Help with FM Broadcast Radio receiver

Thread Starter

ke5nnt

Joined Mar 1, 2009
384
Ok Bill, I'll give it a try, and also I'll pick up some of those parts when I get a chance. I'd like to give a more complicated (better) radio circuit a try after this one. Something that might actually work like a normal radio. Any suggestions? I'm looking at you Audioguru, since you seem so keen on suggesting the best of the best.
 

Wendy

Joined Mar 24, 2008
23,415
Check Tanner's stock of variable caps very carefully. A lot of them are from radios.

Modern tech uses phase locked loops to synthesize frequencies to very high precision. Helps that they can custom design digital chips for the job.
 

Wendy

Joined Mar 24, 2008
23,415
You would do yourself a favor getting a small number of parts, such as a TL072, a JR6545, and a LM386 (total cost of all less than $2) next time you visit Tanner's. Op amps are an experimenters godsend, making everything from active filters to audio amps to super simple voltage regulators, both digital and analog. You could add a LM393 and a LM339 for another $1. Any one of the first three parts would have made are really good audio amp with relatively low distortion, for example.
Already did that. I know Tanner's stock intimately, they have no TL082's (empty bin), though they have TL084's. There is only a slight difference between them and the TL072's and TL084's.

The comparators were there.

:D
 

Wendy

Joined Mar 24, 2008
23,415
OK, responding to your PM, what do you need help with first? The LM386 is a prewired audio amp, it is one good way to go. Look up the data sheet for schematics.

If you want to be more adventurous we could develop an op amp circuit together. Been a while since I've tried this (again). I suspect I've gotten a lot better since the last time. Encouraging, ain't I? :D

I would try a regular protoboard with these project, they are in the frequency range a protoboard can handle.

You could go with the 4 transistor versions I showed earlier.

Pick one and we'll get started! BTW, think about the offer I sent you on my reply to the PM. Transistors really are cheap.

I had to dig 5 pages in the bone yard to find this bone, this is an active board.
 

Thread Starter

ke5nnt

Joined Mar 1, 2009
384
Yes Sir, quite active. I do appreciate your help.

Ok, I have read the Datasheet for the LM386. After looking it over, it brings some questions to mind that I need some clarification on.

I'm opting to bypass the internal 1.35KΩ resistor from pins 1 and 8 with a capacitor between the 2 pins for a gain of 200 (at least to start, I'll see how it sounds for this experiment). This whole section on Input biasing confuses me a bit. This section talks a lot about DC source resistance. What is DC source resistance and how do you find it?

Then it goes on to say that all offset problems are eliminated if the input is capacitively coupled. Can you explain this? Coupled to what?

On page 1 of the datasheet where it shows the schematic for Amplifier with gain = 200, pin 7 is connected to a capacitor that has no marked value. Is this the 0.1μf capacitor that is referenced in the input biasing section?

2 more questions about this same schematic. Both the capacitor connected between pins 1 and 8, and pin 5 and the speaker are shown as polarized capacitors but don't have the typical schematic symbol for an electrolytic (with one side curved). What kind of capacitors would you use here?

And finally, I'm not entirely sure how to read the schematic connection for pin 3. The pin shows an arrow pointing to the center of a 10K resistor. What actual connection is made here? I've never seen this before.

Ok, sorry to load you full of questions, but you are helping me tremendously and I'm learning quite a bit so thank you very much for your time.
 
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Audioguru

Joined Dec 20, 2007
11,248
Re: LM386 amplifier IC:
I'm opting to bypass the internal 1.35KΩ resistor from pins 1 and 8 with a capacitor between the 2 pins for a gain of 200 (at least to start, I'll see how it sounds for this experiment).
Then you should add a volume control to the input of the LM386.

This whole section on Input biasing confuses me a bit. This section talks a lot about DC source resistance. What is DC source resistance and how do you find it?
The volume control can be 10k to 100k ohms. The volume control will be in parallel with an internal 50k resistor to ground (they bias the input at ground) so they become the input DC source resistance. Your Super-Regen radio has an output level that depends on signal strength (because it is a simple AM radio without AGC) so the volume control at the input of the amplifier is important.

Then it goes on to say that all offset problems are eliminated if the input is capacitively coupled. Can you explain this? Coupled to what?
The input DC offset voltage is very small and can be ignored. You might need to use a coupling capacitor to feed the audio signal from the "radio" if it has DC (I can't remember if it has DC on its output).

