Bob Tims wrote: > > Happy New Year All, > > I have an interesting noise problem brought to me. > The noise is a 500-900 mV pulse seen at 50 MHz on a > power lead from a DC to DC converter (48V to 5V) to a > VME cage. > There were two scenarios: first, both the power supply > and the cage are earth-grounded, and the pulse is seen > at both the power supply and the cage. Second, the cage > is left floating, and the pulse is only seen at the > power supply. > The noise seems to be a ring, and the leads between the > supply and cage are fairly long. > Has anybody seen something like this before, or does anybody > have any suggestions or comments? > We have several ideas, but not much time. > The frequency and the fact that the pulse reaches almost a volt > concerns us greatly.
You didn't say how you are measuring .5-.9 volts but about the only thing you can be sure of is things are not as you have indicated. For instance if we had just one 0.01uf bypass capacitor across this power lead, its reactance at 50 MHz = 1/(2pi f C) = 0.31 ohms It's highly unlikely the noise source could drive almost a volt into this reactance. It is more likely you have a common mode propagation along the cable. It's also likely you are measuring the common rejection factor, or lack thereof, in your measuring instrument. (scope?) If so, slapping a common choke (ferrite block) on the cable should change the reading. You may need to place the converter in a Faraday shield with an appropriate RFI filter on the power I/O to block the noise. I suggests you look up Henry Ott's book on noise. He suggests so methods for tracing your problems. Fred Townsend DC to Light Consulting f...@luxtron.com
