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FM radio circuit question

Discussion in 'General Electronics Discussion' started by serious coinage, Mar 26, 2011.

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  1. serious coinage

    serious coinage

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    Mar 26, 2011
    For an electronics project, I have been looking to build an FM radio. I have been researching and have a reasonably good understanding of the principles behind it, and have found several circuit designs. At first I was researching superheterodyne receivers which seemed quite interesting but the only circuits I was able to find where ones based on an IC instead of discrete components, like this one. However, I don't seem to be able to find any electronics store that stocks an IC like this - it seems very specialized.

    So, i continued searching and I found this design, which seemed quite good. However, down at the bottom of the page it warns that unless you have a good experience with these type of circuits, that you should use the premade PCB that they offer. Why is this? Is this type of radio circuit very sensitive to circuit layout? Would it be impossible to build this circuit on a breadboard or prototype stripboard? I wanted to modify it so the output would be amplified by an op-amp to drive speakers or headphones.

    In summary, is there any good designs for a superheterodyne FM receiver that isn't based on some sort of tricky-to-get-hold-of IC? Also, why might I not be able to build the one-transistor FM receiver on a breadboard or a PCB of my own design? Sorry if these aren't very clever questions, I don't have much experience with radio circuits, or huge amounts of experience with electronics for that matter. Any advice would be appreciated :)
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Google is your friend.

    Here has some comments on that design.

    In general terms, increased performance will equate to increased complexity. A lot of complexity can be hidden in those FM receiver chips.

    Also note that the circuit uses a 431 which *is* and IC, so it can hardly be called a 1 transistor solution.

    If you get the PCB from them, or if you design one with a very similar layout then presumably you will have less problems.
     
  3. serious coinage

    serious coinage

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    Mar 26, 2011
    Looking at that thread, it doesn't seem like such a good suggestion. Apparently it's a pretty shoddy circuit and the guy who made the thread hasn't had much success(although the way he's not within 100km of an FM station probably didn't help...)

    After some more searching, I have found a couple of transistor based superheterodyne receivers. This one looks very good, but as you said, fairly complex. Even though I'm a newbie, I don't mind it taking a while, so I will be able to spend lots of time fiddling with it if I choose this one.

    However, this one actually looks simpler(although it loses marks for the bright magenta website background.) Is there any drawback to it being like this? I don't mind it not having stunning audio quality or whatever, I just want to build a functioning radio so I can learn about this and have something cool to show for it.

    And a final question, according to the thread you linked, breadboards are bad for high-frequency radio circuits. This is probably a stupid question, but why is this? And will it make the circuit flat-out not work, or will it simply reduce the quality? The guy from my first link built the circuit on breadboards, and here is a video of it working, so I'm confused about this...
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    But it's simple, and will be cheap to build, so it may be worth giving it a go.

    What are you doing this for? A result may be more useful than a fancy failure.

    You will need to determine if the parts are easily available and be sure that you can do troubleshooting if (when!) things go wron and it doesn't work first time.

    OK, it looks interesting. Again, make sure you can source all the parts. But the lack of transformers makes it possibly more viable. Having a block diagram will probably make troubleshooting easier.

    There is a lot of distributed capacitance due to the series of parallel conductors. Depending on how you assemble the circuit, you may have more or less capacitance. With RF circuits small amounts of capacitance may matter a lot.

    That's the whole thing. It may do either, both, or neither. It all depend on your (essentially random) choice of layout.

    If you're not random, you may do better -- but knowing *how* to make it better is another issue).

    It may still be very sensitive. Moving his hand around may affect stray capacitance and may change the tuning (you'll notice he keeps his hand very still). Also it may not have stable tuning -- which would be hidden by the continuous tuning across the band.

    On the other hand, all a breadboard does is allow you to quickly make up a circuit. You're not going to bold the breadboard inside a box and use it that way.
     
  5. serious coinage

    serious coinage

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    Mar 26, 2011
    I think I'll give it a shot. I'll see if I can get all the parts, like you said and to make assembly easier I will use a circuit design program to make my own diagram with all the component values on it(his diagram has all the values on a seperate page.) If it doesn't work on the breadboard, I haven't lost anything, right? Out of interest, if I did it on a stripboard, would I still have problems because it has parallell strips like a breadboard?
     
  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    stripboard will have fewer issues because the conductors are flat. On a breadboard they have significant height which increases the capacitance significantly.

