I'm looking for more range

Discussion in 'General Electronics Chat' started by mik3ca, Apr 13, 2008.

  1. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    see attachment for my FM transmitter.

    It works weird.

    When my transmitter is close to my regen receiver and my commercial radio, it floods part of a station. I'm afraid to change the transmitter frequency because some frequencies cause interference to my moms TV.

    When I'm about say 15 meters away from the transmitter, I seem to pick up the signal at two frequency points on my superregen. It seems that as I begin my tuning, the 108Mhz station (Y108 in hamilton) comes in first, then as that fades, my music that I transmit comes in nicely, then I get silence, then I get my music again, and then finally white noise.

    When I am farther away (about 250 or so feet), I can pick it up if I hit the frequency dead-on and the receiver is in the correct position.

    Is there any way that I can increase the range of my transmitter, and maintain the quality of the sound without increasing the voltage or changing the transmitting frequency?
     
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  2. scubasteve_911

    Senior Member

    Dec 27, 2007
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    I think we need to analyse this circuit before we can give any intelligent suggestions. Personally, I find this circuit a bit daunting and cannot quite understand it. Perhaps someone would be kind enough to give a basic functionality description?

    The first stage is what I understand to be the oscillator / modulator. It seems that the tuned circuit would set the frequency, then the current is being modulated and causing the impedance of the inductor to vary;thus, modulating frequency? The second stage seems to be a simple RF amplifier with an appropriate inductance on the collector and the output should be off of the collector. The input is being biased by the 200K resistor.

    I really would like a play-by-play if anyone has the time..

    Steve
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    Hi Mike,
    Yours is similar to my FM transmitter except:
    1) Its oscillator has a resistor from its collector to its base for bias. This loads down the tuned circuit at the collector and causes the carrier waveform to be distorted.
    2) The second transistor has a base current too small. The 200k resistor should be 33kk if the transistor has an hFE of 200.
    3) The second transistor should have a tuned circuit at its collector so its harmonics are reduced. The harmonics cause TV interference.

    I simulated yours and corrected it and simulated it again. Mine has more output power and fewer harmonics.
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    What are you using for the NPN transistors, Mike?

    If you're using other than the 2N3904's in Audioguru's simulation, the numbers are going to be different.
     
  5. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    Pn3563 are my transistors.

    My transmitter frequency could be retuned at any time by me adjusting the capacitor. Having two tuned circuits could make matters more complex for me.

    I guess I should use a low-value resistor instead of the inductor in the buffer stage.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    My FM transmitter uses 2N3904 transistors because they work well up to at least 300MHz and have a narrow range of hFE from 100 to 300 so they can be biased easily.

    The PN3563 transistors have a very wide range of hFE from 20 to 200 so you don't know what it will do unless you measure its hFE then adjust the bias current to force it to work.

    The tuned circuit in the output stage of my FM transmitter has a broad response. If I tune it to the center of the FM band (about 100MHz) then the response drops a little at 88MHz and 108MHz. It reduces the 200MHz second harmonic and the 300MHz third harmonic and higher harmonics a lot.

    The output RF amplifier (buffer stage?) would have low or no gain if it had a low value resistor instead of the inductor which is a fairly high impedance. A tuned circuit is a high impedance at its tuned frequency, creates gain and "shorts" high frequency harmonics. The output voltage with my tuned circuit is almost double the supply voltage. Double the voltage makes 4 times the power.

    Hi ScubaSteve,
    The FM transmitter has an oscillator that has its frequency modulated by an effect of AM modulation. The AM amplitude-modulates the collector voltage of the oscillator transistor which causes its capacitance to be modulated. Since its capacitance is modulated then its frequency is also modulated.
     
  7. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    I measured the hFE of 2 PN3563's and both of them turned out to be between 120 and 150. I think I saw 141 on the multimeter display, but I will double check.

    The second RF transistor is there for buffering, so that waving my hand over the transmitter antenna doesn't change frequency so much.

    I was thinking, wouldn't it be better if we could merge the two tuned circuits that you suggest into one?

    For example: make the the tuned circuit so that one end connects to VCC and the the other end connects to both trasnsistor collectors.
     
  8. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
    1
    Hi ScubaSteve,
    The FM transmitter has an oscillator that has its frequency modulated by an effect of AM modulation. The AM amplitude-modulates the collector voltage of the oscillator transistor which causes its capacitance to be modulated. Since its capacitance is modulated then its frequency is also modulated

    Thank you sir. I still don't understand the relationship between collector voltage and capacitance, isn't capacitance fixed? I can see that if you limit the range of voltages that the capacitor can see, then you would be effectively modulating the energy that can be stored within. This is why I assumed that the inductor was acting like a variable inductance with current, because I am aware of that relationship. I'm going to read up on the subject.

    Steve
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    No.
    All semiconductors are voltage-variable capacitors when they are reverse-biased so that they do not conduct.
    The datasheet for a diode or for a transistor shows the capacitance changing when the voltage is changed.
    The modulation changes the voltage and the changing capacitance changes the frequency of the LC tuned circuit.

