FM radio receiver

bananasiong · Oct 13, 2006

Hi,
I was told that it is very difficult to make a radio. Even I open the radio, the IC's inside cannot be found easily. But I've found a very simple circuit for FM receiver, no IC required, and only a single transistor. I'm not sure whether this transistor works or not, I'm going to try this out. Have you guys seen this before?

This is the original circuit:
**broken link removed**

And this is the audio gain improved:
**broken link removed**

Thanks

audioguru · Oct 13, 2006

It is a regenerative or super-regenerative simple radio receiver. Read about its problems in Google. It is too simple to be a real radio and is just a cheap child's toy. Another guy was in this forum and for hundreds of posts about it he couldn't make it work, except to pickup the sound from a powerful TV station very close to him.

bananasiong · Oct 13, 2006

Okay, Thank you so much.. It's pointless if I make a radio which can pick up the tv sound..

anyway, what is a rfc? I've seen this in VHF module. what's the different between this and the inductor of the same value?

audioguru · Oct 13, 2006

An rfc is an inductor. Its function is to choke radio frequencies, so it is called a Radio Frequency Choke.

stevez · Oct 13, 2006

I do not think it is pointless to construct such a radio - if the point is to learn. It is not a replacement for a good design that has been well constructed and the performance will be poor at best. It is fair to say that this kind of radio represents what many used in the early days of radio. During those early days there was far less RF junk. The full attention of a skilled operator might improve reception but there are limits.

I do not think that the construction of a good radio is within the capabilities of a beginner or even most advanced hobbyists - if "good radio" is defined as what might be commercially available today. Some can do it but not a large portion of that group.

bananasiong · Oct 13, 2006

stevez said:
I do not think it is pointless to construct such a radio - if the point is to learn. It is not a replacement for a good design that has been well constructed and the performance will be poor at best.

I meaned, if it only picks up the signal from tv station but not radio station. But it's good to try it. I'm going to try since it is not a complicated circuit.

audioguru said:
An rfc is an inductor. Its function is to choke radio frequencies, so it is called a Radio Frequency Choke.

Can I use a normal inductor?

audioguru · Oct 13, 2006

bananasiong said:
I meaned, if it only picks up the signal from tv station but not radio station. But it's good to try it. I'm going to try since it is not a complicated circuit.

It has a variable capacitor for you to tune the frequency it receives. It will pickup airplane transmissions, FM radio or TV audio if they are strong enough. It will be overloaded and stop working if they are too strong.

Can I use a normal inductor?

Yes. A 22uH rfc is a normal inductor.

bananasiong · Oct 14, 2006

I've seen inside the radio, the tuner consists of 4 and some 2 variable capacitors inside. Can I use 2 (or 4) single variable capacitors to do perform the same job? For this type of capacitor, every capacitor in it has to be tuned?

What is a high impedance speaker? As I know, many speakers are rated as low impedance, less than 10 Ohm.

Can you tell me which part of the circuit is tank circuit? Are they C1A, C1B, C3A, C3B and L1?

audioguru · Oct 14, 2006

The circuit you found uses two variable capacitors in series with two 10pF capacitors. They can all be replaced with a single variable capacitor, about 5pF to 40pF.

You should replace the TL431 circuit with a real audio power amplifier IC like an LM386 that can drive a real low impedance (8 ohms) speaker.

The "tank" is the coil and the variable capacitance in parallel with it. At 100MHz, stray wiring capacitance adds to the total capacitance.

bananasiong · Oct 14, 2006

audioguru said:
The circuit you found uses two variable capacitors in series with two 10pF capacitors. They can all be replaced with a single variable capacitor, about 5pF to 40pF.

Yes, thank you.

You should replace the TL431 circuit with a real audio power amplifier IC like an LM386 that can drive a real low impedance (8 ohms) speaker.

Can I know what is a high impedance speaker? How to modify it with LM386? I thought the improved circuit can drive a low impedance speaker already?

The "tank" is the coil and the variable capacitance in parallel with it. At 100MHz, stray wiring capacitance adds to the total capacitance.

what is called stray wiring capacitance?

I can replace 2N4416 with BF256 right?

audioguru · Oct 14, 2006

bananasiong said:
Can I know what is a high impedance speaker?

Speakers are not high impedance. They are usually 8 ohms.

How to modify it with LM386?

