Electronic – Hearing only noise and ticking sound with simple AM radio – The art of electronics


I've tried to build the simple AM radio that is proposed at page 56-The Art of Electronics (3rd).
AM radio

The idea is to tune my radio on a frequency of 657 kHz (I'm sure there's a station at that frequency).

C1 is a variable capacitor between 20 to 55 pf.

The constant R1*C2 = 10^(-4) is chosen to be in between the fast carrier wave (~1 us) and the audibile frequency (~ 200 us).

As an amplifier (in the original circuit it's not shown) I use an op amp with a G=100 and a high pass in input (to prune the DC offset).

As antenna I use a straight long wire (20 meters).

The problem when I build the circuit is that, in the speaker, I hear only a very low static and a periodic tick (I think it's the discharge of the capacitor that gives the tick).
I'm unable to make it work. I think that one of the possible problem is the use of a silicon diode that has too much voltage drop, but before buying a batch of germanium diode to try to fix the problem I wish to hear from expert if that's the actual problem of the radio.

P.S: I'm a beginner.

Best Answer

  1. R1 is too low. The forward voltage of a diode depends on the current through the diode. With a low resistance load, the forward voltage goes up to the point that the voltage of your received radio signal won't get above the forward voltage. R1 should be tens of kiloohms, if not hundreds of kiloohms.
  2. You need a germanium diode or a Schottky diode. The forward voltage of such diodes is lower than plain silicon diodes.
  3. The LM741 isn't made to directly drive a speaker. Other opamps aren't either. You might be able to drive a high impedance earphone with it.
  4. Your antenna must be as long as you can make it, and you must connect the ground of your circuit a real earth ground. If your outlets have a ground pin, then that's the most certain ground in your house. Water pipes or radiator pipes may also be grounded.
  5. Use batteries or power supplies with a linear regulator to power your circuit. Modern switching power supplies can easily produce enough interference that the radio station will be blocked or drowned in the garbage from the regulator.