hmcad1511

[serdo]

Hallo, I connected hmcad1511 to FPGA,it make spectrum analyser. patterns looks good. 500MSPS dual channel mode. But in spectrum is 3 spurs at 125, 187.5, 62.5 Mhz with 20 dB up of noise floor ( noise -85 dB). fclk possible do this spurs. I trying tune adc registers ( LVDS loads and currents) no helps. Is way to delete spurs from spectrum ? Thank you

 

[D.A.(Tony)Stewart]

Can you show your work and shielding?

 

[dick_freebird]

I imagine that this is "subharmonic" switching in some of the
digital outputs, throwing their shuttle charge onto ground.

At 500MSPS you ought to have 500, 250 MHz spurs as well
as the lower ones you note - maybe just not visible under
the raw clock / sample noise.

Consider what you could do to minimize the capacitive burden
on digital outputs, and degenerate what's left with as much
series R at the output pin (before Cpcb and Creceiver) to limit
the height of the edge current pulses (trading for width and
eventually impacting downstream timing). Be sure that the
ADC decoupling works at frequencies well higher than 500MHz.
Try instrumenting up the package signal ground and power
ground pins, observe (if you can) the natural width of the logic
switching impulses as you vary amount and type and routing
of the critical VSS-VDD "back stretch" of the AC current loop,
the HF capacitors.