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Need Help in Loop Antenna Building

Discussion in 'Electronic Projects Design/Ideas/Reviews' started by Harros, Feb 11, 2008.

  1. RadioRon

    RadioRon Well-Known Member

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    I would use no voltage divider or perhaps a modest voltage division ratio, but you don't need to attenuate the signal. A better approach is to use the same transistor in your amplifier but alter the bias point by adjusting the resistor values. When you have a lot of voltage swing available like this and you aren't carrying modulation on the carrier, a simple class C bias is appropriate. You can achieve this by removing the base bias resistors and simply letting the input signal turn the transistor on and off. You could also reduce the emitter resistor a little bit too so that more output power can be had.
     
  2. Harros

    Harros New Member

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    Is there any example of this type of amplifier?
     
  3. RadioRon

    RadioRon Well-Known Member

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    When I looked on the web, most of the examples of class C operation are at much higher power levels and a different type of output circuit is used. So I cannot find a good example of the type you are doing. It is not much different from the amp we have discussed before.

    If you google on Class C RF Amplifier you will see examples of high power amps and you may get some inspiration from those.
     
  4. dave

    Dave New Member

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  5. Harros

    Harros New Member

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    Well, I have done etching the pcbs for the system, however, the ground plane is over-etched where the ground plane seems to be a little bit faded out. I have tried several times etching the pcb, but this still happens to the ground plane. Is it alright to have over-etched ground plane? Will this reduce the performance of the block or cause the block to be malfunctioned?
     
    Last edited: Mar 18, 2008
  6. Harros

    Harros New Member

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    I have solved the etching problem, the ground plane in all the circuits are perfect in shape as well as the connectivity... Well, regarding the Class C Amp, I have designed an Class C Amp bu referring to a example model on internet. But the circuit is not working... Did I do any mistakes in designing this amp circuit?
     

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  7. RadioRon

    RadioRon Well-Known Member

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    This is my mistake in not advising in a previous post. The problem is that you still need something on the base to insure it remains at 0V dc instead of accumulating negative charge while rectifying the RF signal. The normal way to deal with this is to place an RF choke from the base to ground. The RF choke should have a relatively high impedance at the frequency of operation, perhaps on the order of 1000 ohms in this case. The choke is simply a way of insuring that the base remains at 0 VDC while not loading the AC signal.
     
  8. Harros

    Harros New Member

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    Is the RF choke the ferrite bead? By the way, i dont really know the correct method to design class C amp, as i never touched this type of amps before. Well, do you have any idea on the determining the value of the components (inductor, rf choke, and capacitors) used in the amp?

    Besides, I can hardly get a exact ferrite bead at my place for the particular ferrite bead used in the simulator, and I am thinking of editing the PSpice model of the ferrite bead... Do you have any idea on the corresponding values defined in the PSpice model of the ferrite bead?
     
    Last edited: Mar 20, 2008
  9. RadioRon

    RadioRon Well-Known Member

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    An RF Choke is an inductor (whose Q is not considered important or is sometimes intentionally lowered) with an inductance that presents an impedance to the circuit that encourages RF power to flow somewhere else. In other words, when used in a circuit, a very low amount of RF current flows through the choke when compared to the other directions that RF current can possibly go.

    Chokes are usually used to provide bias to an RF circuit. You can imagine that in the same way that a capacitor is useful for blocking DC and coupling AC into a circuit, the choke is useful for blocking AC and coupling DC into a circuit.

    Now, in your case your desired RF signal is at 1 MHz. To be a useful choke in your circuit, you have to estimate the input impedance of the base of the transistor and then set the inductance of the choke to be at least five times higher than that. This is quite difficult to do, so we will just guess that you will be inputting an average current of about 1 mA into the base with a voltage of about 0.7 volts, so our guess is 700 ohms. So, you need a choke that is about 3500 ohms of inductive reactance. That's quite a lot and there is no way a ferrite bead will get you anywhere near that. Such a choke will involve many (hundreds?) turns of fine wire on a ferrite core.

    For the component values, I suggest that you leave the collector load of your existing 2n2222 amplifier circuit as it is, perhaps reduce the emitter resistor a little bit, and then change the base bias to be only the choke to ground. All you are doing is taking your other amplifier and reducing the base bias to zero. Of course, this means that your collector bias current is also zero, but otherwise the resonant tank values can remain as they are.

