Learning about FM receivers

[Charl]

Hello folks,

I would like to learn something about FM receivers. I already have
several schematics of some IC-less schematics - I truly want to get to
the basics of a receiver.

I have a schematic right in front of me of a "synchronisable
oscillator". What this thing does is mix the amplified FM antenna
input with the signal of a local oscillator.

However, I don't understand much of it. For instance, the antenna is
wired to the collector of an NPN, and not to the base. Also, how do
they make sure that the signal from the local osc isn't "out of sync"
with the antenna input?

I'd be happy to reproduce the schematic in Eagle if this will be
helpful in assisting me.

Happy holidays,
-- Charl

 

[Brian Gregory [UK]]

"Charl" <charl@home.nl> wrote in message
news:567d7ec3.0312271503.3a97848e@posting.google.com...

I have a schematic right in front of me of a "synchronisable
oscillator". What this thing does is mix the amplified FM antenna
input with the signal of a local oscillator.

This doesn't sound like part of a normal FM receiver.

It might be some sort of mickey mouse untra simple very poorly
performing FM receiver.

--

Brian Gregory (In the UK).
bg@catlover.com

 

[Charl]

This doesn't sound like part of a normal FM receiver.

It might be some sort of mickey mouse untra simple very poorly
performing FM receiver.

Yes, possibly, but it was published in Elektor so I do assume it
works. And since it works AND it is simple, I figure it'd be a good
way to learn about it.

If you could point me to another IC-less schematic with a lot of
information to go with it, I will dump this one right away.

Greetings,
-- Charl

 

[Quazar]

On 27 Dec 2003 15:03:01 -0800, charl@home.nl (Charl) wrote:

Hello folks,

I would like to learn something about FM receivers. I already have
several schematics of some IC-less schematics - I truly want to get to
the basics of a receiver.

I have a schematic right in front of me of a "synchronisable
oscillator". What this thing does is mix the amplified FM antenna
input with the signal of a local oscillator.

Ok, I might know what your talking about. This is a very simple
receiver that simply is a oscillator and maybe a detector that is
simply mixed over the incoming signal, notching the frequency out.

Will have poor performance.

I would suggest looking into superheterodyne radios; I can make a dual
conversion superheterodyne work just as good as the one in your car
and sometimes better if I add two antenna filter stages instead of
one. This is what all manufacturers use today.

However, I don't understand much of it. For instance, the antenna is
wired to the collector of an NPN, and not to the base. Also, how do
they make sure that the signal from the local osc isn't "out of sync"
with the antenna input?

This is called "Tracking Accuracy", you have to combine two with a
dual variable capacitor or "easier" use two NTE618 variable
capacitance diodes and use potentiometers to get them in sync.

I'd be happy to reproduce the schematic in Eagle if this will be
helpful in assisting me.

I would start by making an AM radio if you are learning, the main
difference between AM and FM is the detector. The main reason I'm
telling you this is because the lower frequencies are easier to deal
with when your learning.

A run of the mill, one transistor audio pre amp will amplify low
frequency AM broadcast band.

Here is a good training kit based on transistors to get you started -
http://electronickits.com/kit/complete/radi/ck302.htm

You really need a 20Mhz oscilloscope and a signal generator to do this
properly, you need to be able to see what is going on inside the
receiver in order to learn enough to build a high quality receiver on
your own.

Even with experience this becomes necessary; I need to be able to see.

Most AM superhets have these stages
----------------------------------------------------------------------
Audio amp (amplifies audio)

detector and AGC (removes the oscillation and leaves only the audio;
AGC auto levels the signal so that your radio is sensitive enough to
pick up the lower signal frequencies, but won't overload on higher
ones. -- I Test radio with out AGC first)

IF 1 (notch filter specific to one frequency; usually 455Khz for bands
below 20 MHz, 10.7 MHz for Bands above

IF 2 (Exact same as above; helps filter even better add more IFs for
even more selectivity)

oscillator (Bounces the desired station to the IF frequency; if this
is 1.925 MHz the tuned station is 1470. 1925KHz - 455KHz is 1470Khz)

mixer (All this does is mix the local oscillator and the antenna
signals together.)

RF AMP (Amplifies the RF signal)

antenna filter (If you want your radio to work as good as one you can
buy add this, but it doesn't have to have it; gets rid of most of the
images. I Test the radio with out it first; this is also a frequency
specific notch filter that notches over the baud cast band.)
------------------------------------------------------------------

Happy holidays,
-- Charl

Same, good luck, hope this helps.

 

[Charl]

Hey,

I would start by making an AM radio if you are learning, the main
difference between AM and FM is the detector. The main reason I'm
telling you this is because the lower frequencies are easier to deal
with when your learning.

Yeah, I did realise that AM would be easier, but since it's totally
different from FM and since I have cable which is FM, I decided to go
for FM anyway.

Here is a good training kit based on transistors to get you started -
http://electronickits.com/kit/complete/radi/ck302.htm

Thanks a lot! The PDF with the schematic has been a great help in
understanding FM. I'm sure it'll also be a lot of help in actually
building the damn thing

You really need a 20Mhz oscilloscope and a signal generator to do this
properly, you need to be able to see what is going on inside the
receiver in order to learn enough to build a high quality receiver on
your own.

Yeah, I have a 10MHz scope and I'm definately going to build an HF
generator after reading the PDF from that address you gave me.

When I finish this radio I'll be sure to write an article about it and
publish it on the net.

Cheers!
-- Charl

 

[nemo]

Brian Gregory [UK] <bg@catlover.com> wrote in message
news:3fee12d6$0$13349$cc9e4d1f@news-text.dial.pipex.com...

"Charl" <charl@home.nl> wrote in message
news:567d7ec3.0312271503.3a97848e@posting.google.com...
I have a schematic right in front of me of a "synchronisable
oscillator". What this thing does is mix the amplified FM antenna
input with the signal of a local oscillator.

This doesn't sound like part of a normal FM receiver.

It might be some sort of mickey mouse untra simple very poorly
performing FM receiver.

--

Brian Gregory (In the UK).
bg@catlover.com

Newbie here - to the group - not to electronics.

Sounds like he's got hold of what used to be called a Homodyne rx. Local
Oscillator same as received frequency and a synchronous detector. Can't
imagine how it'd work with FM though. Rather puzzling.

Nemo

 

[Quazar]

On Wed, 31 Dec 2003 21:01:03 -0000, "nemo" <nemo@naughtylass2.wet>
wrote:

Sounds like he's got hold of what used to be called a Homodyne rx. Local
Oscillator same as received frequency and a synchronous detector. Can't
imagine how it'd work with FM though. Rather puzzling.

Nemo

This guy made a FM receiver with one transistor

http://www.somerset.net/arm/fm_only_lowtech.html

 

[nemo]

Quazar <NOSPAM4ME0@Bots.com> wrote in message
news:cif7vv0126ehhf0n3ceu3mgrn5901gtagt@4ax.com...

On Wed, 31 Dec 2003 21:01:03 -0000, "nemo" <nemo@naughtylass2.wet
wrote:


Sounds like he's got hold of what used to be called a Homodyne rx. Local
Oscillator same as received frequency and a synchronous detector. Can't
imagine how it'd work with FM though. Rather puzzling.

Nemo


This guy made a FM receiver with one transistor

http://www.somerset.net/arm/fm_only_lowtech.html

That's nothing. I made a wireless with just two wires. I took one away and

ended up with a wire less.

They don't write 'em like that any more. They wouldn't dare!! )