On page 1 of the datasheet where it shows the schematic for Amplifier with gain = 200, pin 7 is connected to a capacitor that has no marked value. Is this the 0.1μf capacitor that is referenced in the input biasing section?
Pin 7 is not the input. Forget about the tiny input DC offset voltage that needs a cap and resistor at the input to eliminate it.
Pin 7 is a point in the amplifier that can be filtered if your supply voltage is very noisy (usually the capacitor to ground at pin 7 is not needed).

Both the capacitor connected between pins 1 and 8, and pin 5 and the speaker are shown as polarized capacitors but don't have the typical schematic symbol for an electrolytic (with one side curved). What kind of capacitors would you use here?
The 10uf capacitor between pin 1 and pin 8 of the IC is an electrolytic with pin 1 shown as the + wire of the capacitor.
The output capacitor is an electrolytic with pin 5 of the IC shown as its + wire.

I'm not entirely sure how to read the schematic connection for pin 3. The pin shows an arrow pointing to the center of a 10K resistor. What actual connection is made here? I've never seen this before.
The 10k resistor with the arrow at its center is a 10k volume control. The arrow is its slider.
 

Wendy

Joined Mar 24, 2008
23,415
I am not extremely familiar with a LM386, but something that remains true of most audio amplifiers (other types of amps too). The higher the power supply voltage the louder you can make them. Audio transformers can compensate in many ways, but it is a fundamental truth. The amount of P-P swing without distortion is dependent on the supply voltage.
 

sceadwian

Joined Jun 1, 2009
499
Absolutly true, up to the cooling limits of the setup. If you let the smoke out you won't get any audio out after that =) Thermal considerations for audio amplifiers of that like are so... forgotten it's sad.
 

Thread Starter

ke5nnt

Joined Mar 1, 2009
384
I think I'm going to opt instead to start with a gain of 20 and go up from there. I'm going to have to make a trip to the parts store again anyways regardless of what I have to do. Thank God I don't have to mail-order everything. I think I'd have quit a long time ago if that were the case.

Are there any fundamental parts I should definitely have in the parts box I can pick up while there? Spare me another 5 trips. :rolleyes:

This VERY simple "radio" that I'm messing with right now is a super-regenerative radio? I think earlier in the thread Audioguru said something about those being crap, which I can understand, because even though I can barely hear this radio when the battery is connected (hence messing with the amplifier) I can tell that when I'm receiving a station it's loaded with static. The point of me saying this is, I can tell you right now when I'm done messing with this one, I'm going to want to upgrade a step and experiment with a better FM receiver, so I'm asking... what kind of FM radio receiver should I focus on doing next? A superheterodyne? Or something else? I would seriously love to hear suggestions on this. I'm enjoying messing with the RF circuits and I want to keep going. Your input is critical.

On a final note, I have a question about Variable capacitors (tuning caps, gang caps, whatever you call them). Below is 2 photos. The first shows the front of the variable cap I have, which I have been using to tune the circuit. I assume this is the main dial that changes the capacitance?

The second photo shows the back of the variable cap. There are 2 little screws in there that I can turn, but I have no idea what they are there for. Can someone fill the void for me?

When I bought this variable capacitor, it was labeled as "20-80pf + 20-165pf". What exactly does that mean? It's full range is 20-165 or 80-165 or what? I'm sorry about so many questions, thanks for all the help.


 

bertus

Joined Apr 5, 2008
22,270
Hello,

The capacitor consists of two parts.
One is from 20-80 and one from 20-165 pF.
One is used for the oscillator the other for the input filter.
The two screws on the back are for synchonisation in frequency of the oscillator and tuning circuit.



The middle picture looks like the one you have.

This is a picture from this webpage:
http://www.mikroe.com/old/books/keu/02.htm

Bertus
 

Audioguru

Joined Dec 20, 2007
11,248
This VERY simple "radio" that I'm messing with right now is a super-regenerative radio? I think earlier in the thread Audioguru said something about those being crap, which I can understand, because even though I can barely hear this radio when the battery is connected (hence messing with the amplifier) I can tell that when I'm receiving a station it's loaded with static.
Yes, the simple crappy "radio" is a super-regen like is used in very cheap kids Radio Controlled toys. It produces static (amplitude fluctuations) because it has an AM detector (Amplitude Modulation detector that picks up static) not an FM detector (Frequency Modulation detector that ignores amplitude fluctuations).

what kind of FM radio receiver should I focus on doing next? A superheterodyne?
Yes, a super-heterodyne is a real radio.
But a cheap one like in a very cheap clock radio is crap, and a good one like in a high quality home stereo or a high quality car radio is excellent.

