recommend me a low-value resistor

Discussion in 'Wireless & RF Design' started by mik3ca, Apr 19, 2008.

  1. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    It seems that the last stage in my transmitter controls the transmission distance.
    My last stage is the same as the last stage (after the 10pF coupling cap) in the following circuit:

    [​IMG]

    EXCEPT:

    for my first transmission attempt:

    330 ohm resistor is replaced with a 2.2uH inductor.
    150K resistor is replaced with a 36K resistor.
    the NPN is replaced with a PN3563 NPN.

    It turns out that I get a clean signal for the first street block. As soon as I enter the second block, the signal fades in and out. When I make it to the third block, the signal is mostly faded out.

    for my second transmission attempt:

    330 ohm resistor is replaced with a 2.2uH inductor.
    150K resistor is replaced with a 36K resistor.
    the NPN is replaced with a PN3563 NPN.
    and I replaced the short between NPN emitter and ground with a 220 ohm resistor, and I connected a 220uF capacitor across that resistor.

    It turns out that I get a clean signal for 1/2 the first street block. As soon as I finish the first street block, the signal is mostly gone.

    The problem with the first attempt is that the NPN is warm to the touch. It isn't super hot, just warm. The problem with the second attempt is that the range is heavily reduced.

    If I did the math, it seems that the resistor connected to the base in both scenarios is not high enough to inject too much current. (333 uA is nowhere near 50mA which is the maximum current for my NPN).

    I think the best solution for me is to replace the 220 ohm with a ridiculously low value resistor. Although, I could use heat sinks and deal with the warmth.

    What do you guys think?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    2N2222's are milliwatt capable transistors. You might have some luck using the final stage on the schematic as a driver for a poopier final. More power out of 2222's is like more speed out of a Nash Rambler.

    Be a bit cautious, too. The FCC really comes visiting if someone complains about interference coming in on a commercial station.
     
  3. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    I tuned my transmitter and tested my cheap commercial receiver against it. No one will be affected.
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    Hi Mike,
    Of course the emitter resistor of 220 ohms reduced the range because the electrolytic capacitor across it does nothing at RF frequencies and the antenna is about 75 ohms. So the transistor has a loss of 75/220= 0.34 instead of some gain.

    I simulated your circuit. The output transistor clips and is cutoff making a distorted square-wave because it has an inductor instead of an LC tuned circuit. The distortion causes interference harmonics.

    The schematic has a 3V battery but I think your circuit has a 6V battery. Then the output RF power is about 13mW of signal plus 13mW of harmonics.
    The output signal and harmonics have more power if the coupling capacitor to the base of the output transistor is 33pf instead of 10pF.

    Your PN3563 has a very wide variation in current gain but I think you measured yours and they are about 120. The 5.3V across the 36k base resistor creates a base current of 147uA. The collector current is 147uA x 120= 17.6mA. The dissipation in the transistor is 6V x 17.6mA= 106mW which is not much.

    The RF cops might catch you due to the harmonics you are transmitting. If you transmit on the aircraft band (a little higher than broadcast band FM) then you will be in big trouble.
     
  5. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    when my receiver is farther away from the transmitter, I don't pick up harmonics. I only get the signal on one frequency. and my coupling cap is 33pF.

    So I guess my resistor should me like 50 ohms with no capacitor across it?

    and why does the electrolytic capacitor across 220 ohms do nothing?
    does it not make the overall resistance low at high frequencies? After all, lower resistance = more range. right?
     
  6. rwmoekoe

    Active Member

    Mar 1, 2007
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    i don't understand, in the other thread audioguru has told about this, right? the base resistor being to low and all.
    why don't you try it. he posted a simulation plot showing what he meant, is it right?

    but now i understand why. you thought the base resistor follows the same rule you mentioned:
     
  7. Audioguru

    New Member

    Dec 20, 2007
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    Please learn about harmonics.
    The second harmonic of a 100MHz signal is 200MHz. The third harmonic is 300MHz. The fourth harmonic is 400MHz. The fifth harmonic is 500MHz. Etc.
    Your radio doesn't receive them but TVs and other communications (police radios etc) will receive the interference from the harmonics.

