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LM358 to make FM Audio Receiver

Discussion in 'Audio' started by Xyq, Mar 23, 2017.

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  1. Xyq

    Xyq

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    Mar 23, 2017
    and why use 2N3904 ? can use different transistors ?
     
  2. Audioguru

    Audioguru

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    Sep 24, 2016
    What does R1 do? It applies a current to the electret microphone. Why? Look up Electret Microphone in Google to see that it is a condenser microphone that has 48V permanently embedded in its electret material so that when its membrane vibrates with sounds then it is a variable capacitance voltage divider producing a signal that has an extremely high impedance. How can it be used when its impedance is so high? Because it has a Jfet inside that the datasheet of an electret mic says it uses about 0.5mA at a few volts which is provided by R1 in the circuit. The Jfet has an extremely high input impedance and a fairly low output impedance.

    What does C1 do? It passes the mic signal to Q1 and it blocks the differences in their DC voltages.

    What does C2 do? It passes the amplified audio signal from Q1 to Q2 and it blocks the differences in their DC voltages.

    What does R6 do? It applies base current to Q2 so that it can conduct collector to emitter current.

    What does C3 do? It bypasses RF signals at the base to the positive supply (which should be grounded) so that Q2 is a common base RF amplifier but the low value of C3 allows audio signals to modulate the current in Q2.

    What does C1 parallel with L1 do? They are a resonant tuned RF circuit that has a high impedance at their resonant frequency. C1 has a variable capacitance so it tunes the transmitted radio frequency.

    What does C5 do? It applies positive feedback from the collector output of Q2 to its emitter (which is its RF input) so it oscillates at the tuned LC frequency. C5, the internal capacitance of Q2, the capacitance of the antenna and stray capacitance of the wires between them affect the tuned LC frequency.

    The circuit is missing an importance supply bypass capacitor.

    How does the circuit produce FM? The audio at the base of Q2 causes it to conduct more and conduct less which the datasheet of a 2N3904 transistor shows that then its capacitance changes. Then the frequency of the RF oscillation also changes which is FM. Since Q2 conducts more and less by the audio then it also produces some AM that an FM radio ignores.

    Didn't your teacher teach you these basic things??
     
    bushtech likes this.
  3. Audioguru

    Audioguru

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    Sep 24, 2016
    I guess you have not seen the datasheet for the 2N3904 transistor. It show it working well up to 300MHz.
    You can use any transistor that works well up to 300MHz but Asian transistors, European transistors and American transistors all have different pinouts.
     
  4. Xyq

    Xyq

    19
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    Mar 23, 2017
    Hi Audioguru, I studied the each components but I really don't know the circuit.
    Below are my understanding:

    1 is Carrier Oscillator
    2 is Buffer Amplifier
    3 is Reactance Modulator
    4 is Frequency multiplier
    5 is Power output Amplifier

    Is the correct place to add bypass capacitor? what is the value ? 100nF?

    Thanks for your patient !
     

    Attached Files:

  5. davenn

    davenn Moderator

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    Sep 5, 2009
    I really don't know where you got those descriptions from ??

    1 = microphone power supply and audio out
    2 = microphone amplifier
    3 = is just biasing for Q2
    4 and 5 = RF oscillator
     
  6. Audioguru

    Audioguru

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    Sep 24, 2016
    Xyq, it seems that you did not read my descriptions of the parts and that you know almost nothing about simple electronic circuits. Don't you have a teacher and a text book?

    1) Is a resistor that powers the junction field effect transistor inside the electret mic and a capacitor that couples the microphone signal to Q1 and blocks the DC differences. A resistor and capacitor are not an oscillator and their values are for audio frequencies, not for the very high carrier frequency. Q2 and the parts with it is the carrier oscillator.

    2) Is the microphone preamplifier, not a buffer amplifier.

    3) Is a resistor to provide base current for Q2 and a filter capacitor, not a reactance modulator.

    4) Is a simple LC tuned circuit, not a frequency multiplier. A frequency multiplier would be an entire complicated circuit or IC. A frequency multiplier is not needed in this simple circuit because Q2 oscillated at the required frequency.

    5) Is the low power, radio frequency oscillator, not a power amplifier.

    I said that the circuit needs a supply bypass capacitor which is parallel with the battery. You added it parallel to the emitter resistor of Q1 which would increase its audio gain so high that the audio would be extremely distorted.
     
  7. Xyq

    Xyq

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    0
    Mar 23, 2017
    Hi AudioGuru,
    currently we are experiencing great difficulty of getting the trimmer capacitor to the right frequency of the radio receiver. As it is mount on breadboard. Its pins did not stand well and with noise somemore. Our instructor has advised other classmate of doing trimmer resistor, which can just replace the trimmer with a fixed value resistor after find out the right value. So I was wondering can we do the same for trimmer capacitor. Let's say we set the radio frequency at 100MHz, what value of capacitor can we use to replace the trimmer? And how is the calculation?

    As we did try soldering on PCB, however due to lacking of tools, it failed eventually. And some components are burned as I believed. So we only left choice which is to mount it on breadboard.

