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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 don't know why the simulation is behaving that way. The input capacitor is normally chosen to have a low impedance compared to the source and load resistances that it is feeding. This implies that the larger the capacitance,the better, but in practice we find that when the capacitance is too big, it begins to have too much series inductance which is undesireable. So, we usually choose something smaller. For example, if we want the capacitor to have an impedance less than 1 ohm then it would have to be 160 nF or larger. Your choice of 100 nF seems ok.

    I'm not happy with your choice of base bias resistors. The ratio of base resistors to the emitter resistor is simply too large and cannot be supported by a current gain of only 100 such as the 2n222 might have. You need to either increase the emitter resistor, which will reduce the total collector current, or you have to reduce the base bias resistors so that the base current necessary to drive the emitter resistor doesn't cause a significant voltage drop across the upper base bias resistor.

    While the tank circuit topology seems OK, I'm surprised that the tap ratio is so high. You have tapped the 50 ohm load about 90% across the tank circuit, which dramatically reduces the tank's loaded Q. These types of circuits usually work better when the loaded Q is about 10 to 30% less than the unloaded Q.
     
  2. Harros

    Harros New Member

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    I have redesigned the circuitry to lower the resistance of the voltage divider. I tried to tap the 50 ohm load about 10% across the tank circuit ( i mean i tap it with the tapped value in the previous circuitry up side down), the gain was however greatly decreased (to 0.18dB...), and the waveform is shifted up and down... I wonder why this happens...

    By the way, i am planning to add a power amplifier to the output of the crystal oscillator to boost the transmitting power of the transmitter to 20dBm (100mW) if possible. The oscillator output swings around 4 volts... Do you have any suggestion on the type of amplifier that i can build?
     

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    Last edited: Mar 6, 2008
  3. RadioRon

    RadioRon Well-Known Member

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    Oh, I see why that happens. You have the 50 ohm load DC coupled to the tank circuit which messes up the bias very badly. You need to put a 100 nF cap in series with the output load to block DC.

    There are many types of amps you can use. I will consider and repost
     
  4. dave

    Dave New Member

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  5. RadioRon

    RadioRon Well-Known Member

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    I recommend that you use the same amplifier topology as you already have using the 2n2222 but adjust the emitter resistor so that the collector current remains relatively high so that the amplifier does not limit until it can deliver about 2.5 Volts RMS into the 50 ohm output. You may have to adjust the tapping ratio and your base bias, but it should be able to give you the power output you want.
     
  6. Harros

    Harros New Member

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    There is no gain improvement and output improvement even the 100 nF cap is put in series with the output load, unless the load is tapped at 90% across the tank circuit...
     
  7. mneary

    mneary New Member

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    I don't see any coupling between L1 and L2 on the schematic. Is this correct?
     
  8. RadioRon

    RadioRon Well-Known Member

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    Perhaps you have indeed found the correct impedance match point then.
     
  9. Harros

    Harros New Member

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    Well, this is the updated schematic of the circuitry... The amplifier seems working very well, the output is desirable for the pico volt input...

    As I know, the hfe value for individual transistor might differ even they are same model. Should I measure the hfe value of the transistor and rearrange the circuitry to match the hfe value?

    Regarding the phase detector, I cant perform the simulation for this circuit as there is no pspice model for this component: AD8302. Do you have any suggestion on the way that i should determine the values for all the components associated with the circuitry? Or should I refer to the circuitry in data sheets here (AD8302.pdf and Operation of RF Detector Products at Low Frequency.pdf)?
     

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    Last edited: Mar 7, 2008
  10. RadioRon

    RadioRon Well-Known Member

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    Your bias design is the type that insures that HFE variation does not significantly affect the operating point, so you don't need to check each transistor.

    It is common practice to use the application circuit suggested in the data sheet or application notes from the manufacturer. This is especially true for ICs with complex functions such as this one.

    I find it a bit humorous that you say the amplifier works well. Of course I understand that you mean in simulation only. In my experience, something that works well in simulation does not necessarily work well in practice, especially at RF frequencies. The simulation does not account for stray coupling, non-ideal characteristics of components, stray inductance and capacitance, high impedance grounds, magnetic 120Hz induced noise, power supply noise, AM broadcast radio interference, changes in component values due to heating, bad soldering, undesired feedback, and a few other things. The simulation is extremely useful to get the basics worked out though, so it is still a valuable step in design.
     
  11. Harros

    Harros New Member

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    Well, what should I do next? Should I try implementing this amplifier circuitry on the PCB to see the result? By the way, it seems so complicated to have transistor in amplifier design. Is there other option for me to build this amplifier? Besides, I have limited budget on this project...
     
    Last edited: Mar 8, 2008
  12. RadioRon

    RadioRon Well-Known Member

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    This is the kind of amplifier design that gives you the most design flexibility. You could compare this level of design with programming a computer in assembler. You have to deal with many details that a higher level language shields you from, but on the other hand you can do anything imaginable and with much higher execution speed.

    There are easier alternatives. You can use a pre-designed amplifer like an RF gain block IC. Or you can use a CMOS inverter with added negative feedback.