    Edit: If you can't find a choke inductor, go ahead and try it with a resistor to ground from the base instead of an inductor. Perhaps a value of 1K ohms for starters.
     
    Last edited: Mar 20, 2008
  10. Harros

    Harros New Member

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    Well, I have redesigned this circuitry for the amp. The output is better than before, but the output waveform is not desirable. Any comment on this circuitry?
     

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  11. RadioRon

    RadioRon Well-Known Member

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    The output waveform tells me that the collector tuned circuit is not doing its job. In this kind of amplifier the transistor is turned on for approximately one half cycle at the input's frequency. We can see in the output waveform that this is indeed happening. However, when the transistor turns off during the other half cycle, it is the job of the resonant tank circuit attached to the collector to fill in the other half cycle by ringing. Ringing means allowing the stored energy to freely trade back and forth between the capacitance and inductance and in so doing having a current move around the tank circuit back and forth between inductor and capacitor at the resonant frequency of the tank. Since the current is moving back and forth, the voltage is therefore going up and down as well during ringing. The overall effect is that it smooths out your output waveform to become a perfect 1MHz sine wave. However, yours is not.

    You can see some ringing effect during the off cycle. We can use this to estimate the actual resonant frequency of the tank circuit. I see 5.5 cycles of ringing in a period of .5838 microseconds which is a frequency of 9.42 MHz. Assuming that the capacitors in your tank circuit are still fully involved, which is a safe assumption, this means that only 4.4 uH of inductance is involved in the tank circuit. As you can see, you have 4.7 uH on the hot side of your output point at which you have attached a 50 ohm load. So I conclude from this that the load is tapped far too heavily into the tank circuit and the 470 uH inductor is carrying no current.

    You can test this by removing the 50 ohm load and viewing the resulting output voltage.

    To fix this, you need to tap the 50 ohm load much higher towards the "cold" side of the inductor. Then you should vary the capacitor a little bit to maximise the output voltage.
     
    Last edited: Mar 21, 2008
  12. Harros

    Harros New Member

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    Alright, I have redesigned the amp (classcamp.pdf) by following your advices. The output now is desirable, as well as the output power (input+output.jpg)(output.jpg). Any comment on it? Is it alright to put a emitter resistor with such a small value? (5 ohm, it seems too small for me by the way...)

    There are too many rf choke available in the market as well as the rf choke parameters that i should look into (such as test frequency, SRF, Idc, Rdc, Core material...)(70f_series RF chokes.pdf). Do you have any suggestion on the rf choke that i should pick for this amp?


    Edit:

    I have finished cascading the oscillator with the class c amp. The output is desirable. However, the waveform is distorted (distorted.jpg) and shifted when the circuit initializes, and becomes stable (stable.jpg) after certain period (initialize_output.jpg, input_vs_output.jpg). Why this happens? Any comments or improvements on this oscillator?

    Besides, i found that the stable output waveform is not symmetrical (please refer to nonsymmetrical.jpg). Why this happens? Is there any approach to reduce the distortion?
     

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    Last edited: Mar 21, 2008
  13. RadioRon

    RadioRon Well-Known Member

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    I am not sure about the distortion, but I have some possible explanations. During the negative half cycle of output, the transistor amplifier configuration simply is not very linear in its operation. The best way to understand this is to plot the input and output signals on the set of characteristic curves (collector current vs base current) for this type of transistor and see how the current gain is changing depending on the moment by moment input current magnitude. So the output voltage will be somewhat nonlinear and this is what we see on the negative half cycle.

    On the positive half cycle, it would be different since in this half cycle the transistor is turned off. In this case, any distortion in the positive half cycle would be attributed to the inability of the tank circuit to remove enough harmonics to purify the waveform. So in addition to 1 MHz energy in the output tank, there is harmonics which are put there during the negative half cycle nonlinearity of the transistor operation plus the nonlinearity of the input signal to begin with. these harmonics are well attenuated, but not enough to eliminate them completely. The way to purify the output is to cascade another tank circuit to act as a band pass filter. Another way is to design a class C amplifier with a more linear device operation, but this is much less practical than adding an additional filter. All transmitters using class C power amplifiers include a substantial multi-pole (i.e 3 or more) low pass filter just before the antenna to remove harmonics created by the nonlinear class C process. So a bandpass filter would work well for you, or a low pass filter and a low pass filter is sometimes a bit easier to do. If using a low pass type, the cutoff frequency should be above 1 MHz but below the 2 MHz harmonic, so usually these are designed to have minimum attenuation at 1 Mhz but to have the cutoff frequency as close above that as possible.

