>Relays always have some coupling between the coil windings
>and the armature. The armature is near the signal leads.
>Many relays actually have the steel pole piece inside the
>coil actually connected to the armature, they might have
>20pF capacitance directly from the common contact to the
>coil. Other relays have long leads that pass the coil.
>Unless you know the relay has very low RF coupling from
>contacts to the coil, you should have a low impedance bypass
>capacitor from the coil to ground and a very short ground
>lead on the grounded coil terminal.
I have had first hand experience with such a case in my homebrew linear
amplifier's antenna change over relay, where I had noise from the mains
coupled into the receiver through the relay coil.I noticed that when I had
the linear in line, I had a slightly higher noise floor.(even with the
linear turned off because I only switch one side of the mains through the
on/off switch.)By disconnecting the armature wires, the noise was gone.The
problem was solved with one 0.1uf disc ceramic cap from one of the coil
wires to ground.
That solved the problem but I still need to filter the mains where it enters
the amplifier.
Origin of the noise was later traced to a mistake the builders made when I
had the house altered.
73 Raoul
ZS1REC
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