Help with FM Broadcast Radio receiver

Discussion in 'The Projects Forum' started by ke5nnt, Oct 5, 2010.

  1. ke5nnt

    Thread Starter Active Member

    Mar 1, 2009
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    I've searched AAC forums and looked over a lot of topics on this subject (somewhat on this subject). There are a lot of posts with broken links now, they're pretty old.

    I'm interested in building a little FM radio with enough gain to drive a little 2 inch speaker or so. I built the radio on this page using the schematic titled "original" and I don't get anything but static on it. I've tried tweaking it a number of times but just can't seem to get it working.

    On top of the parts for the schematic above, I have all the parts for a little 1-watt amp circuit I was going to add to this thing, schematic for that found HERE.

    I'll be honest, this is my first time messing with RF circuits so I'm not entirely sure what I'm doing, but I'd like to learn as I go. I read in one of the old posts somebody suggesting that 1-transistor radios are crap and that you should build a 3-transistor radio. I'm fine with that, I just don't get any good results back when I try to search for some projects on google. Guidance and suggestions would be hugely appreciated. Thanks so much.
     
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
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    That certainly is a unique circuit using a voltage controlled zener as the output stage. Problem here is the overall circuit gain is derived using the super-regenerative method.

    Dealing with construction around 100 MHz isn't easy, you can't easily breadboard anything unless you use the dead bug method that I never reallly liked.

    FM radio receiver circuits abound on the internet as do single IC solutions. I'd go for something a bit more conventional in design.
     
  3. bertus

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  4. Wendy

    Moderator

    Mar 24, 2008
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    Haven't seen ya in a while!

    AM is a lot easier than FM, so while the little FM receivers are crap, but the hard part, the discriminator, has been done. I don't think you can do a discriminator with 3 transistors (could be wrong though). Plus, old discriminators are a stone cold bear to tune, they have two adjacent tank circuits that work together with detectors similar to AM circuits to make the signal go ± swings. They used superheterodyne to bring everything to a common frequency, just like AM radio.

    [​IMG]

    Modern receivers use phase locked loops with super heterodyne.

    I think 3 transistors are going to be kinda tough to do.

    A long, long while back I came across a cheap Radio Shack kit (long gone now) that used a super regenerative technique (and had less that three transistors) that worked quite well. I've regretted loosing the schematic a lot. It also had the odd characteristic of killing any other radios ability to receive the signal for well over 10 feet, but it had good sound and reception. That's probably the reason I haven't seen it since.

    Do you want this radio to receive over the band?

    Now to go look at your links.

    One last word, RF construction is a different world. I recommend the dead bug technique and the Manhattan_Construction_for_RF. The idea is to keep the leads as short as possible and provide good RF paths.
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    I think Bertus has found something similar to the Radio Shack version. If you build it try using a conventional FM radio next to it. If I get a chance I may build it someday, if you do let us know how it works.

    That 1W audio amp you linked to is totally bogus, in every way. It may produce ¼W, but I doubt it. Paraphrasing Audioguru, the majority of circuits on the web do not work, I'm coming around to his point of view.
     
  6. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Seems almost half the common question links go to ad websites nowadays, or someone's patent idea.
     
  7. ke5nnt

    Thread Starter Active Member

    Mar 1, 2009
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    Hey Bill, yeah... been gone a while. I have a lot of things going on that I wont get into here but most notably I moved (only moved 20 miles away but moving is moving) and I'm gearing up to start school.

    Anyways, back to the topic at hand. So what I'm gathering is, throwing together little projects that deal in RF is a bit more challenging than your everyday tinker project? Not something you can really put down on a breadboard and expect to work?

    As I said, this is my first try at dabbling in RF circuits, so when I threw that circuit down on the breadboard that I linked to in my first post, all I got was static no matter what the tuning capacitor was set to. Is it possible the circuit works, just not the way I have it laid out on the board?

    This dead-bug thing... would it not also be essential where I place components whether they are placed upside down on a substrate or poked down into the holes of a breadboard? I'm not understanding how one is different from the other in terms of component layout.

    Bertus, thank you for the link you provided. It looks very interesting and I'd like to give it a shot. Before I do however, I'd like to figure out the component placement importance so I don't spend more money on more components just to get more static with new components.

    Again, thanks all for the help!
     
  8. marshallf3

    Well-Known Member

    Jul 26, 2010
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    As the frequency of a circuit increases (or rise/falll times of a waveform) all sorts of things that didn't affect DC circuits start to become more and more of an issue.

    Simple component leads become inductors as well as antennas as well as the components themselves. What was once an insignificant 1 pF capacitance between two resistors mounted close to each other enters into things etc.