Replace the TL431 circuit with an LM386 power amp IC. It can drive an 8 ohm speaker.

I thought the improved circuit can drive a low impedance speaker already?

A TL431 is a power supply part, not a power amp IC. It can drive an earphone, not a speaker. The schematic says a small speaker. How small? Like an earphone?

what is called stray wiring capacitance?

Stray capacitance is the capacitance between wires and pcb traces and if fairly short can be as high as about 10pF. Longer wires have more stray capacitance between them.

I can replace 2N4416 with BF256 right?

The BF256 has a very wide range of spec's. Some of them might work. Others will be cutoff and others will be saturated.

bananasiong · Oct 15, 2006

audioguru said:
Replace the TL431 circuit with an LM386 power amp IC. It can drive an 8 ohm speaker.

the -ve input connect as the anode, +ve input as ref, and output as cathode? The rest of the pins? Vs to Vcc and GND to ground? I really don't know about these.

What is the voltage to the earphone? Very low?

I've just bought some variable capacitors, they have 3 pins, are their functions like variable resistor? Short the center with one end?
Let's say I have 10pF variable capacitor, can I add another 10pF capacitor in parallel so that they give variable value from 10 to 20pF?

Hero999 · Oct 15, 2006

Look on the datasheet of the LM386 and it will tell you how to connect it up.

bananasiong · Oct 15, 2006

Should I choose amplifier with the gain of 20? Should R1 and R5 be removed?

What is the voltage to the earphone? Very low?

My variable capacitors have 3 pins, are their functions like variable resistor? Short the center with one end?
Let's say I have 10pF variable capacitor, can I add another 10pF capacitor in parallel so that they give variable value from 10 to 20pF?

My frc has only 3 color code, the last one is 10 to the power of -ve?

Thanks

audioguru · Oct 15, 2006

bananasiong said:
Should I choose amplifier with the gain of 20?

The datasheet for the TL431 shows a gain of about 300.
Use the LM386 with a gain of 200 and add a volume control. A radio is useless without a volume control.

Should R1 and R5 be removed?

Yes. They are for the TL431.

What is the voltage to the earphone? Very low?

Much less signal than goes to a speaker.

My variable capacitors have 3 pins, are their functions like variable resistor? Short the center with one end?

No, don't short it. Two pins are electrically the same and 3 are used to hold it up.

Let's say I have 10pF variable capacitor, can I add another 10pF capacitor in parallel so that they give variable value from 10 to 20pF?

If you can't get a variable capacitor with the correct value (online stores have hundreds to choose from) then forget about making a radio.

My frc has only 3 color code, the last one is 10 to the power of -ve?

Why don't you look on its datasheet?

bananasiong · Oct 15, 2006

audioguru said:
No, don't short it. Two pins are electrically the same and 3 are used to hold it up.

just use 2 pins to connect?

If you can't get a variable capacitor with the correct value (online stores have hundreds to choose from) then forget about making a radio.

I mean, is it possible to make them to have capacitance of 10pF to 20pF?
If I can't get the correct value, I can change the coil right?

If I use veroboard for tx or rx, the metal will be from one end to the other, so the stray wiring capacitance will be added up? Should I cut them to decrease the size?

Thanks

audioguru · Oct 15, 2006

bananasiong said:
just use 2 pins to connect?

Yes. Look at it or measure the pins with an ohm-meter and don't use both pins that are the same. Probably use one middle pin and one end pin.

I mean, is it possible to make them to have capacitance of 10pF to 20pF?
If I can't get the correct value, I can change the coil right?

Then you will need to have a signal generator to find out what frequencies it will tune.

If I use veroboard for tx or rx, the metal will be from one end to the other, so the stray wiring capacitance will be added up? Should I cut them to decrease the size?

I use Veroboard and only the ground metal is the whole length of the board. All the other metal strips are cut with a drill bit to a few holes long for one function then a cut and then a few more holes for another function. The metal strips for the signals must be very short for low stray capacitance.

Nigel Goodwin · Oct 16, 2006

For RF use I always completely remove any unused strips, or portions of strips, and (as Audioguru says), keep what you use as short as possible.

Hero999 · Oct 16, 2006

Another way to lower the inductance of tracks is to run a layer of solder over the top of them when you've fitted all the componants.

bananasiong · Oct 16, 2006

Solder on the unused metal on PCB? Why? I think cut short the size is easier for Veroboard.

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