Radio: Absolute Basics

[FyberOptic]

Howdy folks. I've been really interested in building a simple AM radio
for a long time, but I've always wanted to understand it rather than
just put together something that somebody else designed. I'm
particularly interested in eventually making one which can use a
battery, and not your typical crystal set which requires a long
antenna and earth ground. Problem is, things I find on the internet
tend to go from simplistic crystal sets to complicated amplified ones
(regenerative, super regenerative, super heterodyne, etc) without much
explanation on what does what. I'm no electronics guru by any
respect, so I get lost very quickly. Maybe one day I'll be able to
look at these things and they'll make total sense, but not today.

So far, I have an understanding of envelope detection, and I recently
grasped basically how a diode detector would work (smoothing out the
peaks of the carrier to form a smooth audio wave). But I can't seem
to find out exactly how one comes to the conclusion of what values of
capacitor and resistor one should use in the detector. I believe I
read that you choose a capacitor based on the maximum amplitude of the
waveform, but how would I go about finding out that value, even? And
then with that, calculating the right cap/resistor?

Wikipedia has some formulas for calculating something, but it's all
greek to me. If I knew what the symbols referenced, I might have some
chance of figuring it out, at least. http://en.wikipedia.org/wiki/Envelope_detector

Anyway, I would appreciate any info or reading that anyone can offer
to point a newbie like me in the right direction!

 

[BobG]

On May 24, 3:22 pm, FyberOptic <fyberop...@gmail.com> wrote:

Howdy folks. I've been really interested in building a simple AM radio
for a long time,
==========================================Think about sending a tone... Say 1khz. We know that the carrier must

be modulated (multiplied) by a modulating signal that goes from o% to
100%... so make the carrier 1mhz and make the modulation a 1khz square
wave. This would look like 500usec of 1mhz on, 500usec of nothing,
Match this to something that will drive an antenna. Should be able to
hear it on an AM receiver,

 

[default]

On Sun, 24 May 2009 12:59:40 -0700 (PDT), BobG <bobgardner@aol.com>
wrote:

On May 24, 3:22 pm, FyberOptic <fyberop...@gmail.com> wrote:
Howdy folks. I've been really interested in building a simple AM radio
for a long time,
===========================================
Think about sending a tone... Say 1khz. We know that the carrier must
be modulated (multiplied) by a modulating signal that goes from o% to
100%... so make the carrier 1mhz and make the modulation a 1khz square
wave. This would look like 500usec of 1mhz on, 500usec of nothing,
Match this to something that will drive an antenna. Should be able to
hear it on an AM receiver,

Yeah but . . . That's what you get when you try to go over 100%
modulation and it causes adjacent channel interference.
--

 

[default]

On Sun, 24 May 2009 12:22:00 -0700 (PDT), FyberOptic
<fyberoptic@gmail.com> wrote:

Howdy folks. I've been really interested in building a simple AM radio
for a long time, but I've always wanted to understand it rather than
just put together something that somebody else designed. I'm
particularly interested in eventually making one which can use a
battery, and not your typical crystal set which requires a long
antenna and earth ground. Problem is, things I find on the internet
tend to go from simplistic crystal sets to complicated amplified ones
(regenerative, super regenerative, super heterodyne, etc) without much
explanation on what does what. I'm no electronics guru by any
respect, so I get lost very quickly. Maybe one day I'll be able to
look at these things and they'll make total sense, but not today.

So far, I have an understanding of envelope detection, and I recently
grasped basically how a diode detector would work (smoothing out the
peaks of the carrier to form a smooth audio wave). But I can't seem
to find out exactly how one comes to the conclusion of what values of
capacitor and resistor one should use in the detector. I believe I
read that you choose a capacitor based on the maximum amplitude of the
waveform, but how would I go about finding out that value, even? And
then with that, calculating the right cap/resistor?

Wikipedia has some formulas for calculating something, but it's all
greek to me. If I knew what the symbols referenced, I might have some
chance of figuring it out, at least. http://en.wikipedia.org/wiki/Envelope_detector

Anyway, I would appreciate any info or reading that anyone can offer
to point a newbie like me in the right direction!

