I'm about to build an FM crystal radio, but the schematic calls for a
50 pF variable capacitor, would a 2 section 233/86 pF capacitor work, if I only use the 86 pF section?Radio page:
Sure, I see no reason it won't work. Assuming that the cap you have has a terminal common to both sections, you could leave that terminal unused and use the 2 other terminals. That would result in about 63pf max capacitance. Stretch the coil out a bit or make it slightly small diameter and you should be able to get it to tune the FM band.
Mike If there is no absolute truth then nothing can be known.
OK, I just had to look at this circuit to see how a rectifier (the "crystal") could demodulate FM. But it looks pretty much like an ordinary AM circuit. So, what's really going on?
My best guess so far is that this relies somehow on the rectifier output dropping off as the incoming carrier frequency gets farther from the tuned frequency. But if you tuned to the nominal carrier frequency, and the modulator was a sine wave, I'd predict a horrible cuspy-looking mess from the output... certainly not the "high fidelity" that the Website claims.
So, maybe you tune off-channel, to the peaks of the deviation? Nope, just a different trashy signal, it would seem.
Anybody have the explanation?
Best regards,
Bob Masta DAQARTA v3.50 Data AcQuisition And Real-Time Analysis
I agree with you; it is a slope detector.
"Bob Masta" wrote
Slope detection.
See
It's not a "new" thing.
The concept is commonly referred to as "slope detection". Get the signal along the curve of the selectivity, so as the FM signal varies, the slope of the selectivity turns that frequency variation into amplitude variation, which the diode can then turn into audio.
It was used when you wanted to receive FM signals, and didn't have a receiver with an FM detector. But, of course you get no limiting that is the norm in pretty much any real FM receiver. It's also finicky, since you need selectivity that isn't nice and sharp, and it's no perfect system so recovered audio will be low in amplitude.
I used to do it with an Hammarlund SP-600 shortwave receiver, that had variable selectivity up to about 13KHz, and a converter to receiver VHF signals. It was never a great thing, had to fuss with tuning, low audio, noisy. A real FM receiver right next to it would meanwhile show a nice quiet signal on the same frequency.
Michael
Maybe I should just stick with AM crystal radios.
Maybe I should just stick with AM crystal radios.
In the 50s, narrow-band FM was permitted on the high freq ham bands, It was easily listened to without a NBFM converter by doing slope detection.
Or build an AM/FM crystal radio. It doesn't need to be great - have fun.
I have an old radio that I can steal the tuning cap from, so I don't have to look around for one of them. I couldn't even find an Australian source for them.
Aww no, Play with it and hve some fun.
Yeah, it's probably a 2 section cap, so use the smaller section with a series cap to lower the capacitance range for the FM band.
Use the larger section for the AM band and switch your earphone for AM/FM.
You'll have to experimnet with the coils to get in the bands, but that's fun too.
The AM will require a million meter antenna, well maybe not quite that long and decent ground connection. Don't forget, you need a high impedance (crystal) earphone for it to work right.
Mike
If there is no absolute truth then nothing can be known.
too.
earphone
It does look like a 2 section cap, even though the radio has only one band. What value capacitor do I need? It already has a ceramic disk connected that reads 82 or 8.2.
too.
earphone
You can figure out the size cap you need using math. For capacitors in series you can use either
1/Ct = 1/C1 + 1/C2 or Ct = (C1*C2)/(C1+C2)where Ct is the total capacitance you want. You know Ct = 50 and your C1 is 86, so you can stuff estimated values for C2 into the equation until you find the value you need.
When you put capacitors in series, the total will always be lower than the smallest cap. So if you put 86 pF in series with say 63 pF, the value *must* be lower than 63 pF. As it turns out, those two in series would give you about 36 pF.
Ed
The large section is the RF tuning cap and is about 365uuf fully engaged. That¹s good for the AM band.
The smaller section is used for the AM oscillator tuning; I don't recall it's value. It is too large for the FM band unless you remove all but one rotor plate, then it should be usable.
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The capacitor I just pulled from the radio (a 1960's 5 valve AM set) has two sections about the same size. Looking at it now it has 21215 stamped on the back (sounds like a serial number) and J C4 on the front Not sure if this is suitable for FM (probably not) but I'm looking towards building an AM set instead. Does anyone know of a good AM crystal set that would use this cap? Has to be easy to build, with no hard to get parts (well I've already got the tuning cap, that was the only hard to get part) Picture of the cap:
From that photo it looks like both sections are identical. Usually the cap from a superhet has one section with smaller and/or fewer plates. The smaller section was used for the "local oscillator" and since it almost always ran at 455KHz higher than the station being received, it had a lower value. See
Just about any germanium diode will work just fine. In a pinch even a schottky diode will work ok if you have a good long antenna. The only difference the variable cap will make with any crystal set is the tuning range, so you'll have to experiment with the coil to tune AM broadcast band. I would start with this simple crystal radio.
Mike If there is no absolute truth then nothing can be known.
cap:
from a superhet has one section
oscillator" and since it almost
value.
care.
diode will work ok if you have
crystal set is the tuning range,
radio.http://www.techlib.com/electronics/crystal.html
The author gives lots of good info
with after you get it working.
That's exactly the same circuit I was looking at. Uses a small diameter coil so I can build a set into a plastic project box. How many meters of wire would a need for the coil? I plan on winding it on a 4 inch long PVC pipe.
Well, let's see. 1.5" X Pi X 65 turns = 306" and 306/39.34 = 7.8M. Make it 9M to allow for the taps and lead wires. You'll have to plug in the actual OD of your PVC.
Mike
"The scientist is possessed by the sense of universal causation...His religious feeling takes the form of rapturous amazement at the harmony of natural law, which reveals the intelligence of such superiority that, compared with it, systematic thinking and acting of human beings is an utterly insignificant reflection." Albert Einstein (theoretical physicist)
Roughly 8 1/2 meters. There are two ways you can figure out how much:
1) Wrap 1 turn around the former. Then lay that turn out in a straight line and measure it. Multiply by the number of turns to get the total. Then add a bit more, as there are taps on the coil which use up some length, and the lead length at the ends of the coil. 2) Math - multiply the circumferance (pi*d) of the former by the number of turns. Then you add a bit for lead length, the fact that each turn will be slightly longer than the circumference since it is wound at a slight angle, length needed for the taps etc.At the cited url, the former is 1 1/2 inches, so don't forget to convert from inches to meters.
Ed
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