FM transmitter project using DPLL - Help

Discussion in 'The Projects Forum' started by dumindu89, Feb 12, 2012.

  1. dumindu89

    Thread Starter Member

    Oct 28, 2010
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    I am going to design a FM transmitter which can transmit an audio signal in a desired frequency with 100 kHz steps (but within 88MHz-108MHz range) using digital phase lock loop for my final year project. Here I have some questions to resolve.

    1. How to choose the best DPLL IC for the modulation part.
    I have found some ICs such as 74HCT9046, 74HC4046. But I don't know which one will suit for my purpose. I need to have a 100% accurate output frequency in the transmission.

    2. Will the output of the DPLL produce harmonics to filter the required frequency range (88MHz - 108 MHz), if the DPLL output is not in the required frequency range?

    3. What are the other facts I should concentrate when I doing this project?

    Your replies/suggestions will helpful to me.

    Thanks in advance
     
  2. DickCappels

    Moderator

    Aug 21, 2008
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    1. How to pick a chip: You need to search for PLLs designed to cover the range of interest (88 MHz to 108 MHz), has enough sensitivity on the phase comparator so that you don't have to come up with a high amplitude signal from the oscillator -the oscillator will usually be separate from the PLL chip in this frequency range. Power supply compatibility might be an issue, but the main thing is to find one that is suitable for your operating frequency, then design your circuit around it. There used to be a few PLL chips for use in cell phones that can handle your frequency range, but such chips are becoming more scarce as the PLL function is absorbed into the large scale cell phone chips. In other words, go to the TI and ON Semiconductor and do a search.

    2. Will the output have harmonics? This question is probably not relevant since you will most likely get a PLL chip that has counters and a phase comparator and use the phase comparator's output to control an external voltage controlled oscillator. Just make your VCO clean and be sure to filter noise out of the PLL and power supply, and you should be good to go.

    3. Other facts you should know: You might want to look at something very similar that I did several years ago. Yours doesn't have to be as complicated as mine, but a microcontroller is a very good way to load the registers in the PLL chip.
    Don't laugh at the construction! http://tinyurl.com/by4lhk
     
  3. dumindu89

    Thread Starter Member

    Oct 28, 2010
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    I couldn't find such IC working in this frequency range. I found programmable PLL IC; FS7140 which has output frequency range of 0-150 MHz via ON semiconductor and programmable IC; CDCE913 via TI which has output frequency up to 230 MHz , but I have to face into problems in fabrication due to their pin/packaging. :(

    Any Idea or suggestion?
     
  4. DickCappels

    Moderator

    Aug 21, 2008
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    Yep, the On Semiconductor part would be a pain to program. The TI part looks pretty good, except you would have to live with their oscillator and its harmonic-rich output.

    I would have mentioned National Semiconductor, but I thought their parts would show up in the TI product listings by now. You might want to have a look at National's (TI's) LMX2485E.

    You would need to double-check the counter arrangement on whatever PLL you select to make sure that it will cover the frequency range and channel spacing you want.

    I would be really surprised if you find a modern circuit in package that's easy to work with in hand-built circuits. But if you find one, let me know!
     
  5. dumindu89

    Thread Starter Member

    Oct 28, 2010
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    Finally I decided to pick 74HC4046A IC for the digital phase locked loop and 74HC4059 IC for the programmable divided by N counter. However, this selections may change if I couldn't meet the required specifications.

    However, I have few questions.


    1. What are the benefits/advantages of using digital phase locked loop for FM modulation?
    2. What things should I consider if I using 74HC4046A IC for the design?
    3. How do I make the 74HC4059 IC as programmable to select the desired channel? ( The modulator should be programmable to generate FM carrier frequencies in the steps of 100kHz)
    4. What are the other things I should concentrate when I designing this?
     
  6. DickCappels

    Moderator

    Aug 21, 2008
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    1. The benefit over an L/C oscillator is that the transmitter frequency will be very stable (as stable as your quartz reference).

    2. The CD4046 is very sensitive to noise. Make sure your power supply is very clean. In one application, I had to use two voltage regulators -a 7812 followed by a 7805 in order to achieve an acceptable amount of ripple on the power supply.

    3. Left for you to figure out; I think it will be a challenge as the maximum input frequency of the Philips 74HC4059 is 40 Mhz. You would have to start out with a divide by 4 prescaler.

    4. Grounding, put a balun on your power supply leads, use adequate bypassing. I recently went through my box of PLLs. If you send me a PM with your mailing address, I can send you an LMX1601 or SAA1057 -no charge if you are willing to tell me how it works out.
     
  7. radiohead

    Active Member

    May 28, 2009
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  8. dumindu89

    Thread Starter Member

    Oct 28, 2010
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    I talked about philips 74HC4046A, not CD4046 :)
     
  9. DickCappels

    Moderator

    Aug 21, 2008
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    Sorry to confuse you, I date back to when the CD series was the only SSI CMOS on the market. The comments for the CD4046 also apply to the 74HC4046.
     
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