Electronic – Why would I use a transformer instead of one inductor in a crystal set

crystal-setham-radiopassive-networksradioRF

I'm reading up on A Simple Radio Receiver. I have a question about this image:

enter image description here

The signal from the antenna (perhaps a few tens of microvolts, or hundreds for a nearby transmitting station) is introduced to the LC circuit either through a small capacitance, or, as in this case, by means of a second coil L2 wound on top of L1, with its other end connected to earth. This behaves like a transformer – currents flowing in L2 generate a changing magnetic flux which cuts L1 and induces an emf in it.

I don't understand why L2 is needed. It forms a transformer with L1, to "cut L1 and induce an EMF in it". What is meant by 'cutting' in this case? And, when they need EMF in L1, why not just connect it to the antenna input directly? Like this:

schematic

simulate this circuit – Schematic created using CircuitLab

Sorry for the ugly schematic, it would've be too large otherwise.

Anyway, why is a transformer needed in this crystal radio set? I actually built a crystal set without a transformer, with this circuit:

enter image description here

I also added a 100n capacitor parallel to the crystal earphone.

Why would or should I use a transformer instead of going into the LC circuit immediately?

Best Answer

There are three advantages to driving the resonant L-C tank via tranformer coupling than directly from the antenna:

  1. It changes the impedance. The impedance of the signal coming from the antenna is probably in the 50-300 Ω range. Crystal radios are usually listened to with high impedance headphones, usually around 2 kΩ. Having a 100 Ω source driving a 2 kΩ load is inefficient, which ultimately means lower volume.

  2. The detector diode drop is a smaller fraction of the output voltage. The transformer steps up the voltage, making it higher than what comes out of the antenna directly. The forward drop of the dector diode is fixed, so it is less relative to the signal after that signal has been increased in voltage by the transformer. This means lower strength radio stations will still produce some output, and the output you get will have less distortion.

  3. Better selectivity. The relatively low impedance of the antenna dampens the resonance of the L-C tank circuit. Another way to say this is that it lowers the Q of the resonant circuit. This means its resonant peak is more spread out, which means that the radio can't be tuned as well to a single station. The effect is that strong stations will appear to spill over into nearby frequencies.

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