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FM Crystal Radio

M

Mike

Jan 1, 1970
0
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:
http://solomonsmusic.net/FM_CrystalRadio.html

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.
 
B

Bob Masta

Jan 1, 1970
0
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:
http://solomonsmusic.net/FM_CrystalRadio.html

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
www.daqarta.com
Scope, Spectrum, Spectrogram, FREE Signal Generator
Science with your sound card!
 
D

Don Bowey

Jan 1, 1970
0
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
www.daqarta.com
Scope, Spectrum, Spectrogram, FREE Signal Generator
Science with your sound card!

I agree with you; it is a slope detector.
 
M

Michael Black

Jan 1, 1970
0
Bob said:
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.
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
 
D

Dave.H

Jan 1, 1970
0
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.
 
D

Dave.H

Jan 1, 1970
0
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.
 
D

Don Bowey

Jan 1, 1970
0
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

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.
 
D

Don Bowey

Jan 1, 1970
0
Maybe I should just stick with AM crystal radios.

Or build an AM/FM crystal radio. It doesn't need to be great - have fun.
 
D

Dave.H

Jan 1, 1970
0
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.
 
M

Mike

Jan 1, 1970
0
Aww no, Play with it and hve some 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.

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.
 
D

Dave.H

Jan 1, 1970
0
<snip>>> > Maybe I should just stick with AM crystal radios.

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.

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.
 
E

ehsjr

Jan 1, 1970
0
Dave.H said:
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.

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
 
D

Don Bowey

Jan 1, 1970
0
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.


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.
 
D

Dave.H

Jan 1, 1970
0
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.

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: http://s222.photobucket.com/albums/dd237/ozguy89/?action=view&current=IMG_6405.jpg
 
M

Mike

Jan 1, 1970
0
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: http://s222.photobucket.com/albums/dd237/ozguy89/?action=view&current=IMG_6405.jpg

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 http://www.ittc.ku.edu/~jstiles/622/handouts/Superhet Tuning.pdf if you care.

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. http://www.techlib.com/electronics/crystal.html
Note how much smaller the unused section of the tuning cap is in the photo. The author gives lots of good info
& tips the beginner. The improvments later on the page might be fun to play with after you get it working.

Mike
If there is no absolute truth then nothing can be known.
 
D

Dave.H

Jan 1, 1970
0
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 http://www.ittc.ku.edu/~jstiles/622/handouts/Superhet Tuning.pdfif you care.

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.http://www.techlib.com/electronics/crystal.html
Note how much smaller the unused section of the tuning cap is in the photo. The author gives lots of good info
& tips the beginner. The improvments later on the page might be fun to play with after you get it working.

Mike
If there is no absolute truth then nothing can be known.

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.
 
M

Mike

Jan 1, 1970
0
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)
 
E

ehsjr

Jan 1, 1970
0
Dave.H wrote:

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.

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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