Custom BNC adapter plug for SPM instruments with 3-pin CF1 input.	Nederlands?
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 75 Ohm output impedance, but the measuring circuit is optimized for 50 Ohm and my standard test leads are also 50 Ohm, it is useful to put in the adapter plug that is on the output of the generator (and only in THAT one!!) an R of 22 Ohm - ½ watt. Then that's the right Z reasonable everywhere and there is no longer any need for doubt about that. Besides this messy construction with only 1 R, the solution with 2 resistors is slightly better. Here you can see how that works. My schematic is over here An even nicer solution with less loss, but NOT the maximum frequency range from DC, is with a toroidal transformer in a plug, or better in a mini box. Ferrite 3F3 or 3C11 or FTxx-77 material seems OK for this. The 50 ==> 75 Ohm N = 1 to 1.22. For a range from 10 kHz to hopefully 2 MHz, a minimum Z of 250 Ohm at 10kHz is OK, the L is then somewhere between 4 and 5 mH. Depending on the core size (14 - 36 mm), the N for 50 Ohm is between 25 and 75. Do NOT use an autotransformer, but use a full set of windings over the entire surface for each Z-side. You can also keep it galvanically separated!

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

Walter Geeraert PE1ABR

 

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The SPM CF1 to BNC adapter connector Compilation by Walter - PE1ABR - 2023-12-04