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Does a radio receiver need power from a battery in order to receive a signal, or is the power just used to amplify the signal? If you wanted to simply detect a radio signal of a certain frequency (not demodulate or amplify the signal), would an antenna and a tuner be enough?

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    \$\begingroup\$ As demonstrated by a crystal set, no, you do not need a local power source to receive AM modulated signals (or measure the power of those) which are strong enough. However, amplification, active filtering, and many types of demodulators (for example, FM (with the exception of slope detection)) require local power. There's also the RFID/"power thief" type of design, where received power charges a capacitor to briefly power active circuitry for processing and/or a reply. \$\endgroup\$ Commented Feb 4, 2013 at 17:06
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    \$\begingroup\$ @Chris: make that an answer. \$\endgroup\$ Commented Feb 4, 2013 at 17:09
  • \$\begingroup\$ @ChrisStratton "AM modulated" is redundant \$\endgroup\$
    – vicatcu
    Commented Feb 4, 2013 at 17:58
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    \$\begingroup\$ @Chris - I agree with Wouter, that's a perfectly good answer to the question. Maybe add a schematic of a typical crystal receiver if you move it from the comments. \$\endgroup\$
    – Oli Glaser
    Commented Feb 4, 2013 at 18:08
  • \$\begingroup\$ @ ChrisStratton - Thank you, a good answer. Based on this, I am assuming the distance range limits on passive RFID/"power thief" type devices are governed by the power available from the charged capacitor to send a strong enough signal back to the transmitter. \$\endgroup\$
    – Eddie
    Commented Feb 5, 2013 at 8:00

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You can receive a signal without a battery, but it has to be strong enough to drive whatever output device you have. It would take a VERY strong signal to drive an LED, not so much to move a very sensitive meter, and even less to be audible in a sensitive headphone. If course to be audible it has to be demodulated. So the crystal receiver that has been a kid's first electronic project since the 1920's: an LC circuit to tune a long-wire antenna, a crystal diode to demodulate, and a high impedance headset to listen. AM only. RFID tags use the same principle but they actually create a modulated field load to the transmitting antenna to "answer" the reader. They depend on a very-low-power IC that can take the received power and reflect it back to the antenna with the ID as modulation. That's the limit of my knowledge, there may be RFID's that retransmit on a different frequency.

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A project I was involved with "received" power from a loop antenna at 600kHz and continuously transmitted data for 16 strain gauges at 80MHz F.M. back onto the same loop antenna - it would work at 40mm distance from the power source. On this job the power not only fed the RF o/p stage and modulator but also fed a PIC, a small Lattice PLD as well as providing 3mA current for all the strain gauges, amps, filters and ADCs. Why so complex - it was spinning at 3,000 rpm on a steam turbine. Batteries wouldn't last 5 minutes.

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While it is possible to receive signals using no power [crystal radio] they aren't very efficient. Signals need to be strong enough to forward-bias the diode detector: 0.6V for silicon, 0.2V for germanium diodes.

The second point is the "Q" or quality of a tuned circuit: The antenna and tuner may pick up a signal quite well, BUT to measure it you need something hanging off the tuned circuit. The lower the impedance of the measuring circuit, the more it will impact the tuning circuit.

One of the low-power circuits out there is simply a solar cell, feeding just enough voltage to a diode, to Almost forward-bias it. You then get improved reception [although you still need high impedance earphones / metering circuit to avoid loading down the tuned circuit].

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