Custom BNC adapter plug for SPM instruments with 3-pin CF1 input.

SPM = Selective Level Meter / Selektiver Pegel Messer

Standard the SPM input (e.g. the SPM-2 or SPM-3) is a symmetrical input. So floating from ground. To avoid disturbing ground loops this is very useful. However, there could be an unwanted side-effect, the sensitivity to interference from the environment because the input is not hard to ground. When applied in a measuring arrangement in which multiple devices are tied together everything is in one way or another on multiple locations connected to ground.

It is possible that the SPM input (or the amplifier circuit, which is externally attached to it) unintentional has to do with EMC influences, so the need arose to either connect or NOT connect to ground.

Because of this I designed an adapter plug that is an improved variation on a familiar theme, namely the rectangular caps from Harting used for 25 pin RS-232 connectors, and install it in there.

I have arranged in it both additional shielding as well as a switchable ground connection. The piece of circuit board where the banana plugs are attached to is with double sided copper clad, for greater strength both sides have received a "solid roasting" with solder. I was unfortunately not able to find threaded banana pins, to secure it with nuts.
Please note: The banana pins are intentionally off-center in the cap's because it would not fit on all SPM CF1 inputs otherwise!

Because the PS3 generator is designed for 0 - 75 ohm output impedance, but the measuring circuit is optimized for the standard 50 Ohm and also for 50 Ohm test leads, it is useful to add to the adapter on the output of the generator (and only in THAT one!) an R of 22 ohms - watt. Then reasonably everywhere is the right Z and there is no doubt about that.

Throw a glance at the pictures. Click for larger.
By clicking them both, they stand side by side.




Walter Geeraert


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The SPM CF1 to BNC adapter connector
Compilation by Walter - PE1ABR - 2012-03-04