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I have this circuit from college (almost 15 years ago) that is through-hole components wired together and it picked up the strongest local radio station. I now can't get it to work, so I'm wondering if it's because it's tuned for a certain frequency and there is no radio station where I live now broadcasting at that frequency or some other issue. I could have something incorrect on the external amplified speaker.

So here is my LTspice model of what I believe I have. The way this circuit worked is you would hold onto the wires of the antenna section and the person holding it would become the antenna.

enter image description here

When I plot this as a bode plot I get a flat line, so I'm not sure if this the correct way to check for what the circuit is tuned to. Any suggestions on how to check?

I've ordered a TinySA that has a signal generator on it, so I'm going to check and see if I can check that way too.

The amplifier I'm using is this one from Amazon.

I've previously wired this circuit to a headphone jack so I have ground and the Vout connected to the L on the amplifier via line out cable. I've ensured the ground is connected from the circuit to the ground at the headphone jack on the amplifier and Vout is connected to the L on the amplifier. I put a speaker on it and can hear buzzing when I connect myself to the antenna, so I think it's working, but I'm not sure.

I recently fixed this headphone jack as I think I had ground connected to the R on the headphone jack, but now I'm wondering if that was wrong from the get go or correct.

Another option I can check is driving back to college and checking it, but who knows if the radio station that was in use back then is still in service today, so I'd like to check with simulation if possible.

I have the potential to show this to some younger people to get them involved in electronics. Having the simulation working will help me understand this bit better (I was really bad with RF stuff in college), plus if I need to tune the circuit for a local radio station it will be helpful.

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  • \$\begingroup\$ That class-D audio amplifier likely generates much radio frequency itself. It is possible that its radiation will mask weaker broadcast AM radio signals. You might try substituting a more linear style amplifier. \$\endgroup\$
    – glen_geek
    Apr 16 at 17:27
  • \$\begingroup\$ Is it possible the antenna is actually connected on the opposite side of the diode (i.e., at the non-grounded junction of L1 and C2)? \$\endgroup\$
    – K. A. Buhr
    Apr 17 at 2:35
  • \$\begingroup\$ @K.A.Buhr no that antenna location is correct. I have a piece of paper that the circuit goes thru to ensure component placement is correct and locations of ground, antenna, and output are labeled. \$\endgroup\$
    – tlab
    Apr 17 at 2:47

2 Answers 2

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First put in the right diode model for the diode you have into SPICE. If you want to find the resonant point, it would be at the point I have labeled RP (which is a resonator attached to a rectifier diode).

enter image description here

I don't do radio, but you could most likely tune this circuit by adjusting the capacitance of C2 or L1, a variable cap, or some way to vary the capacitance of C2 would probably be easiest.

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Regarding your question title, this circuit is commonly known as a "crystal radio receiver". The name comes from the original rectifier components -- actual crystals -- that have been replaced by silicon diodes in modern versions. This name is still commonly used, though.

It is a type of AM detector. The theory of operation is that a tuned circuit, usually some variant of an LC "tank circuit" consisting of an inductor and capacitor in parallel, is made to resonate at a target RF frequency, where the amplitude of the frequency corresponds to the encoded audio signal (i.e., AM or "amplitude modulation"). The voltage across the tank circuit averages zero -- it carries both the positive and negative voltage peaks of the RF signal. The rectifier passes only the positive half of the RF signal on to the rest of the circuit, and so the average voltage passed by the rectifier is proportional to the amplitude. A low-pass filter can be used to "average out" the RF, passing the amplitude modulated audio signal. The low-pass filter can be an explicit RC circuit, or -- as is the case in the simplest version -- a side effect of the physical characteristics of a ceramic earphone.

As I mentioned in a comment, your version of this circuit is a little weird. If the antenna was connected at the alternate point (the other side of the diode), it would be a traditional crystal radio circuit. The signal from the antenna and ground would be applied across the L1/C2 tank circuit, the diode would rectify the RF signal, and the R1/C1 low-pass filter would pass the audio to "OUT". (I'm not surprised your circuit still works as shown, and the principles of operation are going to be more or less the same.)

With respect to your LTspice simulation of this circuit, you can't model the antenna input as a plain voltage source. See this answer for example. In your circuit, if V1 is just a pure RF voltage source, then D1, L1, and C2 aren't doing anything. They are just a bunch of components connected between V1 and ground, and might as well be in the next room. All you're modeling is a radio frequency input V1 connected to R1 and C1, which form an audio frequency RC low pass filter, and that's why there's nothing interesting at OUT. (The reason @VoltageSpike gets useful plots is that they're looking at RP which is in the middle of the network of otherwise useless components.) The simple solution here is to model your antenna as V1 in series with a small capacitance -- the above-linked answer suggests 160pf (1000 ohm impedence at 1MHz) -- and you should get some more reasonable output. The peak resonance will match the peak resonance observed by @VoltageSpike, for the reasons below.

You don't need to simulate this circuit to figure out what's it tuned to, though. The primary resonant component here is the LC "tank" circuit, consisting of L1 and C2 in parallel. It has a peak resonant frequency given by f(Hz) = 1/(2 pi sqrt(LC)), which works out to around 1300kHz for the L1 and C2 values you've listed. I wouldn't take this too seriously though. Unless you used high precision components, the actual values of L1 and C2 are probably only within 10-20% of their nominal values anyway.

As long as the station is strong enough and you've got enough antenna, you ought to be able to pick up anything from 1200-1400kHz or maybe even a wider range. Note that adding another 2.2nF in parallel with C2 will tune down to 920kHz, so that might be an option if you have something down there. (This is all assuming that you don't want to try to incorporate a variable capacitor, which would allow you to tune more carefully.)

The key to getting these circuits working, especially when you can't tune them perfectly, is a big enough antenna and a good enough ground. A person only makes a so-so antenna. If there's a really strong radio station nearby, you might pick it up. If not, a couple dozen feet of copper wire strung across the ceiling will work better. Also, the ground you describe might not be adequate. You're relying on the amplifier ground, so you're relying on the power supply ground, but what ground is that? If you're using a two-prong wall wart, the output ground is very likely going to be isolated from the AC neutral through the switching transformer for safety reasons. Since this isolation isn't intended to isolate RF, you might get some effective grounding at RF frequencies, but it's hard to say.

Consider attaching a "real" ground directly to the LC tank. Traditionally for old crystal sets, you either ran the wire out the window and into a ground rod, or you at least alligator clipped it to all metal plumbing. If you're worried about ground loops, you might be able to put the RF ground in series with a midsized bypass capacitor, say 0.1uF. If you're in a location where it's not practical to connect a real ground, a long piece of wire strung along the ground could work well, too.

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  • \$\begingroup\$ This is very helpful. I was thinking it was FM, but makes more sense that it's an AM circuit. I got a TinySA and from what I can tell even though it can generate a AM signal there is no tone or audio signal sent. I'm actually traveling tomorrow and will take the kit with my to try back in the college town, hopefully the station is still up being AM. I did a radio search there is 1350kHz station in that area versus where I am now the closest frequency is 1010kHz. I'll have to order components to retune for the area. \$\endgroup\$
    – tlab
    Apr 17 at 22:38
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    \$\begingroup\$ I added the 160pF cap and when I plot Vout it's peaking around 1275kHz. I'm guessing this would pick up the 1350kHz like you are suggesting. I will update this when I get it all working or tested out. \$\endgroup\$
    – tlab
    Apr 17 at 23:02

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