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What causes noise in the receiver if I tap or hit the case of the transmitter? Or if I tap the mic cable?

This was a general question from the RAMPS group about various wireless transmitters generating low frequency noises when struck.

In general, there is mechanical coupling from the case into the inductors in the main oscillator in the transmitter. A thump on the case moves or bends the inductor, changes the inductance value by a tiny amount and changes the frequency of oscillation. Since a changing frequency is just FM, the FM receiver picks it up as a low frequency thump. There are various ways of reducing the mechanical sensitivity. Most involve very rigid coil assemblies such as inductors wound on ceramic forms. In our case, we use solid quarter wave ceramic resonators. 

The cutest trick I've seen, was a (brand) unit that used a miniature Teflon insulated coaxial line as a resonator. They wound the coax stripped off the outer insulation in a tight cylindrical coil with about 6 turns. The entire shielded coax coil was then soldered on the outside into a solid mass. This made a nice rigid assembly with the center conductor acting as the inductive element since a short coax line with one end shorted looks like an inductor.

The other way to generate a thump is to use a capacitor in the audio circuity that is sensitive to mechanical stress. The wrong kind of ceramic capacitor with DC voltage on it can really generate a lot of voltage when stressed. NPO ceramic capacitor types are as good as most film caps or tantalums but X5R types are bad and Y5Z are horrible. NPO's have the least capacity for a given size and the other types have 5 to 50 times more capacity in a given size and that's why they exist. I tried 50 Volt Y5Z type capacitors in the design of the 48 Volt phantom supply for the UH200C. You could get about as much audio talking into the transmitter PC board as you could using a microphone. Fortunately some small 50 Volt tantalums came on the market that would fit in the same space and saved my bacon. I knew the problem existed, but the severity surprised me.

My advice is to whack the case of a transmitter with both your finger and with a pencil sized object. If you know how a transmitter is going to react to mechanical shock, you can prepare for it.

On the subject of mechanical stress and audio, try the same thing with your electret mic cables. Some are much worse than others. If you tap the cable close to the mic (6") you will get mechanical noise transmitted directly to the mic element. In the middle of the cable, it is due to flexing of the mic cable. Phantom powered are sensitive to this since there is DC voltage on the cable and flexing the cable changes the dimensions and the capacitance of the cable. The pro mic manufactures have taken this into consideration in the choice of cable.

Posted 1 year agoby LectroAdmin