic signal theory relations. TDS5034 has a bandwidth of 350 MHz. To display a 250 MHz square wave different from a sine, you'll need at least 750 MHz bandwidth to see the 3rd harmonics.

Forum Discussion

PLL output square wave distorted

7 Replies

Recent Discussions

Inquiry regarding purchasing FPGA licenses

Agilex 3 BB18A package - patching MSEL1 and MSEL2 on interposer

Operating temperature for 10M08DCF256A7G

Question for LPDDR5 power sharing guideline

Cyclone 10 LP's Extended Industrial parts