You don't really state a goal here. Is your aim to learn how to build
a superhet or learn some of the intermediate versions of AM detection
first.

A single tube/transistor regen or super regen or reflex receiver is
only a step up from a crystal set and isn't way more complicated like
you seem to indicate.

If you wanted simple you go from crystal detector to amplified crystal
detector, then to tuned radio frequency (amplify the RF with 1-3 tuned
stages before you go into the detector) then on to regen, reflex,
superhet (which you can get on a single chip these days with minimal
external parts).

Check out:
http://www.melektronikos.com/en/books/rrbook/chapter3/chapter3h.htm
Single active part TRF receiver

http://home.pacbell.net/lengal/ip/amregen.pdf
Here's a simple single transistor regen (leave out everything east of
the 4.7 UF cap and connect earphones from the cap to ground - or add
the amp and drive a loudspeaker).

http://www.qrp.pops.net/tf2.asp
Not significantly more complicated (if you leave out the audio amp
stage - again). And some good explanations.

--

 

[default]

On Mon, 25 May 2009 18:53:25 -0400, default <default@defaulter.net>
wrote:

http://home.pacbell.net/lengal/ip/amregen.pdf
Here's a simple single transistor regen (leave out everything east of
the 4.7 UF cap and connect earphones from the cap to ground - or add
the amp and drive a loudspeaker).

Oops - back in the day, we had earphones with a mile or two of very
fine wire in them with a 2K impedance - today's 8-40 ohm "ear buds"
are probably not going to cut it - a crystal earphone may still be
available, and work however.

I used to use tube matching (plate) transformers to drive low
impedance earphones and speakers but even those are scarce today.
Closest thing in production today are public address line matching
transformers (attached to the public address speakers throughout a
building to lower the 25 or 70 volt signal to something an 8 ohm
speaker can deal with)

Bite the bullet and build an audio amp (or buy one at Radio Shack) -
then experiment with "front ends" would be my recommendation.
--

 

[Jon Kirwan]

On Mon, 25 May 2009 19:03:22 -0400, default <default@defaulter.net>
wrote:

On Mon, 25 May 2009 18:53:25 -0400, default <default@defaulter.net
wrote:

http://home.pacbell.net/lengal/ip/amregen.pdf
Here's a simple single transistor regen (leave out everything east of
the 4.7 UF cap and connect earphones from the cap to ground - or add
the amp and drive a loudspeaker).

Oops - back in the day, we had earphones with a mile or two of very
fine wire in them with a 2K impedance - today's 8-40 ohm "ear buds"
are probably not going to cut it - a crystal earphone may still be
available, and work however.
snip

Yes. I dearly remember those headsets. I actually built one that
kind of worked okay, too. But even Radio Shack had the old style
earphones. These days, I suspect that they set you back an arm and a
leg when you find them.

I'm kind of sad to see their ready availability go away.

Jon

 

[default]

On Tue, 26 May 2009 02:58:25 GMT, Jon Kirwan
<jonk@infinitefactors.org> wrote:

On Mon, 25 May 2009 19:03:22 -0400, default <default@defaulter.net
wrote:

On Mon, 25 May 2009 18:53:25 -0400, default <default@defaulter.net
wrote:

http://home.pacbell.net/lengal/ip/amregen.pdf
Here's a simple single transistor regen (leave out everything east of
the 4.7 UF cap and connect earphones from the cap to ground - or add
the amp and drive a loudspeaker).

Oops - back in the day, we had earphones with a mile or two of very
fine wire in them with a 2K impedance - today's 8-40 ohm "ear buds"
are probably not going to cut it - a crystal earphone may still be
available, and work however.
snip

Yes. I dearly remember those headsets. I actually built one that
kind of worked okay, too. But even Radio Shack had the old style
earphones. These days, I suspect that they set you back an arm and a
leg when you find them.

I'm kind of sad to see their ready availability go away.

Jon

I know what you mean. A lot of neat stuff goes obsolete.

Found on ebay:
http://cgi.ebay.com/VINTAGE-C.F.-CANNON-ALNICO-NO.-15-MAGNETIC-HEADSET_W0QQitemZ310141627607QQcmdZViewItem

Old dual coil Cannon earphones. Looks older than the one I had. Mine
had chromed end caps with stamped manufacturer's data on them.

