Re: Rail Probes?

John Gord

In addition to good connection methods (very short grounds, etc.), a valuable technique for these measurements is to loop the probe cable a few times through a large ferrite core (split cores are easiest). This minimizes the added high frequency noise from ground currents between the DUT and measuring scope. I learned this from a designer of multi-kilowatt power converters.
--John Gord

On Thu, Apr 23, 2020 at 02:43 PM, Ed Breya wrote:

I had never heard of these, but from the data I'd say their usefulness is
mostly in the form of convenience. The exact same kind of measurements have
been done before, by many, for decades - I've done it. You measure the DC and
low frequency content with a regular scope input, and the high frequency
content with an AC-coupled 50 ohm wide BW input. These "power rail" probes
just combine the measurements into one channel, and add DC offsetting - look
at the 7A13 for an earlier example.

They are intended for low impedance sources, like power supply outputs, to
look at interference, ripple, and noise. There is a generalized term for all
of this, called PARD. If you want to see really high frequency content, you
use 50 ohm wide BW gear - a scope for time domain, or an SA for frequency. The
important thing is to isolate the DCV of the supply by AC-coupling, and most
importantly, to protect the equipment from surges that occur during
connect/disconnect, power up/down, and PS or load faults, etc.

In the block diagram, it may be shown simply as AC-coupling, but in the
details you would find that the HF signal path is all RF, 50 ohm environment,
including the RF coupling cap, with sufficient voltage rating.(not just any
old cap), and various protection circuitry on the 50 ohm output side going to
the equipment.

So, if you can live with the inconvenience of using a regular scope input or a
7A13 for DC and LF, and having to rig up a fairly simple AC-coupling and
protection circuit (the fanciness and complexity depend mostly on the required
BW) for HF, you can save the 4 grand or so.

Also note that the key to HF performance and CMRR is to carry the signals
directly in coax, with minimal impedance connections. You can get some ideas
looking at the optional accessories and how they attach to the DUT - that's
the part that really makes it work.


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