    I think that you can use any rf choke or inductor that fits your criteria. The most important includes the series resonant frequency, which must remain well about your frequency of operation, your be careful of the maximum DC current above which you might burn the choke.

    edit: The behavior of the oscillator after power-on is normal. All oscillators build up to their stable operating condition rather slowly (that is, not instantaneously, but rather over periods that range from tens of microseconds to hundreds of milliseconds) after DC power is applied. This is because when DC power is first applied, the oscillator is in some random state. Imagine that the amplifier part of the oscillator is immediately ready to amplify but it has not received any feedback from the output yet because there is no oscillation to start with. Now, imagine that it is a very high gain amplifier and will amplify any tiny bits of noise or AC of any sort at the input and in fact is not happy to remain at a stable voltage because of its unstable design. When it does amplify the tiny bit of noise at the input (there is always a tiny bit of noise in everything), it then can feedback some part of this to the input thorugh the resonant circuit and begin its real function of amplifying only one particular frequency.
     
    Last edited: Mar 23, 2008
  14. Harros

    Harros New Member

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    Is it better for me to put the low pass filter after the power amp rather than between the power amp and the crystal oscillator?
     
  15. RadioRon

    RadioRon Well-Known Member

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    There is no choice, you must put it after the power amp.
     
  16. Harros

    Harros New Member

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    The output now is not distorted anymore and is desirable... Is it ok for us to amplify the square wave rather than the sine wave (the harmonic-removed siganl)? Or should we put two filter in the circuitry (1 for crystal oscillator harmonic removal, 1 for power amp harmonic removal)?

    By the way, i dont know which rf choke i should buy here, could you please give some recommendations of it? (a list of rf chokes is enclosed here, thank you)

    I am thinking of power up this transmitter using 2 9v battery. Will the battery provide enough power for the transmitter?
     

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    Last edited: Mar 23, 2008
  17. Harros

    Harros New Member

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    Hi, back to the tx loop antenna, i found that both the radiation resistance and loss resistance is very low for the tx loop antenna. I wonder the way that i should start with to get the impedance matching between the tx antenna and the power amp. Do you have idea on it? By the way, there is no network analyzer in my college's lab, this has made the impedance measuring to be difficult.

    Should i provide a ground plane to the rod antenna ( i learn from the internet that a ground plane is provided to the monopole antenna usually)? Does the impedance buffer amplifier provide the virtual ground to the rod antenna?
     
    Last edited: Mar 24, 2008
  18. RadioRon

    RadioRon Well-Known Member

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    It is OK to amplify the square wave from the oscillator. At worst, the output waveform before the low pass filter might become a bit more distorted, but it will be operating more efficiently, and we normally trust the filter after the final amp to clean up the waveform.

    The RF choke for the base of the final amp can be the 200 uH part in that list. It is normal practice when building experimental circuits to buy several values of a component so that if tuning is necessary, you have some alternates on hand.

    I prefer not to use 9V batteries except if the current draw is quite low, like less than 50 mA. I don't know what your current draw is going to be exactly, but it might be more than 50. You get a lot more lifetime out of AA or C alkaline cells, but of course you either have to deal with the lower total voltage available.
     
    Last edited: Mar 26, 2008
  19. RadioRon

    RadioRon Well-Known Member

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    The impedance matching to the loop antenna is probably best done using transformer technique. If you resonate the loop with a capacitor you increase the terminal voltage and the impedance, then you can couple into the this with a small number of turns of a secondary loop on the same form. You can check the effectiveness of your coupling by putting a receiving loop nearby (like 3 feet away) and hook that up to a scope or other 1 MHz receiver and see what matching gives the most signal.

    The rod will work better if you provide some sort of ground plane or counterpoise. Any mass of metal is a help, the bigger the better, within reason. No matter what you do though, you will not achieve a resistive impedance match to 50 ohms because the rod is very short, electrically speaking so it will always have a high impedance. The purpose of the impedance buffer amp is to convert this to a low impedance.
     
  20. Harros

    Harros New Member

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    Is it possible for me to use the inductor instead of RF choke? As it costs me a lot to buy several rf chokes...
     
  21. Harros

    Harros New Member

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    Is it ok for me not to put the ground plane in this system? As it is quite expensive to get a metal plate at my place...
     

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