Only $12. uses the old pin terminals for fahnestock clips
--

 

[Baron]

default wrote:

On Mon, 25 May 2009 18:53:25 -0400, default <default@defaulter.net
wrote:

http://home.pacbell.net/lengal/ip/amregen.pdf
Here's a simple single transistor regen (leave out everything east of
the 4.7 UF cap and connect earphones from the cap to ground - or add
the amp and drive a loudspeaker).

Oops - back in the day, we had earphones with a mile or two of very
fine wire in them with a 2K impedance - today's 8-40 ohm "ear buds"
are probably not going to cut it - a crystal earphone may still be
available, and work however.

I used to use tube matching (plate) transformers to drive low
impedance earphones and speakers but even those are scarce today.
Closest thing in production today are public address line matching
transformers (attached to the public address speakers throughout a
building to lower the 25 or 70 volt signal to something an 8 ohm
speaker can deal with)

Bite the bullet and build an audio amp (or buy one at Radio Shack) -
then experiment with "front ends" would be my recommendation.
--

The transformer from an old mobile phone charger or wall wart 240v to
3.4v makes a reasonable audio transformer.

--
Best Regards:
Baron.

 

[default]

On Tue, 26 May 2009 20:11:13 +0100, Baron
<baron.nospam@linuxmaniac.nospam.net> wrote:

default wrote:

On Mon, 25 May 2009 18:53:25 -0400, default <default@defaulter.net
wrote:

http://home.pacbell.net/lengal/ip/amregen.pdf
Here's a simple single transistor regen (leave out everything east of
the 4.7 UF cap and connect earphones from the cap to ground - or add
the amp and drive a loudspeaker).

Oops - back in the day, we had earphones with a mile or two of very
fine wire in them with a 2K impedance - today's 8-40 ohm "ear buds"
are probably not going to cut it - a crystal earphone may still be
available, and work however.

I used to use tube matching (plate) transformers to drive low
impedance earphones and speakers but even those are scarce today.
Closest thing in production today are public address line matching
transformers (attached to the public address speakers throughout a
building to lower the 25 or 70 volt signal to something an 8 ohm
speaker can deal with)

Bite the bullet and build an audio amp (or buy one at Radio Shack) -
then experiment with "front ends" would be my recommendation.
--


The transformer from an old mobile phone charger or wall wart 240v to
3.4v makes a reasonable audio transformer.

Never occurred to me. I guess at the levels we are talking about
there's not much chance of ever saturating. Audio transformers had
all the E's on one side I's on the other with a paper spacer - mains
transformers use interleaved lamination stacks.
--

 

[Baron]

default wrote:

On Tue, 26 May 2009 20:11:13 +0100, Baron
baron.nospam@linuxmaniac.nospam.net> wrote:

default wrote:

On Mon, 25 May 2009 18:53:25 -0400, default <default@defaulter.net
wrote:

http://home.pacbell.net/lengal/ip/amregen.pdf
Here's a simple single transistor regen (leave out everything east
of the 4.7 UF cap and connect earphones from the cap to ground - or
add the amp and drive a loudspeaker).

Oops - back in the day, we had earphones with a mile or two of very
fine wire in them with a 2K impedance - today's 8-40 ohm "ear buds"
are probably not going to cut it - a crystal earphone may still be
available, and work however.

I used to use tube matching (plate) transformers to drive low
impedance earphones and speakers but even those are scarce today.
Closest thing in production today are public address line matching
transformers (attached to the public address speakers throughout a
building to lower the 25 or 70 volt signal to something an 8 ohm
speaker can deal with)

Bite the bullet and build an audio amp (or buy one at Radio Shack) -
then experiment with "front ends" would be my recommendation.
--


The transformer from an old mobile phone charger or wall wart 240v to
3.4v makes a reasonable audio transformer.

Never occurred to me. I guess at the levels we are talking about
there's not much chance of ever saturating. Audio transformers had
all the E's on one side I's on the other with a paper spacer - mains
transformers use interleaved lamination stacks.
--

The phrase "Well stocked junk box" springs to mind !

--
Best Regards:
Baron.