    Your radio's input overloads when your transmitter is near. A cheap radio overloads from a strong signal. a good radio doesn't.

    It will just reduce the range of the transmitter.

    I told you before that an electrolytic capacitor has too much inductance to be a capacitor at VHF frequencies. A 1nF ceramic disc capacitor has very low inductance and has a reactance of only 1.6 ohms at 100MHz. A 0.1uF (100nf) ceramic disc capacitor begins to be inductive at 100MHz.

    A 220uF electrolytic capacitor works well at audio frequencies. Its reactance is 1.6 ohms at 455Hz. Its reactance rises when it becomes inductive above about 200kHz.
     
  8. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    Is there math that can tell me how much inductance a capacitor has at high frequencies?

    In the mean time, I'll try a capacitor valued between 100pF and 1nF across the 220 ohm resistor.

    With regards to harmonics, the higher the harmonic level, the weaker the signal right?
    It would be nice if I can eliminate harmonics in my transmitter without having to use another tuning capacitor.
     
  9. Audioguru

    New Member

    Dec 20, 2007
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    Every make and size of capacitor is different.
    Wikipedia says that an electrolytic capacitor is unsuitable at higher frequencies.
    A capacitor manufacturer says,
    "Capacitance Frequency characteristics

    The effective capacitance decreases as frequency increases.
    Self-resonance is typically below 100 kHz depending on
    capacitance. At self-resonance the device is resistive and
    beyond it is inductive."
    http://electrochem.cwru.edu/ed/encycl/misc/c04-appguide.pdf

    A bypassed 220 ohm resistor in series with the emitter of your output transistor will reduce the supply voltage to it to 645 and its output power will be 41%. its heat dissipation will also be 41%.


    A perfect square-wave has the same amount of power in its fundamental frequency as in all the harmonics combined. So a square-wave has 50% distortion. Half of its output power is wasted.

    The amplitude of each harmonic diminishes at higher numbers.

    Like I said before, the tuned circuit at the output is broad enough to cover the entire 88MHz to 108MHz FM broadcast band when it is tuned to the center at 100MHz. Then the second harmonic at 200MHz is reduced a lot and higher harmonics are reduced more.
    The transistor needs the proper amount of drive level and proper biasing so that it doesn't produce harmonics.

    Isn't the supply voltage of your transmitter 6V? The last circuit you posted is designed for 3V.
     
  10. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    yes, mine is 6V.

    before your last 2 posts, I modified the final stage, so that the emitter is grounded, and the collector has the 220 ohm and a capacitor (that I'm trying to figure out the best value for) connected across it.

    Before, when I was screwing the TV up, I realized that I was actually transmitting on the wrong frequency. It turned out that the first harmonic (base frequency) was at around 400 - 600Mhz, just because I had a colpitts design using a 0.1uH inductor, a variable capacitor between 2 and 7pF (across the inductor) and a 10pF capacitor (across the transistor).

    as soon as I changed it to a 0.1uH inductor, a 57pF - 62pF variable capacitor (connected across the inductor), and no capacitor across the transistor, the TV interference is completely gone, and I went through channels 35 through 60 to verify it.

    I'll continue to play around with it. If I could squeeze the harmonic frequencies together more, then it would be a bonus to me. Something like 110Mhz as base, 2nd harmonics of 109Mhz and 111Mhz, and 3rd harmonics of 108Mhz and 112Mhz, etc, would be better than the standard harmonics like you were mentioning.

    also, Don't standard radio station towers transmit harmonics?
     
