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a strong radio signal

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mstechca

New Member
I am trying to make an FM radio that allows me to select a station and listen to it clearly using only transistors, capacitors, resistors, diodes, antenna, inductor, and a speaker. I want to connect the inductor and the capacitor in parallel so that I can easily select a station.

So far, I only managed to pick up one station with the receiver I got. There are three stations in my community.

Is there a way I can use a potentiometer instead of a variable capacitor for tuning?

I also read in a book that transistor amplifiers produce a frequency that depends on the resistance from the base (of the NPN) to the power source and the coupling capacitor.

What happens to the output frequency if I join two R-C coupled transistor amplifiers together? Does the frequency get added or multiplied?

I want this radio of mine to work.

By the way, my antenna is a 1M coaxial cable. Most of it is shielded except for one inch at the end.
 

kinjalgp

Active Member
1) You can't tune a radio using variable resistor because the Q-factor of the R-C circuit is very poor which results in poor selectivity.
2) When you cascade two "SIMILAR" RC coupled amplifiers, the frequency does not change but the gain is multiplied. IF AV1 and AV2 are respective gains of 2 stages then, Total gain AV = AV1 * AV2.
 

john1

Active Member
You could use a rotary switch to choose
different capacitors,
Or,
you could try a move-able insert in the coil(s),
some radios do that to tune.

If you intend to make an FM reciever,
i think you will need more than one inductor.

Two speakers are common because most FM stations
transmit in stereo.
 

Chippie

Member
what happened to the use of vari-cap diodes......? 8)


voltage controlled to give change in capacitance.......BB105's etc....
 

john1

Active Member
Hi chippie,

Mstechca has given a list of components he wants to use,
in his post.
Quote:
"using only transistors, capacitors, resistors, diodes, antenna, inductor, and a speaker.

I dont know why he has limited himself to these components,
but my guess is that by using discrete components, he
expects to learn more about circuit behaviour than he would
by using a silicon chip that someone else has designed.

Yes, maybe varicap diodes could have been included,
in fact that might be an easy way to tune it.

Maybe he is restricted to the parts available.

With the parts he has listed, i would have gone for a
regenerative AM first stage, and audio amp to speaker,
with no tuning cap i would have some switched preset stations.

With other inductors available, maybe better circuits
come to mind, but with just one i think he'll be stuck.

John
 

mstechca

New Member
I dont know why he has limited himself to these components,
but my guess is that by using discrete components, he
expects to learn more about circuit behaviour than he would
by using a silicon chip that someone else has designed.
You got that right!

Yes, maybe varicap diodes could have been included,
in fact that might be an easy way to tune it.
is a varicap diode a capacitor and a diode in series?

Maybe he is restricted to the parts available.
That is partially right.

With the parts he has listed, i would have gone for a
regenerative AM first stage, and audio amp to speaker,
with no tuning cap i would have some switched preset stations.
How is this done? I am basically between a new and an average electronics user.

With other inductors available, maybe better circuits
come to mind, but with just one i think he'll be stuck.
I looked on the internet and a few sources claim that the resonant frequency = 1/ (2*pi*sqr(L*C)). sqr is square root. Why will I need more inductors? I have alot of ceramic disc capacitors ranging from 1pf to about 0.1uF.
 

mstechca

New Member
here are my two circuits I plan to use. (see attachment)
 

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J0HN1

New Member
Hi mstechca,

That looks like a straightforward A.M. reciever to me.
It looks like basically a crystal set feeding an amplifier.

As a first build its ok.
The coil, if it is for medium wave ( i suspect it is)
would be around 100 turns of thin insulated wire,
on a cardboard tube of about one and a half inches diameter.

select the diode carefully,
it has to be a 'signal diode'
most diodes need a bit of forward voltage before conducting,
usually half a volt or a bit more,
but a 'signal diode' is only meant for very weak voltages
and will conduct forward at much lower voltages,
this is necessary because in this arrangement the voltages
are only from the power that comes in from the ariel.
Signal diodes should be kept separately from ordinary diodes,
they are usually made of clear glass (but not always)

As for that 1 nano-fahrad capacitor, ive seen this in many
circuits but ive never known it make any difference.
I would omit that, the wiring would probably be more than
that anyway.

