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Altera_Forum
Honored Contributor
Honored ContributorGenerating 400MHz external clock (Cyclone V)
Hello,
I'm trying to generate a 400MHz clock signal from my Cyclone V in order to drive the clock input for an external ADC, but I'm getting severe attenuation on the clock signal. Details: -I'm using the PLL megacore wizard, with output frequency set to 400MHz, and operation mode set to LVDS. Input clock is 50MHz. -The LVDS PLL output clock is going to diffio_tx_p&n pins (but I've also tried outputting a single-ended clock to a gpio pin and got similar problems). -I'm measuring the output clock using a 1GHz scope and 15pf probes to probe these diffio pins. The output clock works well for lower frequencies such as 60MHz, but attenuates as frequency increases. By the time I hit 400MHz the Vp-p of the out clock is only 100mv but I need it to be >500mv. Question: Are there different pins on this fpga I should be using instead that would drive those 400+MHz frequencies, or what else might I be doing wrong? Thanks14 Replies
Altera_Forum--- Quote Start --- It does look like a sine wave yes, but shouldn't the amplitude of the fundamental 400MHz sine wave still not be severely attenuated with 1GHz scope? --- Quote End --- The quality of the signal you observe depends on your ability to probe the circuit with a scope probe, with a ground return path right next to it. If I want to capture clean looking waveforms for documentation, I will generally solder a wire loop to ground (eg., decoupling cap pad) next to the probe point, and wrap that wire around the scope probe tip. That provides the least inductance, without having to use fancy probe tips. If you can see a 100mVpp sinusoid, then the LVDS receiver on the ADC clock input pin should still work ok. If the ADC is not working now, chances are you have another issue you have yet to find :) Cheers, Dave- Altera_Forum
Thank you Dave,
It does look like a sine wave yes, but shouldn't the amplitude of the fundamental 400MHz sine wave still not be severely attenuated with 1GHz scope? The fundamental should have higher amplitude than the overall square wave. edited: changed first harmonic to fundamental - Altera_Forum
I concur with Dave's observation about scope bandwidth. Also, your 15pf looks like about 26 ohms at 400Mhz, a stiff load for an FPGA output.
- Altera_Forum
Your scope probably does not have enough bandwidth.
The Fourier Transform of a (perfectly symmetric) square wave has odd harmonics http://mathworld.wolfram.com/fourierseriessquarewave.html so your 400MHz square wave has a fundamental at 400MHz, and the first harmonic at 1.2GHz. Hence, your 1GHz bandwidth scope attenuates the harmonics, and makes your square wave look like a sinusoid. Cheers, Dave
