Understanding Modulation in 4-20ma Transducers/Transmitters

[Adder_Noir]

Dear All,

I have today been introduced to the workings of a 4-20ma transducer/transmitter. My boss described to me the workings of one and he stated that a transducer could be used to modulate the signal coming out of the primary cell.

I understand everything about the transducer but I have one problem with it. I do not know what he means by modulation. What is modulation and why would we use it to modulate the signal coming out of the primary cell

Thanks in advance

 

[faust9]

Modulation is the combining two signals, a carrier signal and a data signal. If you listen to the radio or use a cell phone or watch TV(cable Dish or broadcast doesn't matter) then you are benefitting from the modulation process. There are a couple of reasons to use modulation. For very very small signals you can use amplitude modulation(AM as in AM radio) to help you boost the signal while adding only a small amount of distortion vice directly amplifying the signal which could add a lot of distortion(a nuclear power plant I worked at use this technique to allow for accurate neutron monitoring when the plant was shutdown). You can use modulation to boost a data signal for wireless data transmission (AM, FM, PM(FM stations use PM--phase modulation--because the circuitry is generally simpler)). Again, with radio, cell phones, and TV you take a data signal boost it to very high power levers a and frequencies to send data around the world and out into space. This signal boosting is done in such a manner as to add only minimal amounts of distortion.

Here, a good little page. You might try how stuff works as well.
http://cbdd.wsu.edu/kewlcontent/cdoutput/TR502/page21.htm

So why use it? Low distortion signal boosting, Data transmission over long distances, wireless data transmission, transmission of data at discrete frequencies i.e. the radio spectrum TV Radio and Cell phone companies fight over so much there's also security reason (there's a side product of AM called single side band--SSB which can be used to transmit data much more securely well, FM and PM create sidebands too but are not as useful).

Hope my brief synopsis helped a little.
Good luck.

 

[Adder_Noir]

That is a fantastic explanation mate Cheers a bundle. My boss was telling me how it is used on a transducer which is transmitting data to a drive. It operates on a 4-20ma range. He told me that we only have 4ma entering the transducer but a possible maximum of 20ma leaving the transducer. I am guessing it is the modulator which is responsible for boosting the current up to 20ma going away from the measuring parts of the transducer. Hope that makes sense Thanks very much anyway that helped a lot