I have a question about Variable capacitors.When I bought this variable capacitor, it was labeled as "20-80pf + 20-165pf".
A good FM radio uses a variable capacitor that has three sections. One section for the oscillator is 1pF to 16pf (or 4pF to 70pF) and the other two sections are 2pF to 35pf. Your high capacitance tuning capacitor is made for a Short-Wave radio, not an FM radio. Your tuning capacitor might work in an AM radio.
 

Wendy

Joined Mar 24, 2008
23,415
I was thinking about that TL084, and drew something that has been rolling around my head. It might be able to do 2W distortion free, but the transistors are not very protected and could get hot (and smoke) pushing higher power levels of sound.



I have no idea if this will work. I might try it someday. The gain is way high, around 10,000. Low frequency roll off is around 10Hz. It also has more parts than I like to use.

If you want to drop the maximum gain go for R3 and/or R5, 1MΩ will reduce each by 10.
 

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Audioguru

Joined Dec 20, 2007
11,248
Hi Bill,
You like TL084 opamps because they are sold at RadioShack? But a TL074 is a TL084 selected for low noise and costs the same.

Your amplifier has a voltage gain of (gasp!) 10,000. The 1k resistors pull up or pull down the bases of the output transistors with about only 2mA then their current gain must be at least 200. But power transistors (TIP31 and TIP32 for example) usually have a minimum current gain of only 30 at 400mA.
If you use darlington transistors then you need more diodes and lose more voltage swing.
 

Wendy

Joined Mar 24, 2008
23,415
It is mainly a whimsical circuit. I was thinking of 2N2222 and 2N2907A's. The problem of no protections is real, and I'm absolutely depending on RMS to keep the heat down (not a good thing). It is something a person can breadboard and play with.

I might try something with a dual op amp, I really don't like the resistor count on this one.

I tend to stock both in my parts box, I try for 3 of each type. Gives me some smoking space.
 

Audioguru

Joined Dec 20, 2007
11,248
A TDA7240A amplifier IC costs $2.51 at Newark today (Digikey has run out of them).
Its output with a 13.2V supply is 14W at clipping (low distortion) into a 4 ohms speaker.

A TDA2822M amplifier IC costs $.60 at Newark today. It is a low power stereo amplifier IC or a 0.9W at clipping into 8 ohms with a 6V supply bridged amplifier.

Both are not available at "RadioCrap". If RadioShack sold them then they would cost $20.00 each.

Then you don't need opamps, power transistors and many resistors.
 

Thread Starter

ke5nnt

Joined Mar 1, 2009
384
Ok, wired up the following circuit, which is the original "crap radio" with a LM386 chip set up with the gain-20 circuit from the datasheet. Schematic:



Please note the trim-pot. The following image shows the pins, which I numbered to show how I connected it to the circuit above (the circuit is also numbered). Not 100% sure I made the connections correctly so I'd like it double checked.


Here is a pic of the amp circuit on the breadboard connected to the rest of the "radio" built on the cardboard shown earlier in this thread:


It seems that after adding the amplifier circuit, reception on the radio is worse than before, and the volume hasn't changed at all. Still very quiet. I'm positive I've done something wrong.

Also, on the variable capacitor that I asked about earlier from this circuit, how do I know what the proper positions for the screws on the back side are? Thanks.
 

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Wendy

Joined Mar 24, 2008
23,415
You don't really. They are to make the tank circuit that tunes the radio frequency and the local oscillator that heterodynes the RF to the local oscillator synchronous. If you have test equipment you can measure each (the tank circuit and the local oscillator) where they are together, but most people just tweak both until it works. Time consuming, but traditional.

Have you powered up the circuit yet? The schematic is the thing, if it is correct and you make a wiring error no big deal. If it isn't right then you have problems.
 

Audioguru

Joined Dec 20, 2007
11,248
The "radio" project shows two variable capacitors. One has a 70pF in series with a 140pF one and the other has a 30pF in series with a 70pF. I doubt that either will tune in the FM broacast band and your variable capacitor is again very different.

The "radio" project uses an excellent supply bypass capacitor that is a 0.001uF ceramic disc. But your schematic has a 220uF electrolytic capacitor C10 that is useless at radio frequencies. Use a 0.001uF ceramic capacitor for your C10 and use the 220uF electrolytic capacitor between pin 4 and pin 6 of the LM386 IC.

Your 22uF value for C9 is much too high. Use a 330nF (0.33uF) film capacitor instead.

The LM386 has plenty of gain at 1MHz so it will probably oscillate (and not work as an amplifier) if it is made on a breadboard. The "radio" also will not work when made on a breadboard.
 
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