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    help about noise!pls


    The above circuit is fm receiver.
    when i used pin supply 9V, this circuit had quite nice performence. But when i used AC/DC rectifier or switch supply on same pcb , the received audio has a tone ooeee (noise). I don't know why?
    pls help

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    Re: help about noise!pls

    you need to filter the voltage supplied from your AC/DC rectifier... just put a capacitor parallel to the output of the rectifier and then ground it to the -ve

    ---------- Post added at 16:30 ---------- Previous post was at 16:27 ----------

    wait, what kind of rectifier to you use by the way?

    ---------- Post added at 16:32 ---------- Previous post was at 16:30 ----------

    you may start with the basic rectifier with a filter with this link:
    Filters

    ---------- Post added at 16:34 ---------- Previous post was at 16:32 ----------

    you may start with the basic rectifier with a filter with this link:
    Filters



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    Re: help about noise!pls

    i use the switch supply of computer (ATX) and put filter capacitor but not better. have else reason for this noise?



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    Re: help about noise!pls

    i suggest that you check the value of your inductor...



    •   Alt12th January 2011, 09:54

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    Re: help about noise!pls

    I would not recommend using a SMPS for this receiver because of the risk of noise from the switching process. A simple analogue power supply would not give this problem.

    However, if you really need to use a SMPS then it can be done, but it should have good output filtering using an LC filter from the link suggested by Joelalfuerto. When SMPS are used in professional audio equipment which carries very low level signals (such as your receiver) then it is common for the DC output of the SMPS to be followed by a traditional analogue regulator to assist in providing a stable and quiet DC rail.



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    Re: help about noise!pls

    please use filtered/regulated power supply or add a regulator circuit and connect to c4(please observe the polarity) and later connect your existing power supply unit.



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    Re: Help me fix the noise in a FM receiver with AC/DC rectifier

    The problem is that this is not an FM receiver, it's a cheesy
    AM receiver that manages to crudely demodulate FM as well.
    It has no rejection of CW, AM or burst RF energy that is
    in-band.

    So it does a swell job of picking up any power supply
    harmonic that comes along.

    Look for ringing on supply edges and add a snubber
    across any diodes / switches. Put ferrite beads on
    the power and return legs at the supply. Put the
    supply or the radio in a can. Etc.


    1 members found this post helpful.

    •   Alt13th January 2011, 21:42

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    Re: Help me fix the noise in a FM receiver with AC/DC rectifier

    You could try to decouple the junction of L3/ R3 with 100 uF capacitor.
    Frank



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    Re: Help me fix the noise in a FM receiver with AC/DC rectifier

    Quote Originally Posted by dick_freebird View Post
    The problem is that this is not an FM receiver, it's a cheesy
    AM receiver that manages to crudely demodulate FM as well.
    It has no rejection of CW, AM or burst RF energy that is
    in-band.

    So it does a swell job of picking up any power supply
    harmonic that comes along.

    Look for ringing on supply edges and add a snubber
    across any diodes / switches. Put ferrite beads on
    the power and return legs at the supply. Put the
    supply or the radio in a can. Etc.
    Thx.
    i think this circuit makes a simple PLL in case it's FM receiver. I made it and play well.



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    Re: Help me fix the noise in a FM receiver with AC/DC rectifier

    Quote Originally Posted by contau View Post
    Thx.
    i think this circuit makes a simple PLL in case it's FM receiver. I made it and play well.
    There is no loop so it is not a PLL.
    The transistor is an AM detector. It poorly detects FM with "slope detection".
    AM has amplitude changes and noise also has amplitude changes.
    An FM detector detects frequency changes and ignors amplitude changes.



    •   Alt14th January 2011, 22:11

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    Re: Help me fix the noise in a FM receiver with AC/DC rectifier

    Quote Originally Posted by Audioguru View Post
    There is no loop so it is not a PLL.
    The transistor is an AM detector. It poorly detects FM with "slope detection".
    AM has amplitude changes and noise also has amplitude changes.
    An FM detector detects frequency changes and ignors amplitude changes.
    why not? i prove this is a PLL.



