Using signal generator in place of PLL - is that possible?

Discussion in 'Wireless & RF Design' started by tomshong, Aug 28, 2012.

  1. tomshong

    Thread Starter Member

    Oct 6, 2011
    36
    0
    Hi experts,

    So I have a National Instrument MyDAQ which can generate up to 20 KHz signal. And I have some questions/ideas that I wanna ask the experts here......

    1) is it possible use a signal generator such as a MyDAQ in place of PLL as a LO to a RF system for testing purpose?

    2) if yes, and I am wondering, if it's possible to cascade two 10x frequency multiplier to get the frequency up to 2MHz? sort to make it a poor man's function generator.

    3) if 1 and 2 are both yes, then that really got me to wonder, why do we ever need a PLL in a RF system at all if there's a way to vary the crystal oscillator and simply multiply the frequency by multipliers?

    Probably dumb questions to the experts, but I haven't seen any discussion on this. :p

    Tom
     
    Last edited: Aug 28, 2012
  2. Papabravo

    Expert

    Feb 24, 2006
    10,166
    1,797
    1) Only if the signal generator has a control voltage input.

    2) 20 kHz is too low a frequency to be practical for RF work. DDS or a crystal oscillator is much more common.
     
  3. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85
    (1)Yes,it is,but the MyDAQ is likely to be disappointing,as its frequency stability is probably not good enough--the on line specs don't say what it is.
    The other point here is that the fundamental frequency range of the MyDAQ is inadequate in the practical case,which brings us to the next question!

    (2) Yes, & this method was used extensively for many years---BUT!
    Frequency multiplication is not a wideband process like frequency division.

    The normal way is to distort the fundamental frequency,then feed it into a circuit resonant at some harmonic of that frequency.

    Multiplying by more than 4 in one stage becomes quite tricky,so higher factors are achieved using multiple stages.

    Normally multiplication is performed in multiples of 2 or 3,which tends to leave your X10 out in the cold!

    Another major problem is that outside a quite small range,the multiplication stages need to be retuned with each change in frequency.

    Amplitude modulation modes,such as full carrier AM,SSB,etc cannot be multiplied in this manner,as the modulation is distorted by the non linear stage needed to create harmonics.

    Such signals are normally shifted in frequency by the use of converters (mixers) with the output from a multiplier or PLL used as a local oscillator signal.
    Frequency modulation can be multiplied,however.

    (3) From the above,it may be seen that frequency multiplication,although a simple concept,becomes quite complex in execution.
    PLL systems are frequency agile,so although they appear more complex,they make life a lot easier if you need to shift a modulated signal up or down in frequency,while retaining the ability to vary the ultimate frequency.
     
  4. tomshong

    Thread Starter Member

    Oct 6, 2011
    36
    0
    Thanks.

    1) How would I measure frequency stability on a MyDAQ? Can you direct me to a link?


    2) I see there's a 12x multiplier on Minicircuit. What you think of it?

    http://www.minicircuits.com/pdfs/ZX90-12-63+.pdf
    I don’t understand what you mean by “multiplication stages need to be retuned with each change in frequency.” Can you elaborate?

    I also don’t understand the part when you said AM can’t work with an LO like this. How’s the data in amplitude get distorted by the frequency harmonics?

    3) So if I understand the concept correctly, frequency multiplication is done by harnessing the harmonics of the fundamental frequency, which means at the output of these multipliers I would get not just get the desired multiplied frequency but also the harmonics, which makes it hard to use as a variable LO. Right?
     
  5. Papabravo

    Expert

    Feb 24, 2006
    10,166
    1,797
    If you have a x12 multiplier and you change the input by 1 MHz., the output changes by 12 MHz. For a given GBW product, the circuits of the next stage will have to sacrifice one or the other.
     
  6. vk6zgo

    Active Member

    Jul 21, 2012
    677
    85

    For (3),Yes,but the resonant circuit is tuned to the harmonic you require.

    This does bring up a further point,in that you could,if you need to tune a lot of multipliers to get to your final frequency,mistune one,pick up an incorrect harmonic,happily tune all the others up,& be on the wrong frequency at your ultimate output.

    Many years ago,I worked at a Radio Broadcasting site with large HF Transmitters,which needed to operate on multiple frequencies so as to get the best day & night coverage.

    To this end,they employed several switchable crystal oscillators,followed by tuned multiplier stages,& then RF amplifiers,up to the large output stages which were high level modulated by equally big modulators.

    It was quite possible to inadvertently select the wrong crystal,& tune the things up on the wrong output frequency,if you weren't paying attention.
    Usually,the tuning "just didn't feel right",which would alert you to the problem,but there was a story that one of these had been tuned in the middle of the "20 metre" Ham band on one occasion,& sat there for about an hour before anyone noticed.:eek:

    You really need to read up on this stuff from the RF point of view.
    If you can get your hands on an older ARRL Handbook or similar for the information on multipliers.
    The newer ones don't quite cover it as well,but have some good stuff on mixer/converter stages.
     
    Last edited: Sep 1, 2012
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