The 1 MFD, make sure its not an electrolytic, and not leaky.

The 510 ohm? dunno what thats for ... miss it out.

Get a decent earth, and a decent ariel, and try it on an
amplifier that you know works to start with.

If you can get a trimmer, thats a small adjustable capacitor
then you should be able to tune something in, but dont
expect the tuning to be sharp, if you can get a station to
come in, and reject the rest thats about normal.

The amplifier you have drawn there may be suitable just
for making it work, but the linearity will be poor.
I will assume the output goes directly to a speaker.
The resistor in the emitter of the output transistor is
hardly necessary, but may have been included as a current
limiter, i would suggest a few ohms, maybe two or three
and a high value electrolytic (1000 ?)across it, or omit it.

The resistor on the base of the output transistor would
have to be a lower value than a collector load would
normally be, i would suggest using that 510 ohm resistor.

That leaves the two resistors on the base of the first
transistor. They call that arrangement a divider.
I suggest 800 to 1000 ohms from the first base to ground.
To see what value for the one from base to positive,
you will need the output load connected (the speaker)
and a voltmeter across it.

With no resistor from the input to positive, there should
be no reading on the voltmeter (ten volt range)

Start by touching a 1 meg from input to positive.
You should see a reading, maybe not much, it depends on
too many variables to guess what reading.

Aim for about half to a third of the battery voltage,
by trying lower values from input to positive.
starting at maybe 1 megohm and reducing a bit at a time.

that circuit is going to be a bit dependant on its
battery voltage, and its class A, a poor choice for
batteries because it uses the same current soft or loud.
Typical output circuits use less at low volume and
more at higher volumes.

With radio reception there is more involved than just
audio amplifying, the 'front end' thats the tuning part
is usually helped significantly before it gets to the
audio stages.

with this i would suggest a regenerative cicuit which
would mean some additional small windings on to the
tuning coil, and a few more components. That would
boost your signal and improve the selectivity a lot.

Its a lot to take in for a beginner, i hope you have
someone you can call on if you get stuck.

Best of luck with it,
John
 

mstechca

New Member
That answer was one of the best I have seen.

How do I make it regenrative? I plan to have multiple transistor amplifier stages connected together. I got only one FM station in. Three FM stations are within 5 km away from me.

I dont have any friends or relatives who know this well.

I know a voltmeter is very helpful, but unfortunately, I dont have one. Where can I borrow one or get one for free? or, is there an equation I can use to determine what resistor I can use between the base of the 1st transistor and the positive battery terminal? Equations save me money.
 

john1

Active Member
Hi mstechca,

Thank you for your kind words.
However, it seems i have got it wrong,
you say these are F.M. stations.

I had thought that might just be an error,
and when i saw the RF circuit, i still thought
that they must be A.M.

You see F.M. is transmitted differently to A.M.
The arrangement you have drawn is for A.M.
I am told that an A.M. reciever can recieve a rough
version of F.M. if its held slightly off-tune,
thats cos the AM circuit will respond on one side.

However the single tuned cicuit you have shown,
would in my opinion have such poor selectivity that
i dont think it could recieve F.M. in the proper way
at all.

Yet you say you have reception from one of these F.M.
stations?
You say ..."with the reciever I got"

Is that with an F.M. reciever?
I guess it must be.

You will have to alter that front end to recieve FM,
in my opinion.

Also you would be looking at 80 to 110 megacycles
roughly.
Working with tuned circuits at these frequencies is
rather specialised, and to get the high selectivity
several stages of RF(IF) amplification are used.

A fairly high degree of selectivity is needed in a
standard FM reciever because the reciever is tuned
to either side of the station, and the response from
each is used, as the station frequency moves toward
one of the sides, it also moves away from the other.

with a slight increase in frequency from the station,
the higher one responds more, and the lower one
responds less,

with a slight decrease in frequency from the station,
the lower one responds more, and the higher one
responds less.

The circuits used as 'detectors' with FM are called
discriminators i think.

Unfortunately ive never tried to use regeneration
above about 6 megacycles, i have a feeling that reaction
depends in some way opon the amount of energy that is
held in the ariel and first stage.

With the tiny capacitances and tiny amounts of micro
henries used in the RF reception at those frequencies
it is quite possible that regeneration can not be used.

