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Originally Posted by PocketBrain
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Well, not a transmitter on purpose, but yeah, a frequency mixer to extract intelligence from the radio signal. Instead of racial profiling, people being pulled out of the line for closer inspection are now well-manicured.
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Actually, any radio that has a mixer stage will have what is known as 'receiver conducted spurs'. Basically what that means is that the radio signals that are picked up at the antenna, when mixed with the internal local oscillator (LO) inside the receiver to produce an intermediate frequency (IF), also produce the sum and difference frequencies of all the incoming signals mixed with the LO. Simple, eh :approve:
In simple terms, your receiving antenna actually will radiate (ie:transmit) very low level signals produced inside of the receiver. The FCC has maximum allowable levels for conducted spurs. Well designed radios will have virtually no conducted spurs.
This is actually used in England, where the government insists that every radio receiver and TV is licensed and to enforce that, they have inspectors that drive around in vans that have very sensitive antennas that 'sniff' out the local oscillator frequencies in the consumer gear that can be picked up due to radiation from the equipment.