help me remove interference

Discussion in 'Wireless & RF Design' started by mik3ca, Jan 30, 2008.

  1. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    so I was experimenting with my homemade superregen radio yesterday, and it was acting funny.

    What I ultimately want to do is run my radio using a wall adapter as a power supply, and connect the output directly into my laptop.

    I am only partly successful.

    When my laptop is running on batteries, it generally emits a very high pitched sound (that I can hardly hear), but when my radio is connected to my laptop through the microphone socket, I hear the laptop sounds quite well, AND it even overrides the faint stations. In all tests, the laptop is at least 12 inches away from the radio.

    another thing I find crazy in another test, is this:

    My antenna was about 1 meter long of thin wire connected to a thick antenna stick (about 30 cm long), and that stick was on my bedroom floor. the radio was on one end of my bed. The laptop was at the other end. The AC adapters of both devices were connected to different wall sockets, and no cables were touching each other. Remote radio reception was O.K., but I think there is still some interference on my end that can be removed.

    then, I extended the length of my antenna to about 3 meters. I hooked it up to a non-conducting screw on my room light, but the radio itself was in the same location. Reception was somewhat worse, and there was more noise.

    In both cases, the antenna was pointing in the same direction.

    In another test, I had the antenna pointing in a different direction (away from all sources of noise, and farthest point away from the local station tower). this time, I get no remote station signal.

    so I think the problem narrows down to the wire connecting the radio to the laptop.

    the output of my receiver is connected to an audio output through a 1500uF coupling capacitor.

    I experimented with capacitors and 1500uF is the best for volume.

    Please help.
     
  2. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    a source claims that it is my speaker cable (that connects the radio to my laptop) that is causing the problem, but here's the kicker.

    if I make my cable ridiculously short, I would have a problem because my radio is connected directly next to my laptop. If I make it long, I can move it farther away from my laptop.

    what do I do here? make it really long, or really short? and why?
     
  3. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    You were picking up noise from your room lamp.
    Antenna is directional, and you were not aimed at sources?


    Speaker cables don't pick up much RF noise. If you are concerned about it anyway, use shielded twisted pair for the speakers and ground one end of the shield.
     
  4. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    but I read somewhere that speaker cables can act as antennas that can cause interference. I want to eliminate it.
     
  5. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    The shield will indeed eliminate it, if you ground one end of the shield. The twists help cut it down all by themselves even without the shielding.
     
  6. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    I was curious, since I live across the street from a hydro-tower, (electricity tower), I was wondering how far away I have to be from it in order to pick up the station. It is blocking the path between me and the station.
     
  7. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    Research now tells me that it is the power lines across the street that may be causing the problem. now it seems that I need an excellent filter.

    Is there anything better than a PI filter that I can make without using complex calculations?
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    896
    People use radios, televisions, car radios and cell phones with electricity towers all around and don't have interference nor weak signals, because their circuits have enough tuned circuits for good selectivity to reject interference and have enough amplifier stages to be very sensitive. They also have automatic-gain-control so that strong signals don't overload them.

    Doesn't your super-regen radio have a single tuned circuit and a single transistor?
    What frequency are you trying to receive?
    Is it AM or FM?
     
  9. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    then maybe i should add more tuned circuits between the power supply and the radio.

    Only for the detector portion. Then it gets amplified by a push-pull 3-transistor amplifier.

    103.5 Mhz in Toronto


    FM.
     
  10. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    No.
    Tuned circuits should be at the inputs and outputs of RF amplifier transistors.
    A cheap ordinary FM radio tuner has a tuned LC circuit at the antenna that feeds an RF amplifier transistor that has automatic-gain-control. The output of the RF amplifier transistor is also tuned. In a super-heterodyne radio, there is a high frequency oscillator that is also tuned and this oscillator's output and the amplified RF are mixed together in a mixer transistor stage to produce a lower IF frequency. The IF (Intermediate Frequency) is tuned with a few LC circuits and/or crystal filter circuits and amplified with a very high gain multi-stage amplifier. Then the IF amplifier feeds the AGC for the RF amplifier and feeds a demodulator.

    A good FM radio circuit does not pickup interference.

    Wikipedia lists Z103.5 with a studio in Toronto. The transmitter power is 30,700W (it was 100kW then 50kW and is now a little less) from their transmitter in Orangeville, Ontario. Their antenna is slightly higher than the CN Tower in Toronto and the station is heard all over southern Ontario. Most FM radios in your city will receive it perfectly. The $1.00 scanning FM "radio" from The Dollar Store might not.

    Your super-regen circuit has an AM demodulator. It might receive an FM station if it is tuned to one side and uses slope detection. An FM radio is tuned to the center of the radio signal for good selectivity and good sensitivity.
     
  11. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    oops, I forgot you were talking about tuned circuits.

    Mine does the same, but there really isn't any AGC involved.

    Mine is set at a fixed frequency, since my receiver is a superregen.

    mine is amplified with a transistor push-pull amplifier.

    I won't enter AGC until I can tune into the remote station.

    My radio receives many FM stations. I just have a problem with Z103.5
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    An FM radio has a very high gain radio-frequency amplifier before the signal is demodulated, not a high gain audio amplifier. Then it is very sensitive. AGC allows it to avoid being overloaded by strong local signals.

    It has many tuned circuits for good selectivity. Its IF amplifier provides most of its selectivity and sensitivity. Your radio doesn't have an IF amplifier.

    A weak FM station has the same volume as a strong local station because their frequency deviation is the same.
     
  13. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
    189
    0
    My first amplifier uses a PN3563 transistor which is meant for RF applications.
    This helps demodulate the signal.

    my high gain amplifier follows my RF amplifier.

    this is the part I need to work on. High-end sensitivity.

    I managed to make it so that there is no signal between 102.9 and 104.5, but that isn't what I want. I can't see AGC helping me yet.

    I don't want to have to play with multiple capacitors.

    If there is a simple version of this IF amplifier that I can tie in, then I could probably bind it in.
     
  14. Audioguru

    New Member

    Dec 20, 2007
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    An IF amplifier has a few fixed tuned circuits that are tuned to a single frequency. The oscillator's capacitor and the capacitors at the input tuned circuit and at the output of the RF amplifier are tuned to change stations.

    Then the radio is a super-heterodyne design, not a super-regenerative design.
     
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