    It's not terribly bad around the 100 MHz area but more than enough to make a difference sometimes when dealing with sensitive circuits such as a receiver.

    It's all kind of hard to explain, but it comes down to stuff like you don't want the output of something close to the input, nor extra copper on a board unless it's part of the design etc.

    Layout could be your problem on the original circuit, do you have some pictures?
     
  9. marshallf3

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  10. Wendy

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    The main thing about RF is negligible factors aren't. SMT works very well for RF, but with through hole components you need to keep wire lengths as short as you can (they are inductors), preferably without creating a rats nest you won't be able to probe and troubleshoot. Two leads running next to each other is a capacitor.

    One of the nice things with both methods is you can lay out the parts similar to the schematic. This makes it easy to trace the circuit.

    The Radio Shack circuit I built in my dorm room was a plastic perf board, so it's layout wasn't too critical. They did provide a picture somewhat similar to what I like to draw though, so the layout was provided.

    About the move, did you get closer or further away from Tanners? Someone quoted elsewhere on this site, 3 moves = 1 fire.
     
  11. ke5nnt

    Thread Starter Active Member

    Mar 1, 2009
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    After reading what you gentlemen have said, I'm going to assume lead lengths and "bird's nest" effects are part of my problem. Pictures below. So maybe I should try doing this upside down on some cardboard or something, clipping leads as short as possible, etc? Assuming I use the dead-bug method, there is one thing that confuses me slightly on it. What do you use to make your connections between leads? Pieces of wire? If that's the case, wouldn't the wire pieces essentially create capacitance and a general "bird's nest" effect just the same as long leads? What's the difference?

    To answer your question Bill, I'm closer to Tanner's now. I now reside in Carrollton.

    Here are the pics of the breadboard setup.

    [​IMG]
    [​IMG]
    [​IMG]
    [​IMG]
     
  12. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    I think the main problem is the capacitance between the lines in the breadboard.
    These lines are made if metal strips laying next to eachother.
    When you look at the construction of a breadboard, you will see where these capacitances are.
    I have attached a PDF with some more info.

    Bertus
     
  13. Wendy

    Moderator

    Mar 24, 2008
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    Protoboards are notorious for not handling RF well, as Bertus has said. 1Mhz is pushing it, and you have a 100 of the little buggers!

    Back when I was doing the majority of my home RF experiments I used a chunk of wood, 2X4 scraps, to mount terminals on. Weren't pretty, but it worked well enough to have my Dad aggravated to what it was doing to our already weak TV reception.

    I was a pure beginner, and resources like the web hadn't been thought of yet.
     
  14. Audioguru

    New Member

    Dec 20, 2007
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    A simple super-regen "radio" was used long ago when there were few radio stations. Today it is overloaed by all the radio stations in town and cannot separate them so you might hear a few stations all at the same time.

    A super-regen radio has an AM detector that detects FM if you tune it to one side of the station's frequency. Then there is pretty bad distortion.

    Modern FM stations are stereo and have SCA "restaurant music". These extra signals cause interference with the "super" part of a simple super-regen "radio".

    A normal super-heterodyne radio works much much better than a simple super-regen.
     
  15. marshallf3

    Well-Known Member

    Jul 26, 2010
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    No doubt the ProtoBoard is the problem as well as the components being too far apart for somehing designed to operate in that frequency range. If you get some common perfboard (holes with no copper) and build the circuit tighter it would probably work.
     
  16. Wendy

    Moderator

    Mar 24, 2008
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    I prefer perfboards with copper pads myself. Tends to hold parts more securely, and tends to have fewer cold solder joints (especially in the early days of me soldering).
     
  17. ke5nnt

    Thread Starter Active Member

    Mar 1, 2009
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    Ok, I got a few things to try and I appreciate all the input from you folks. I really do! One last question/request if I may. Know any good links to a tested super-heterodyne schematic I can try if they work out better in modern times compared to a regen? I'm going to rebuild the original circuit first using some perf board I think I've got lying around. I'll report back in a few days (time is short).

    Thanks again all.
     
  18. Audioguru

    New Member

    Dec 20, 2007
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    Many years ago the ganged variable capacitors, transformers and crystal filters were sold to hobbiests for making radios. Not any more. Now you get the parts from a radio that you buy and hope the parts are not too small to work with.
     
  19. sceadwian

    New Member

    Jun 1, 2009
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    Easy to find Audioguru, if you need parts of that nature simply hunt through local thrift stores, older electronics tend to end up there and often in really good (sometimes working) condition.
     
  20. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Best finds are if you can ever catch an old service shop that's closing down, and many are due to the non-fixable nature of most modern electronics. In addition, sometimes on eBay or CL someone will have "inherited" a bunch of electronic stuff when their father died, don't know what it is and sell it.
     
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