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):



So with an output impedance of 50 Ohm, the series resistor should be R = Z₀ - 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

Hi Matthais,

I think your interpretation is correct.

And it makes sense that the oscilloscope produces a clean waveform when its input is 50 ohms. The big question is "What is the amplitude of the waveform from the generator when the oscilloscope is directly connected to the LVCMOS generator and the input impedance of the scope is 50 ohms?"

The difference between the 50 ohm measurement with and without the cable should be an indication of the loss of the cable if the input signal is a single frequency square wave.

If the signal is random data, then determining the loss is slightly more complicated because the signal is spread out over frequency and the cable loss generally increases with increasing frequency. You would need to look at a FFT of the data (hopefully the scope has a fft function) to see if there is some slope over frequency reducing the amplitude of the signal when the cable is used.

Postscriptum: The manufacturer of the signal generator, has recommended me another 50 ohms in series on the load end. I guess it makes sense this way: of course, the transmission needs to be terminated on both ends. Works like a charm now. Thanks to all responders!