    Yeah, try the breadboard first. If it works, then you're on your way. If it doesn't, it does not necessarily mean the circuit is at fault. I wouldn't expect that you'd have huge problems. Try *NOT* to keep things compact. Using every second row of connectors wherever possible will reduce the capacitance considerably.

    Just for interest, are you doing this for fun, or is there a purpose for this project?
     
  7. serious coinage

    serious coinage

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    Mar 26, 2011
    A bit of both. At my school we have a special semester subject where you get to choose a topic that interests you and work on it for the semester, and give a presentation about what you did, what problems you encountered and other exciting things like that. I chose to build a radio and research in depth how they work.

    Anyway, I have another question. I haven't had much trouble finding parts, except for one particular transistor, a 40673 dual-gate MOSFET. It seems to be discontinued, and the two other parts recommended as equivalents(the 3N159 and the 3N201) by the site also seem to be unavailable. Does anyone know of a suitable replacement or equivalent of this part?
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I would search for any dual gate mosfet. You may find some still available.

    Ooh, look. Digikey have some.

    Find the specs on the one in your circuit and see if any of those listed appear similar.

    I know this isn't a trivial thing to do -- maybe someone else will come up with an obvious replacement. I may be able to take a closer look later.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I can't guarantee it is a perfect replacement, but this may be OK.

    I can't find any other than surface mount -- except on eBay. The ones on eBay are ridiculously expensive though.
     
  10. serious coinage

    serious coinage

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    Mar 26, 2011
    Hmm. It seems to be a reasonably good match, but it's not possible to use surface mount devices on a through hole circuit board, is it? The MOSFET is being used as a frequency mixer in the circuit, is a dual-gate MOSFET the only way to do frequency mixing? Would it be possible to modify the circuit so it uses a different type of mixer?
     
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    If you're making a printed circuit board, you just mount the surface mount components on the copper side of the board. The problem is soldering them :)

    If you're using strip board or a prototyping board, you can get small adapter boards that you mount them to, then attach pins to. The pins have the correct spacing for strip board or a breadboard.

    The answers to the other questions are yes, and yes. The problem is that you would need to design something yourself and it may be somewhat more complex.

    It's a shame because these dual gate mosfets were once a dime a dozen, now they are almost impossible to source in other than surface mount packages.
     
  12. serious coinage

    serious coinage

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    Mar 26, 2011
    I found this, and it looks like a good equivalent at a good price. If I find any other sources of similar parts, I'll give links in case it helps anyone else.
     
  13. blackmask

    blackmask

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    Apr 15, 2011
  14. blackmask

    blackmask

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    Apr 15, 2011
    And as I suggest, use breadboard or multi purpose board for your test design. If your design operates then only design a PCB. Most of site who provides circuit diagrams offers you to use their PCB...this is nothing just their business...if depends upon you if you wish to use their PCB or design your own...
     
  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    blackmask, he's after a receiver, not a transmitter.
     
  16. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    OK, equivalent parts again...

    See here for someone who is apparently still selling them.

    See here for a datasheet.

    and for that transistor that you found... They actually say that it's electrically similar to the 40673, and here is it's datasheet.

    A quick comparison suggests that it's pretty similar.
     
  17. serious coinage

    serious coinage

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    Mar 26, 2011
    I actually ordered the part I posted just after I posted it because it said it was a good replacement for the 40673, and a quick look at the datasheet suggested that it would do the job, just like you said. Good find on the supplier of 40673s, though. If the MPF131 doesn't work for whatever reason, I might look into that. Thankyou for all your help!

    edit: If the circuit calls for a 20-30pf trimmer, there's no harm in using a 5-30pf trimmer, is there? It shouldn't matter as long as the trimmer's range includes the desired range, right?
     
    Last edited: Apr 16, 2011
  18. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Yeah, sorry it took so long to get back to you. I didn't notice at first that they actually listed it as a replacement for the 40673.

    Remember when you get them that they are static sensitive and should be handled appropriately!
     
  19. serious coinage

    serious coinage

    13
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    Mar 26, 2011
    I know I should be using an antistatic wriststrap and grounding myself before touching a MOS device, try not to touch the leads and keep static sensitive stuff in some sort of conductive foam. Any big points I'm missing?
     
  20. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    They're the big ones. In addition, you should solder the MOS device(s) in last using a soldering iron that has a grounded tip.

    Once in the circuit they are far less sensitive as there are typically low enough impedance paths between the gate(s) and the other terminals so that static can't build up -- although aiming your finger at them after shuffling along carpet in runners is still to be avoided!
     
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