    You've been great help of us, once again thanks for your kind help and explanation.
     

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  8. Audioguru

    Audioguru

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    Sep 24, 2016
    You were told before that the very high stray capacitance between the rows of contacts and the wires all over the place on a solderless breadboard cause a radio circuit not to work. Also the long wires have inductance that the circuit does not want. Also your parts are too far apart. Also your coil is spread out too much.

    Since you burnt some parts then your soldering is terrible and/or the soldering iron is cheap without temperature control so it gets too hot.

    It is impossible to change the frequency of an LC tuned circuit with a trimmer resistor.

    Here is comparison of yours and mine:
     

    Attached Files:

  9. davenn

    davenn Moderator

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    Sep 5, 2009
    as AG said, you were advised earlier by others of us that you cannot breadboard this circuit ..... IT WONT WORK

    so not surprising you are having problems, you need to listen to the advice you are being given and follow it
     
  10. Xyq

    Xyq

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    Mar 23, 2017
    we will solder on PCB board. I have done a transmitter simulation on LTSpice. I don't know what is wrong for my LTspice configuration? Do I need to add frequency formula? the step command is wrong. I really don't know. You've been great help of us, once again thanks for your kind help and explanation.
     

    Attached Files:

  11. Audioguru

    Audioguru

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    Sep 24, 2016
    What do you expect to see in your simulation? The timebase is set to see the carrier AM modulation but the oscillator is very overloaded by your 300mV audio signal that is 30 times higher than a mic signal and the value of C4 is almost nothing.

    I think you should first get your oscillator working without modulation and without the impedance of an antenna like this:
     

    Attached Files:

  12. Xyq

    Xyq

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    Mar 23, 2017
    Audioguru, I have tired just RF oscillator but something is wrong

    other simulation is for whole circuit
    Location 1 is my measurement point
    What I expect to see is : Location 2: can we see the Sine Wave after microphone preamplifier ?
    Location 1: can we see the original Sine wave from mic phone?
    we have measured location 2 and 1, but only see the straight line (voltage line) - don't know why

    and C4 is trimmer capacitor. we tired use STEP command to change capacitance as the frequency(? ) (or voltage) will change on simulation.
     

    Attached Files:

  13. Audioguru

    Audioguru

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    Sep 24, 2016
    You must reduce the timebase to see audio at your locations 2 or 3. Use tran 0 100ms 95ms 1ms.
    It is impossible to see an audio signal at the antenna.
     
  14. Xyq

    Xyq

    19
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    Mar 23, 2017
    Thank you so much. it is working.
    Can I use STEP command to change Trimmer Capacitor as audio is changing? but I cant see the waveform (only trimmer capacitor). what is wrong? Thanks
     

    Attached Files:

  15. Xyq

    Xyq

    19
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    Mar 23, 2017
    We have done other simulation and block diagram of this transmitter.once again thanks for your kind help and explanation.
     

    Attached Files:

  16. Audioguru

    Audioguru

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    Sep 24, 2016
    There is no "modulator". The mic preamp modulates the current in the RF oscillator transistor which produces a little AM that an FM radio ignores. The changing current in the oscillator transistor also changes its capacitance which varies the frequency of the oscillator producing FM.
     
  17. Xyq

    Xyq

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    Mar 23, 2017
    Thanks . btw can I ask u how to simulate the trimmer capacitor and Frequency?
     

    Attached Files:

  18. Audioguru

    Audioguru

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    Sep 24, 2016
    Sorry, I make design and build circuits, not simulations. I have never stepped a capacitance in an RF circuit when the RF is a sweep, and I think 100M is not 100 Megahertz, I think 100m should be used.
     
  19. Xyq

    Xyq

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    Mar 23, 2017
    Thanks a lot . Audioguru.

    I have drawn the block diagram , can u help me check? is a low-pass filter?
    what mic preamplifier is doing? mic preamplifier is increasing a microphone signal and preventing induced noise that would otherwise distort the signal

    Thanks again
     

    Attached Files:

  20. Audioguru

    Audioguru

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    614
    Sep 24, 2016
    C3 grounds the base of Q2 at RF frequencies so that it can be a common base amplifier that oscillates. Calculate the reactance of the 0.01μF capacitor C3 at 100Mhz: 0.01 ohm.

    Is C3 part of a lowpass filter that cuts audio high frequencies? Simply calculate the cutoff frequency: Yes, 3200Hz which is audio and is another error of this defective circuit. My fix of this circuit uses 470pF, not 10,000pF so it does not affect audio and cuts ultrasonic frequencies above 41kHz. At 100MHz my 470pF capacitor has a reactance of 3.4 ohms which is low enough.

    The mic preamp increases the 10mV or less level of the mic almost 10k ohms / 100 ohms= 100 times which is too much. But the circuit is defective again since it is missing a capacitor to block the RF from the oscillator from also being amplified. My fix of this circuit adds a 100pF capacitor to the base-emitter of Q1 so that it does not amplify the RF from the oscillator.

    Amplifying the mic signal does not prevent noise, but the amplification in this defective circuit is too much which causes distortion. My fix has the mic preamp gain with a little less than 17 times.
     
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