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Filter resonator

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  1. Apr 22, 2015 #1
    I want to replace a dielectric resonator in a satellite LNB to change the frequency of the LO.
    Can I use a filter to replace this resonator? a Helical filter maybe.
     
  2. jcsd
  3. Apr 22, 2015 #2

    davenn

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    a filter isn't a resonator

    You still need the/a resonator as that is your oscillator source

    what is the current freq of the LO ? what LO freq are you wanting ?

    There are ways to directly change the resonator freq ... many of them are already tuneable over a reasonable range 500MHz to 1 GHz
    Another is by altering the size of the dielectric puck in the resonator

    Dave
     
  4. Apr 22, 2015 #3
    Short answer: Not likely.

    An oscillator resonator can take different forms to meet different needs. If phase noise and stability wasn't much of an issue, you could use a microstripline. If you were interested in a low noise VHF oscillator, a helical resonator wouldn't be a bad choice. Moving toward UHF, you might do okay with a SAW filter. At microwave frequencies, however, your choices are somewhat limited. You can build a cavity oscillator or you can use a dielectric resonator. The DRO designs are actually quite stable and pretty good low noise sources. They are somewhat adjustable as well.

    How far do you want to move off frequency?
     
  5. Apr 23, 2015 #4
    Thank you for your answers.
    i want my oscillator to operate at 868 Mhz so I thought that an helical resonator could do it.
    The problem is that I cannot find helical resonators, I only find helical filters.
    the current frequency is around 11Ghz
     
  6. Apr 23, 2015 #5

    davenn

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    OK well, 868MHz is pretty low freq in comparison to the 11GHz of a satellite LNB so that isn't going to happen
    Mini-circuits have an awesome range of oscillator blocks

    here's just one from the search I did that would cover your wanted freq
    http://www.minicircuits.com/pdfs/ROS-1015-119+.pdf

    I have used a lot of their mixers, filters and VCO's over the years .... seriously great stuff!!

    any further questions, just ask :)

    Dave
     
  7. Apr 23, 2015 #6
    The VCO replaces all the oscillator in the LNB.
    Isn't just simpler and cheaper to replace the resonator?
     
  8. Apr 23, 2015 #7
    Oh dear. I'm afraid I have bad news:

    Do you KNOW what kind of mixer you have? It may well be a Rat-Race Mixer on stripline. Do you know how the LNA stage is biased or matched? It probably isn't characterized well for 868 MHz. And now you merely want to replace the resonator?

    I suggest reading at least one book on RF design from cover to cover. You are way back on the shallow end of the knowledge curve here. I can't help you in just this one message, or even a series of messages. You need a much deeper understanding than I can convey here.

    I have the first edition of this book: https://www.amazon.com/RF-Circuit-Design-Second-Edition/dp/0750685182
    I found it it to be very useful. The Second Edition should be even better. You might also consider some of the related books on the page.

    I'm sorry, you have a long learning process ahead. This isn't the simple problem you may think it is.
     
    Last edited by a moderator: May 7, 2017
  9. Apr 23, 2015 #8
    Those are really bad news.
    what i need is a dual LNA and downconverter Tu operate around 900Mhz...
    Are you saying that the LNB can possibly not work in lower frequencies? (Assuming that I take off the filters..)
    If not.. what can I use?
     
  10. Apr 23, 2015 #9

    davenn

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    the input to the LNA I assume is waveguide and if so will be for the around 12GHz range
    the front end amplifier GaAsFETS and surrounding circuitry will be optimised for 12GHz

    none of which is any good for 900MHz ..... what are you actually trying to achieve ?
    ie. what are you wanting to receive on 900 MHz ? ... its mainly a cellular phone band

    you really need to start a fresh and get a circuit design for a RF amp front end for the 900MHz band
    follow that with one of those VCO modules I linked to. What IF freq do you want ?
    what are you doing with the IF ? are demodulating a digital or analog signal ... specifically what type ?

    as you can see, you have a lot of work to do :wink:

    Dave
     
  11. Apr 24, 2015 #10
    Thank you for your help
    I want the IF frequency to be around 2Mhz, I'm going to sample it so it is going to be a digital demodulating.
    I think that what I want is named a diversity or a MIMO receiver because I want two antennas and a dual LNA/Mixer, do you know a good one suited for me like the one in dual LNBs?
    Maybe active mixers? I found the ADL5802 interesting but i dont know if it is exactly what i want.
     
    Last edited: Apr 24, 2015
  12. Apr 24, 2015 #11
    First, Keep It Simple and Stupid (known as the KISS principle). Build something that works first, get some experience, and THEN consider a MIMO system.

    Second, for digital demodulation of a signal, you probably want to have both an in-phase and a quadrature (I and Q) input to the A/D system and the DSP software behind it.

    Third, don't re-invent the wheel. If you have never designed with RF before, buy some pre-made components from companies like Mini-Circuits. They have quadrature power splitters, mixers, amplifiers, voltage controlled oscillators, and the like. This will save you a lot of time, effort, and learning the hard way.

    Fourth, learn to solder well. Given the sort of questions you're asking, I get a very strong impression that you may not have ever used one. It's not that easy or obvious. Pay attention to RoHS solder technologies and materials. (The geezer in me has a very bad attitude about RoHS solder --it is a classic example of pretending to be Green while making up protectionist trade barriers, installing planned obsolescence, and generally making a mess of the electronics industry)

    Fifth, find some test equipment. Get a frequency counter, an RF signal generator, an oscilloscope and some attenuators. Learn to use them. You'll need this stuff to know whether your gear is actually working the way you expect it to.

    Sixth (and this is important) some governments regulate radio receivers to be limited as to demodulation and frequency limits. Personally I think the notion is based upon Luddite grade ignorance --but those laws exist in many countries, including the US. Make sure you're not violating those laws. Some have NO sense of humor about such activities.

    Again, read up on this. There are many more hints I didn't include in this message. If you know any ham radio enthusiasts, you might want to introduce yourself because they may have experience and equipment you might find helpful. And then again, maybe they just like to hear themselves talk on HF (stay away from those people)...
     
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