    Here is the capacitance of a 2N3904 ransistor and a 2N4401 transistor.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Audioguru is correct.

    Something else that may surprise you is that the values of capacitors changes drastically once you get near the GHz range. A ceramic SMT 47pF cap may measure that @ DC frequencies (below 50MHz), but get it up to 500MHz, and it'll triple in apparent value.
     
  11. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
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    Audioguro,

    Thanks a lot for clearing that up, don't tell my semiconductor physics professor that I didn't know that! Before, I was question that mere modulation of a voltage on a capacitor was changing its capacitance, I didn't read into the fact that it was acting like a varactor. Nonetheless, this is really neat stuff! Radio has always been like black magic to me, which is why I stick to very highly integrated devices :(

    Wookie,

    That does surprise me. I wonder why that is.. I know that as frequencies increase, inductive reactance may overcome the capacitance (SRF), but I wasn't aware of this phenomena. Do you have a link? I was briefly in electromagnetic physics and decided to take it next semester instead, but I don't want to wait :p

    Steve
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    We're deviating too much from the original question. Write me offlist.
     
  13. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    Back to my transmitter range topic.

    I was looking at the following schematic that Audioguru made for one of his transmitters.

    [​IMG]

    I was just curious, why is C12 30pF instead of 10pF?
    Why is C5 only 470pF instead of 10nF?
     
  14. Audioguru

    New Member

    Dec 20, 2007
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    For more signal to the output amplifier. My circuit has the same undistorted output level when it has a 6V supply as the circuit I corrected for you.

    The output impedance of the mic preamp is R4 in parallel with R6 and in parallel with the input impedance of the transistor. So the impedance is 6.9k.

    The upper audio frequency of broadcast FM radio is 15kHz.
    If C5 is 10nF then an audio frequency of 2.3kHz would be reduced to -3dB and 15kHz would be reduced to only -16dB like a no-treble muffled AM radio.

    470pF has a reactance of only 3.4 ohms at 100MHz so is fine to be an RF short to ground for the base of the oscillator transistor.
    The 470pf capacitor reduces an audio (?) frequency of 49kHz by -3dB and reduces 15kHz by almost nothing. So the sound is crisp and clear.

    Somebody on another website modified my transmitter and got much more power from it.
     
  15. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    I was thinking, if C12 was high enough, all the low frequencies from the previous stage would pass through. what is the absolute maximum value I should use for C12?

    I normally use higher coupling capacitors to couple stages (at least in nF units).

    and I noticed with my design that when I tune my receiver to the transmitter frequency, and I am 1 meter away from it, I get a loud (partly-distorted) version of my signal. As I slowly tune the receiver cap, the volume goes down to about less-than normal volume of the source (which is my laptop earphone jack). Then it remains constant like that (with little white noise in the background) until I tune the cap far enough, then I get white noise. I'm thinking that could be because I didn't convert the last inductor pull-up to a second tank circuit of the same frequency as the first.
     
  16. Audioguru

    New Member

    Dec 20, 2007
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  17. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    So I should adjust R8 and C12 to make the lowest passible frequency of the carrier.

    I never thought that an extremely high capacitor value would flood the transistor.

    Isn't the goal here to make the capacitor reactance to 0 ohms at the lowest frequency we want to pass?
     
  18. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    I'm doing my circuit on a breadboard. How should I adjust the capacitances? should I lower the values by about 20%?
     
  19. Audioguru

    New Member

    Dec 20, 2007
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    No,
    Tuning the circuit from 100MHz to 120MHz is only a 20% change in frequency. The change in the amount of signal passed by a coupling capacitor in such a small difference of frequency is negligible.

    No.
    A 10pF or 33pF coupling capacitor in a 100MHz circuit is fine. A 1nF or higher won't make any difference because the 33pF capacitor passes a signal that is almost as high.

    Learn about voltage dividers. A divider with 99% is almost 100%.
    A 10pF coupling capacitor in my circuit passes maybe 80% of the signal. A 33pF capacitor passes maybe 95% of the signal. A 330pF capacitor passes maybe 99% of the signal. A 1.5nF capacitor passes maybe 99.5% of the signal. You won't see any difference if the value of the capacitor is larger until it becomes inductive, then it doesn't work.

    Of course not. Make the reactance low enough so that the loss is small.
    Above I explained that the loss with a 33pF coupling capacitor in my circuit is only 5% and you won't notice the tiny loss in signal level. When the circuit has a little less loss then it clips and causes interference harmonics.
    A huge capacitor is an inductor, not a capacitor at radio frequencies.

    Power is logarithmic. You can hardly notice double or half the power. 5% is nothing.
     
  20. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
    189
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    I had a look at this one as well:

    [​IMG]

    What is the advantage to having C6?
    and why is RFC1 trying to pull TR2's NPN's base to ground?
     
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