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    Re: Help me fix the noise in a FM receiver with AC/DC rectifier

    The transistor is simply an emitter-follower with no voltage gain. It passes frequency changes (FM) and also passes signal level changes (AM) including static noise. It has a high input resistance so it does not load down the tuning parts.
    Coil L1 changes its reactance as the input frequency is changed by modulation so its conductance of the DC from the transistor decreases when the signal frequency increases and its conductance of the DC from the transistor increases as the signal frequency decreases. It is called "slope detection".

    A Phase-Locked-Loop has a voltage controlled oscillator and a phase detector. This simple circuit does not have them.

    Please turn off the dots in your Multisim schematic.



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    Re: Help me fix the noise in a FM receiver with AC/DC rectifier

    Quote Originally Posted by Audioguru View Post
    A Phase-Locked-Loop has a voltage controlled oscillator and a phase detector. This simple circuit does not have them.
    voltage controlled oscillator and a phase detector is inside transistor. The varicap Ccb will change due to audio feedback, so the oscillator fre will be changed --> creat a VCO.
    Transistor will multiply signal frequency and oscillator frequency --> creat a phase detector.



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    Re: Help me fix the noise in a FM receiver with AC/DC rectifier

    Quote Originally Posted by contau View Post
    voltage controlled oscillator and a phase detector is inside transistor.
    No. It is just a simple transistor junction. There is nothing else "inside" the transistor. And you've chosen a transistor with a particularly poor gain-bandwidth product for radio frequencies, at 250MHz it will struggle to give you the voltage gain which we'd expect from the front end of a receiver.



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    Re: Help me fix the noise in a FM receiver with AC/DC rectifier

    it's superregenerator circuit, well at least your prototype working like that



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    Re: Help me fix the noise in a FM receiver with AC/DC rectifier

    The transistor might be operating as a super-regen AM detector.
    Inductance of wiring (especially if made on a breadboard) causes the collector to have a high impedance then stray capacitance (especially if made on a breadboard) causes positive feedback from the collector to the emitter.



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    Re: Help me fix the noise in a FM receiver with AC/DC rectifier

    I don't think there's even regeneration; the transistor is
    really just the diode AM detector, with current gain.



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    Re: Help me fix the noise in a FM receiver with AC/DC rectifier

    Quote Originally Posted by DXNewcastle View Post
    No. It is just a simple transistor junction. There is nothing else "inside" the transistor.
    I mean voltage controlled oscillator and a phase detector are inside transistor circuit. oscilator frequency depends on audio signal feedback from E pin.



    •   Alt16th January 2011, 10:26

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    Re: Help me fix the noise in a FM receiver with AC/DC rectifier

    Contau, either you have discovered something we have all missed, or you are wrong. There is no phase detector and there is no audio feedback.
    The circuit may work but it's operation is either as a plain simple rectifier or, depending on stray capacitive and inductive effects, it may oscillate and work super-regeneratively. If you are able to receive FM signals, it is not because it's an FM receiver, it just means you are tuned to one side of the signal and as it 'moves' in and out of tuning range, the change in strength appears as a change in signal level.
    In essence, it's an emitter follower using a transistor beyond it's design frequency and with a strange and inefficient varicap tuned circuit at it's input. Even using similar components it's possible to make something far better.

    Brian.



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    Re: Help me fix the noise in a FM receiver with AC/DC rectifier

    Quote Originally Posted by betwixt View Post
    .There is no phase detector.
    In essence, it's an emitter follower using a transistor beyond it's design frequency and with a strange and inefficient varicap tuned circuit at it's input.
    .
    I think the transistor multiply the fm signal to oscilator signal --> "phase detector". The output signal of this "phase detector" will moved to emitter and pass over lowpass filter. At here, lowpass signal will reverse to base. This circuit will retransmit anything what it received.
    I have no adjustable capacitor, so the varicap i used is junction CB of any transistor.
    Last edited by contau; 16th January 2011 at 16:45.



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