  11. beenthere

    Retired Moderator

    Apr 20, 2004
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    Hardly at all - Uncle Charlie will levy fines until they go away. Any emission outside of the alloted bandwith for the assigned frequency is unacceptable
     
  12. Audioguru

    New Member

    Dec 20, 2007
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    Please learn about how to bias a transistor so it doesn't clip.
    The base current is (6V - 0.7V)/36k= 0.15mA.
    The current gain is 120 so the collector current is 0.15mA x 120= 18mA.
    The voltage across 220 ohms is 18mA x 220= 3.96V.
    So the transistor is almost saturated and clips the bottom of the waveform. The level is much lower than if a tuned circuit is used.
    Adding a capacitor across the resistor shorts it and reduces the level more.

    They are designed to produce a very low level of harmonics.
    If they transmitted harmonics then everything would have interference.
     
  13. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    The answer is yes.

    Typically the harmoics are -60 dB with respect to the carrier as they are not to interfere with other services.

    The FCC doesn't like you overmodulating your signal either, as that bleeds over to adjacent channels in the service.
     
  14. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    Here is the way I see it using an hFE of 140:

    6V / 220 ohms = 27.3mA - Collector current
    27.3mA / 140 = 195uA - Base current
    6V - 0.7V = 5.3V - Base voltage (assuming transistor turn-on voltage is 0.7)
    5 / 195uA = 25.6K - Resistance from +ve.

    So from that math, 36K is too high, and I should go for something between 24K and 27K?
    or am I missing some math?
     
  15. Audioguru

    New Member

    Dec 20, 2007
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    The 220 ohm resistor is supposed to have half the supply voltage across it so the collector voltage can swing the maximum when it has a resistor as its load. You have the entire supply voltage across it so your calculation transistor is saturated with its collector voltage at ground.

    3V/220 ohms= 13.6mA. The hFE is 140 if you measured it so the base current needs to be 97.4uA. The base resistor needs to be (6V - 0.7V)/97.4uA= 54.4k.

    If a tuned circuit is the load for the output transistor then its collector voltage is 6V at rest. If it is biased properly then it can swing from almost 0V to almost 12V. Its total load is the 75 ohms of the antenna. Yours is 220 ohms//75 ohms= only 56 ohms which makes its level low.
     
  16. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    I should knock myself out right now. LOL.

    So now I should take the transistor as 3V, and the pull-up resistor as 3V.
    I see where you are coming from.

    and should I change the 220 ohm resistor to 75 ohms and change my base resistor and apply the transistor biasing math again?
     
  17. Audioguru

    New Member

    Dec 20, 2007
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    I simulated the circuit with a tuned circuit and the output transistor biased correctly. The output level was high and the harmonics level was low.

    Then I replaced the tuned circuit with 220 ohms and re-biased, then tried 75 ohms and re-biased.
    With the resistors the output level was low and the harmonics level was high.
    With a 75 ohm collector resistor the current is 40mA. The transistor and resistor each have a heat dissipation of 120mW.
     
  18. mik3ca

    Thread Starter Active Member

    Feb 11, 2007
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    I want to avoid having two tank circuits containing two variable capacitors.

    I tried connecting the two NPN collectors together, because I wanted the one tank circuit to apply to both stages. That didn't work.

    So if I were to have a 2nd tank circuit like you suggest, and I want to fix the tank circuit at a certain frequency (so I don't have to adjust the 2nd tank ever again), what should it be if I am trying to transmit on frequencies between 108.5Mhz and 115Mhz?
     
  19. Audioguru

    New Member

    Dec 20, 2007
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    I think you are asking for trouble by transmitting on the aviation band's frequencies.
    How far are you from Hamilton's airport?

    The ILS guides planes to the landing runway when the weather is bad. Your transmitter might guide them to your house.
     
  20. SgtWookie

    Expert

    Jul 17, 2007
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    It's going to be expensive for your parents to come visit you while you are in jail for interfering with aviation communications, so they may have to forget visitations.

    No electronics in jail either, I'm afraid.

    Why not put the transmitter idea aside, and work on building a superheterodyne receiver instead?

    Very little chance of getting in any trouble there - unless you start changing parts at random again, causing broadband emissions.
     
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