question about crystal radios

[alpineranger]

I'm wondering why they work. This is why I'm confused:
-A basic crystal set will use a simple LC resonator to tune the signal, and a one diode half wave rectifier to demodulate. The demodulator obviously needs a well filtered signal.
-North american am radio is roughly ~500 kHz to 1.6 Mhz, with 10 kHz channel spacing

My dumb calculation (inductor and capacitor in parallel, no resistance), I get something that looks like:
Z = 1/2*pi * (F/tau)
where tau = L/C
My apologies for any abuses of conventional notation.

Plugging in some numbers, it seems like this thing would totally suck as a turner (tau = 1Mhz, F = 500 kHz - 3db of attenuation!)

I must be wrong somewhere here because I've built these things in the past (albeit when I was like 10 years old) and they've worked.

 

[QuixoticOne]

I'm not going to go through your Z calculation since it is sort of incorrect at first glance and also irrelevant to the main principle involved.

Check out the definition of 'Q' or 'Quality factor' of a resonator. In general
Q = Center Frequency / 3dB Bandwidth
Q = Energy Gained per cycle / Energy Lost Per cycle
et. al.

http://en.wikipedia.org/wiki/Quality_factor

Basically if you have a parallel LC resonator they'll tune at the frequency where X(L) = - X(C).

If you have high quality inductors and capacitors with little loss factor and little parasitic resistance in their connections they are said to have high quality factor.

If you place a resistive 'load' across the LC tank you load the tank and reduce the Q factor which makes the resonance response more broad and less sharply defined / peaked at its center frequency. Thus for maximum frequency selectivity you use a high impedance load on your tuned circuit so it will still have a high Q factor and narrow band operation.

At frequencies under 1MHz Q factors with LC circuits in the several thousands are not infeasible with good quality components and high resistance loads

At higher frequencies you can use things like tuned line resonators or cavities to get Q factors in the ten thousand to one hundred thousand range. Similarly quartz crystal resonators can have Qs in the tens of thousands.

Beyond the selectivity given by a single LC circuit, it is not uncommon to have circuits of cascaded stages of LC -> buffer amplifier -> LC -> buffer amplifier -> LC -> buffer amplifier where your multiple stages of selectivity cascade to further increase the selectivity; having sharply tuned IF amplifiers and audio filters also helps to accomplish that goal.

In the simplest case of AM broadcast band operation, even 10 kHz channel bandwidth at 1600 kHz carrier frequency gives around a 1/160 effective Q needed to receive that channel and significantly attenuate adjacent ones, which isn't so much.

In a primitive LC + diode detector radio you'll only be sensitve to quite strong signals, so even getting 3dB loss of the next channel over will make that signal remarkably weak in comparision to the already weak audio on the primary channel.

Also you seem to be not taking into account that it is not common practice for strong channels that are minimally adjacent in frequency to be allocated to stations in the same geographic area, this even if you could only well discriminate 20kHz, 30kHz, 50kHz differences, you'd still have some added ability to receive only the channel of interest simply because there likely wouldn't be active channels at comparable signal strengths nearby for several channel bandwidths up or down from your center frequency.

 

[alpineranger]

Thanks. I did learn about quality factor in school years ago but I never use that stuff, so it totally escaped me. I also assumed that one needed to consider the resistive load for impedance matching purposes, and didn't consider the possible effect on the tuning circuit. Also, I have no doubt that with enough circuitry one could fairly easily build a tuner with really good selectivity, but the question here was achieving acceptable performance with only a few.

BTW, here in LA the radio bands are pretty crowded. At least the FM spectrum is, can't say for AM because my car radio AM reception is broken.

 

[bobsmith1492]

I discovered years ago (I was maybe 12) that by hooking an earphone across a diode I could pick up the local AM station.

 

[Nathelion]

Heh my speakers do that... don't ask me how or why, but if I turn the volume way up I can listen to what must be a radio broadcast, some kind of we-are-going-to-sit-here-for-ten-hours-and-talk-about-how-bad-teenagers-are-these-days channel. Sound quality is quite good, too.

 

[Modelworks]

Crystal radio was the first project I ever built.
Local Ham operator, Mr.Hathaway, taught me when I was 12, about 25+ years ago.
He told me were going to make a radio from a cardboard toilet paper roll, some wire, a single diode, and a coffee can.

He got me hooked on electronics

Related trivia.
During WWII , soldiers used to take their razor blades and use them as a detector to listen to the bbc broadcast.

 

[bobsmith1492]

Originally posted by: Modelworks
Crystal radio was the first project I ever built.
Local Ham operator, Mr.Hathaway, taught me when I was 12, about 25+ years ago.
He told me were going to make a radio from a cardboard toilet paper roll, some wire, a single diode, and a coffee can.

He got me hooked on electronics

Related trivia.
During WWII , soldiers used to take their razor blades and use them as a detector to listen to the bbc broadcast.

Ok, now you have to explain! Razor blades? I can see the other radio, but... razor blades?

I got my ham license (technician, KB8ZEU) when I was 9 but was already hooked on electronics. I didn't use it much, though; didn't really see the point. All you could do was talk with people...

 

[Modelworks]

Originally posted by: bobsmith1492

Originally posted by: Modelworks
Crystal radio was the first project I ever built.
Local Ham operator, Mr.Hathaway, taught me when I was 12, about 25+ years ago.
He told me were going to make a radio from a cardboard toilet paper roll, some wire, a single diode, and a coffee can.

He got me hooked on electronics

Related trivia.
During WWII , soldiers used to take their razor blades and use them as a detector to listen to the bbc broadcast.

Ok, now you have to explain! Razor blades? I can see the other radio, but... razor blades?

I got my ham license (technician, KB8ZEU) when I was 9 but was already hooked on electronics. I didn't use it much, though; didn't really see the point. All you could do was talk with people...

I haven't built it myself, but the theory is sound.
http://bizarrelabs.com/rtfox1.htm

One of the things that makes television and radio interesting is that you can use right away the information you have learned.
We are going to build a radio that is different from any you have seen. In the radio will be these things:

An old safety razor blade
A safety pin
A broken point from a lead pencil
Radios of this kind were actually used during World War II. American G.I.'s in Italy put several of them together. At night when they sat near the front lines, they listened to phonograph records played on a radio station in Rome. That is why the set you are going to make is called "The Foxhole Radio." If you live within twenty-five or thirty miles of a station, you can hear a program on your foxhole radio, too.
These are the only tools you need to build the foxhole radio:
A hammer
A pair of pliers
A pocket knife
These are the parts that you will need:
A board. Any piece of wood will do. Just make sure it is at least eight inches long and six inches wide. It can be bigger if you like.
A piece of cardboard tubing. This should be two inches in diameter, like the tubes on which ribbon is wrapped in five-and-ten-cent stores. The tube should be six inches long.
A spool of insulated copper wire. You may have to buy this at a radio store. Ask for No. 28 gauge; this is just the right size. The wire will cost about 75 cents.
A pair of earphones which you can put over your head like the pilot of a jet airplane. The phones can be bought at a radio store for two or three dollars.
Three new nails. The nails can be either an inch or two inches long. Anything you have will do.
Four metal thumbtacks, not plastic tacks
A used blade that fits a safety razor. A plain white-looking blade often works better than "blue" blades.
A big safety pin
A pencil with a fat lead
Let's put all the tools and parts on a table like this and see if we have everything before we begin:

More with pics on the webpage