    There are many RF gain block ICs available. These are ICs in which they have done all the work for you and you only need to make A simple pcb with a few components and it will work. Here are some examples:

    http://www.minicircuits.com/products/amplifiers_monolithic.html
    http://media.digikey.com/pdf/Data%20Sheets/Avago%20PDFs/MSA-3111,%20MSA-3186.pdf
    http://www.sirenza.com/documents/products/2858/SBF-4089(Z)_Green_Datasheet.pdf

    These kinds of amplifiers have much much more bandwidth than you need which can be a danger. Because the bandwidth is so large, you have to design your circuit with a layout and parts suitable for 1 GHz operation so that the amplifier doesn't oscillate.

    I have seen people make amplifiers successfully at 1 MHz using simple CMOS (HCMOS) logic gates. If you put a feedback resistor from output to input, these gates will operate in a linear way and make an easy amplifier. You still would have the problem that the gain might be too high and the amp will oscillate. The lower the feedback resistor value the better I think.
     
  13. Harros

    Harros New Member

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    So, should I implement the BJT transistor amplifier circuitry on the PCB for result and troubleshooting? What if we implement the op-amp in the amplifier design? Do you have any suggestion on the op-amp that we may use in the design? By the way, I am running out of time... (5 weeks to go before presentation)
     
    Last edited: Mar 9, 2008
  14. RadioRon

    RadioRon Well-Known Member

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    An op amp would work too, but might be somewhat noisier. The key to the op amp approach is to choose one with good bandwidth. You need at least 8 MHz gain bandwidth product to get reasonable gain at 1 MHz. This one is typical:

    http://www.national.com/ds/LM/LMV851.pdf

    but there are many to choose from. Any more bandwidth than 50 MHz and you are risking problems with stability.

    In most projects there are many ways to solve a problem. You are faced with choosing from several alternatives, none of which you have built before. In this case, you should assume that the first prototype that you build won't work well and will need some adjustment. For this reason it is often best to build something very early in your schedule to get a feel for how far away from completion you are. It is not wise to use up most of your time available studying the design on paper and in simulation and assume it will work when you finally build it. It won't. You will make minor mistakes, and with little time left risk panic and further errors.

    In your case, you should build rough prototypes of each stage using hand cut pcb and then test them individually. Using a piece of un-etched copper pcb, you can sketch out a simple block layout in pencil right on the copper, and using a sharp knife or a rotary tool with a grinding head (ie. dremel) you can cut simple straight strips out of the copper to isolate islands of copper which are the connection nodes between components. Do this for the BJT amplifier and you can have it built in less than one hour. If you use double sided copper board, the uncut side can become a good ground plane which is useful in this kind of design. (always connect the ground plane to the top side ground) Then you can test it using a generator and an oscilloscope. Once it is adjusted and working, you can consider this little pcb a building block to use in your system. It might look ugly compared to a commercially designed product, but that is not important.

    If you are able to work quickly like this, it won't be hard to make a choice of which type of amplifier to use because you can choose any one type, build it and decide quickly if it will be OK or if you need to try another. Working quickly in building prototypes takes some pressure off of how you make your choices.

    Of course, to work quickly may mean to make the wrong choice and waste a few dollars on the wrong parts. This is a normal risk in our world and we can take some solace in the fact that the electronic parts for prototypes are the least expensive part of the game.
     
    Last edited: Mar 9, 2008
  15. Harros

    Harros New Member

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    Thank you for your advices, I know what should I do now.

    For the rx ferrite loop, I am thinking of making it to be a single block, where terminals from primary loop are connected to the tuning capacitor, and the terminals from secondary loop are connected to the BJC connector. Is there any thing that should i pay attention to?

    My rx rod antenna is about 1 meter high, is it too big for the buffer amp?
     
    Last edited: Mar 9, 2008
  16. RadioRon

    RadioRon Well-Known Member

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    Nothing comes to mind about the ferrite loop. Go ahead and build it. I think you have the right idea.

    The rod antenna is not too big for the buffer.
     
  17. Harros

    Harros New Member

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    I have designed the phase detector circuitry, but I wonder if those are the suitable value for the components used in the circuitry. Any comment on this circuitry?
     

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  18. RadioRon

    RadioRon Well-Known Member

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    I would reduce the bypass and coupling cap values to 0.1 uF instead of 1 uF. Otherwise, seems OK. You would be wise to copy the application pcb layout in the data sheet, page 21.
     
  19. Harros

    Harros New Member

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    Well, I am thinking of providing the power to the Phase Detector using a power supply, thus I have made changes on the diagram... Do I need to add extra capacitors to the power supply part as what have been done in previous design (as shown in the picture below)?
     

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  20. RadioRon

    RadioRon Well-Known Member

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    The only critical point where you must have a ceramic bypass capacitor is from Vcc to ground as close as possible to the chip.
     
  21. Harros

    Harros New Member

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    Well, for the transmitter part, the output swing of the crystal oscillator is around 4Vpp, so, is there any suitable amplifier for this situation? This is because I want to increase the transmit power of the transmitter... Or should I use voltage divider on the output of the crystal oscillator and implement the small signal amplifier (just like the type that i build for the receiver) to increase the transmitter power?
     
    Last edited: Mar 14, 2008

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