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I am making an AM radio with the specifications of: Part 1. Design a circuit with an antenna for tuning your receiver to a signal of a specific carrier radio frequency so that you can select a certain radio station. The frequency range is from 526 kHz and 1706 kHz at 10 kHz intervals.

I found a site with some information about a resonator https://www.instructables.com/id/How-to-Build-and-Tune-an-AM-Radio/ and I have read some stuff about the detector, antenna, tuner, etc. Parts of an AM Radio, but I don't know what kind of tuner, detector, or amplifier I need for my circuit specifically. I'll start with asking what kind of tuner would I need for my specifications(Answered)? Also, is a tuner and a resonator the same thing(Answered)? Can someone explain the detector and/or amplifier part of the am radio with regards to my specifications?

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  • $\begingroup$ You might want to try to find a copy of the Handbook for Radio Engineers -- it explains everything you could possibly need to know about AM receivers and transmitters (ableit at a vacuum tube tech level), from the context of assuming you build a breadboard circuit as you read. $\endgroup$
    – Zeiss Ikon
    Mar 25, 2020 at 17:59
  • $\begingroup$ Welcome to ham.stackexchange.com! $\endgroup$
    – rclocher3
    Mar 25, 2020 at 23:43

4 Answers 4

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The tuner is whatever allows the frequency to be selected. Tuners usually involve some kind of resonant circuit.

For a very basic AM receiver like you describe, the tuner is probably nothing more than an LC circuit, with one or both of the inductor or capacitor being variable.

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To answer the last part of the question, the detector for an AM receiver is basically a rectifier, which rectifies the RF and the signal that it was modulated with.

Once the RF has been rectified, it has superimposed on it the original signal that the carrier was modulated with. This is then usually put through a low-pass filter to turn the "rectified RF modulated with a signal" into just the signal, by removing the RF. The lazy man's low pass filter in these cases is our ears, but it doesn't hurt to put a tiny capacitor across the signal going to ground.

And the last stage is an amplifier - there are many amplifiers you could use for this. A simple transistor would do the trick, or you could use an LM386 IC if you wanted a decent sound out of a speaker or a pair of headphones.

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Simple crystal radio schematic.

The way I understand it, the tuner is an LC circuit forming a parallel tank that is designed to give maximum impedance at the desired frequency. This mean at that frequency, more current will flow through the detector and headphones than through the tank. At other frequencies, the tank circuit presents a low impedance and those signals are sent to ground through the tank, bypassing the detector.

The selected frequency is given by 1/(2*pi*sqrt(LC)). Making either your capacitor or inductor adjustable lets you tune around.

The diode acts as the detector. This circuit has no amplifier, so it'll only be good for picking up strong signals. The amplifier would come after the detector/diode.

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    $\begingroup$ Would it make a difference as whether the inductor, capacitor, or both are variable? Also, would you mind providing an answer about what the detector and/or amplifier is, especially with regards to my specifications? $\endgroup$
    – JackLalane1
    Mar 25, 2020 at 18:00
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    $\begingroup$ For this circuit to work, the "headphones" should be a "crystal earpiece", which has an extremely high impedance compared to regular moving-coil headphones. $\endgroup$
    – Scott Earle ♦
    Mar 26, 2020 at 2:05
  • $\begingroup$ @JackLalane1 Normally you don't make both variable. Which one you make variable just depends on the frequency range you need. I don't have a lot of experience, but from what I've seen variable capacitors are more common. $\endgroup$
    – byl
    Mar 26, 2020 at 19:32
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You would need the following:

  1. Antenna - an inverted 'L' antenna using insulated wire strung between supports about 30' apart and 30' high.

  2. Earth - an underground metallic water pipe would make a good earth connection.

  3. Tuner - a parallel-tuned circuit (a coil with a variable capacitor across it) tuneable across the medium wave broadcast band.

  4. Detector - a Germanium diode, a capacitor and a resistor to rectify and demodulate the signal. It is the process of extracting the audio from the modulated signal.

  5. Audio amplifier - requiring a transistor, a resistor, a capacitor, audio output transformer and a battery to amplify the audio.

  6. Speaker - for audio output.

enter image description here

A representative schematic of the radio is shown above.

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