Dear all,
Over night, I found myself in the middle of a project that requires somewhat more careful RF design than what I'm used to doing on a day-to-day basis (rendering algorithms). Maybe some of you experts can help me out.
My problem doesn't sound all that hard: I would like to take the LVCMOS output of a function generator (
Novatech 409B/02; click for spec sheet) and pipe it through a coax cable of 4-6 meters length into a
TI CDCV304 clock buffer for further use. The signal is a square wave between 10 and 150 MHz.
According to the spec sheet, the LVCMOS output has an impedance of 50 ohm. The manufacturer further specifies "V
oh >=2.4V and V
ol <=0.4V
when series terminated". Given a 50 ohm BNC cable and how I understand series (source) termination, wouldn't this mean that no additional series resistor is needed? See for instance this image here (click image for source article):
![](http://www.pcb-prototype.net/images/a/991005o.gif)
So with an output impedance of 50 Ohm, the series resistor should be R = Z
0 - 50 ohm = 0 Ohm, right? However, when I run a 100 MHz square signal through said long BNC cable into a 500 MHz, 2Gs/s scope, I see severe signal distortion until I activate an additional 50 ohm termination on the scope's end. The wave will then be very clean but at an amplitude reduced to less than 1 V p-p.
Any ideas how I can get this sorted out? I appreciate any hint, however slight. Thank you very much in advance!
Cheers,
Matthias