Radio Frustration

Discussion in 'Wireless & RF Design' started by tobyw, Apr 30, 2013.

  1. tobyw

    Thread Starter Member

    Mar 21, 2013
    37
    4
    I am new to electronics and radio, but I have managed to create a basic receiver which looks like the attached circuit. The main radio part is all on a breadboard, the LM386 is on a breakout board soldered together with 3 header pins to attach with jump wires to the radio receiver.

    When I attach my aerial (several feet of wire) I can pick up stations quite loudly and all is good. However, there are some things which confuse me, and I am hoping someone might be able to explain.


    • How can I remove the low hum in my circuit? It disappears when I touch the metal handle of the potentiometer volume control
    • If I remove the lead connecting the output from the transistor to the LM386 breakout board, and just hold it by the insulated part (not the end), not only can I still hear the radio station, but I can hear it LOUDER. How is this possible?!
    • If I remove the coil, I can pick up Romanian English language radio. But only that station. Again, how is this possible since the oscillator is gone? Is it just reacting with the overall impedance of the circuit?

    Any light that anyone can shed on these questions I'd be grateful. It's confusing me a bit..
     
    Last edited: May 1, 2013
  2. tubeguy

    Well-Known Member

    Nov 3, 2012
    1,157
    197
    One thing, you need a coupling cap between the LM386 output and the speaker. At least a 220uf or larger electrolytic would work.

    What are you using for a power supply and what is the voltage ?
     
  3. tobyw

    Thread Starter Member

    Mar 21, 2013
    37
    4
    You are right Tubeguy - I knew I would mess up the schematic somehow. I do have a 220uF cap between the LM386 output and the earphones. I have updated the image to reflect this.

    I am using a 5v battery. It's all working fine as far as getting stations, just the hum and the weirdness!
     
  4. davebee

    Well-Known Member

    Oct 22, 2008
    539
    46
    At radio frequencies, all kinds of components exist that are not drawn neatly on the schematic.

    Wires act as capacitors and inductors and antennas. What appears as ground to DC may no longer be a clean ground for AC. Parts resonate and transfer energy at all different unexpected frequencies. Your body picks up energy at both power line frequencies and radio frequencies and feeds it as a signal into anything you touch.

    To deal with that, you have to track down and change the circuit wherever it does not work the way you want.

    You might have to add bypass capacitors at more places, improve the grounding, add shielding to sensitive parts of the circuit. If you look inside any commercial radio, you'll see that all of these things have had to be done to get it to work satisfactorily.
     
  5. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    There is also something strange with your battery connections.
    The - pole is connected to the top of the tuning circuit.
    I think this should be connected to the ground connection.
    Pin 4 of the LM386 would work, as that is connected to ground.

    Bertus
     
  6. Ramussons

    Active Member

    May 3, 2013
    557
    92
    There seems to be another mistake too. Diode D2 should be connected directly to the Transistor Base; C2 between Transistor Base and Ground.

    Ramesh
     
  7. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
  8. tobyw

    Thread Starter Member

    Mar 21, 2013
    37
    4
    Thanks for the replies.

    Bertus

    You are right about the battery - I put it wrong in the schematic. I'm not very good with Eagle. It is connected correctly but wrong in schematic.

    Ramussons - What is the reason for the capacitor being between transistor emitter and ground? Thanks
     
  9. Sue_AF6LJ

    Member

    Mar 16, 2013
    45
    27
    Addressing the hum.........

    The one thing you don't have that you do need is a ground.
    In order to get rid of the hum you need to connect the common / negative side of your circuit to an earth ground.

    I can see not many here have built a crystal set before. ....
     
  10. joeyd999

    AAC Fanatic!

    Jun 6, 2011
    2,693
    2,763
    Hey, Sue.

    I built my first crystal set back when I was 5 (around the same time that I started that fire...) That is what got me into electronics.

    But, I am not a radio guy. I've always wondered why do crystal sets need a ground, but transistor radios do not? Since you've presented yourself as a radio expert (and produced photos to prove it!) I'd like to hear your explanation.
     
  11. k7elp60

    Senior Member

    Nov 4, 2008
    478
    69
    I disagree on this. The transistor is biasing from R2.
     
  12. k7elp60

    Senior Member

    Nov 4, 2008
    478
    69
    You need a filter between the diode and the 10uF capacitor. The filter consists of a .01uF capacitor from the diode cathode to ground. See the attached diagram, A ground as Sue mentioned may be necessary. An antenna wire about 20 feet long may also be necessary. The values you have for the tunned circuit will tune a portion of the standard broadcast band.
     
    Sue_AF6LJ likes this.
  13. Ramussons

    Active Member

    May 3, 2013
    557
    92
    Yes, the transistor is biasing from R2. In the original ckt, C2 would be charged over time and because there will be no discharge path, and will finally block.


    The thumbnail shows how it should be. The detector OA81 conducts the +ve amplitude to the base (the bias does not change), C2 grounds the RF component and the transistor amplifies the AM envelope.

    Ramesh
    [​IMG]
     
    Last edited: May 6, 2013
  14. Sue_AF6LJ

    Member

    Mar 16, 2013
    45
    27
    The antenna in a transistor radio is a loop antenna, it is a complete circuit.
    The crystal set needs a ground because without it the antenna circuit is not complete.
     
  15. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    (1) I think the "hum" you are getting is actually a phenomenon known as "motor-boating " (because it more commonly appears as a "put-put" sound.
    It is due to unwanted coupling between input & output circuits of high gain audio stages via the power supply.

    You need to use by pass capacitors on the power supply rails.

    (2) In this case,it sounds like you are receiving a local station directly into the LM386 input--this is not unknown,high gain audio amplifiers are often affected by RF interference.

    (3) Removing the coil means you no longer have a tuned circuit,but all the rest of the components of a radio are present,so a strong local station can still be received.
     
  16. copper dog

    New Member

    Apr 15, 2013
    7
    0
    Yes I noticed the lack of a bypass cap as well. The circuit should have a .1uF bypass capacitor located as close as possible to the op amps power supply pin. You have no gain configured on this system as well. A simple way to get a gain of 50 on the LM386 (assuming you want gain) is to install a 1.2K resistor and a 10uF cap in series between pins 1 and 8. The data sheets for this op amp also recommend a .05uF cap and 10 ohm resistor in series to ground at the output (pin 5). This would be prior to the coupling cap (C5 on your schematic) although to be honest I took that out of my last project because I saw no performance differences and I was trying to keep the parts count down.
     
Loading...