But this is not really my field, others will have to
advise you about that.

If you want to build an FM front end, i think you will
need to include transistors in the RF stages,
i dont think there is an equivalent of the 'Crystal Set'
for F.M.

As for the meter,
well you will need one.
The resistor on the input to positive really cant be
calculated.
My guess would be in the area of 300k, but you have
to choose it, by looking at the output voltage, start
high and come down a bit at a time.
To do this sort of thing you will need many resistors
to play with. My personal collection is many cardboard
boxes-full, as im sure others are too.

An AM reciever, as i had thought this was, would be a
more traditional beginners project.

Ive never built an FM reciever, and tuning double IF
cans still 'phases' me, and i have the right gear!

Im sorry if this has assumed a bit of a negative
aspect, maybe you would consider a medium wave
reciever instead?
Unless other members have any suggestions ... ?

But you will still need a meter ....

Regards John
 

john1

Active Member
Cheers Pilot,

I didnt know regeneration could be used at the higher
frequencies, obviously it can!
I thought that 'Low Tech FM page' was most interesting.
Especially the tuned cavities pictures!

Regards, John
 

mstechca

New Member
The radio circuits you have shown will not work well. 1) I only have BJT transistors (NPN and PNP). 2) I dont have the diode required in the schematic. 3) The voltage they use is different. I want to use 5 volts because I want to turn this radio into a packet radio modem which I can use for my computer.
 

mstechca

New Member
I have been working on my radio. The good news is that the amplifier works nicely. When it is used as an AM radio, the maximum volume is a little bit above normal volume. In other words, the signal comes in loudly.

The bad news is that I can't get FM in perfectly. I want to know where I can get the best tank circuit or crystal circuit so that I can have a working FM radio. If I can get a loud AM radio, I should be able to get an FM radio (with more than one station) at least 1/2 the volume of AM.

What I have been getting is radio noise, and metronome sounds at the speed of at least 4 beats a second.
 

stevez

Active Member
I admire your persistence. As suggested in an earlier post, FM is different than AM in terms of how the audio is derived from the received signal. As much as 15 years ago I purchased a book by Texas Instruments, sold at the time via Radio Shack, that did an excellent job of explaining many concepts related to electronics including AM and FM transmission and reception. I would give you the book but I've already given it away. If someone could point out a similar book or send one your way it might help you in your endeavor. You still won't have all the answers but the help that others offer here might be more useful to you.

Unless you build a receiver with multiple stages you are not likely to get the performance you need. It does sound like you are and will continue to learn much from your effort. If you could explain a little more about what you are trying to do we might be able to point you in the right direction.

FYI - all diodes exhibit some capacitance, the capacitance changes with the voltage across the diode, varactors are diodes that are made with a greater and more predictable amount of change.

FYI-#2 - they do make/sell transmitter and receiver modules for a few dollars each that trans/receive at around 432 mHz that are used for short distance data transmission. MCM Electronics used to have them. I've not used them but they appear to be simple/low cost. You would then use the electronics components that you have for interfacing with whatever it is that you are doing.

Again, good luck and keep at it.
 

mstechca

New Member
So far, I have three cascaded amplifiers (see circuit on one of my previous posts in this thread) ganged together. I managed to pick up some AM stations (with distortion) without an antenna.

Here is what I am trying to do:
I want to make an FM radio that can pick up at least two stations in clearly. with the least about of radio interference.

I am not going to buy a radio kit because a) I can't afford it, b) It is usually more expensive than building your own receiver, c) The kit takes up more room space than if I built my own receiver. d) The only way to customize a number of features in the unit is to desolder components which wastes time. e) The kit may require a special adapter. I dont want to use any special adapters.

Once I get FM working, then I will turn my radio into packet radio and use it as a wireless modem.

I am tempted to use every single transistor I have (about 30) into the receiver alone.
 

Richard Lim

New Member
About Strip Antenna For CAR Radio

Instead of conventional CAR Radio using a long dipole antenna.
This causes a lot of problems whenever when you drive u have to raise the antenna upwards to gain better reception. Else poor reception.

My idea of using a strip antenna can be mounted at the front windscreen making use of the car chasis as ground. How can we get a better reception here.

Please feedback any comments.

Bgrds,
Richard Lim
 
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