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basic electronics question: types of oscillators

Discussion in 'General Electronics Discussion' started by Qualcomm_inside!, Feb 10, 2017.

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  1. Qualcomm_inside!

    Qualcomm_inside!

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    Jan 27, 2017
    I have a question about types of oscillators in electronic circuits.
    I want to know what is the difference between actual RF oscillators used in radio devies, especially UHF, and ' digital ' oscillators used in computer logic?
    What I mean is, if I have a radio or a phone/wi-fi device etc transceiving at say 2.4GHz and I have a computer with the same clock frequency, is this not the same type of RF oscillator? Can a radio hear a computer processor talking? vice versa?

    In mobile devices like smart phones, how do the different osc work for the digital logic and the RF side? do they use the same or same type of clock gen for both ?
     
    Last edited: Feb 10, 2017
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    No:
    - A radio operates on modulated sine waves.
    - A computer operates on square waves normally not modulated (apart from a technique called spread spectrum)

    A radio can "hear " a computer. That's called interference and is undesired. Any device using high frequencies needs to undergo some testing to ensure this doesn't happen under normal operating conditions.
    A computer can "hear" a radio, too. It depends on where you're looking. The audio inteface of a computer is especially prone to interference from strong radio emission. A recent example is the issue LG's 5k monitors have with WiFi interference.
     
  3. LvW

    LvW

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    As Harald has pointed out - there are some fundamental differences.
    For this reason - some authors (and I strongly support them) apply the term "oscillator" to harmonic (linear) oscillators with a sinusoidal output only (Frequency in Hertz).
    All devices producing squarewaves should be called "wave generators" (repetition rate in 1/s).
     
  4. BobK

    BobK

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    Ah memories...

    When I was 19, summer of 1971, I was an intern at a U.S. army research laboratory. We had a Wang programmable desk calculator. This thing was about the size and weight of 3 typewriters. I discovered that an AM radio would pick up audio at the frequency of any loop it was running. The machine had a row of function keys that would each execute a user defined program. So I programmed each key to be a musical note and you could play it like a piano. This was at the time when analog synthesizers were new technology. So I credit myself, with at least being one independent inventor of the digital music synthesizer.

    Bob
     
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  5. davenn

    davenn Moderator

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    and if analog radio modulated with sine waves .... if digital as so much is these days ... TV, FM broadcast, cellphones etc
    then the digital modulations are square waves
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

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    The modulating signals are square waves (or the equivalent data, depending on the construction of the modulator) while the carrier is still sinusoidal. A square wave as carrier will generate way too much noise outside the intended frequency range due to hatmonics.
    If for whatever reason a non-sinusoidal carrier should be used in the generation of the transmit signal, it is imho sure to be followed by a waveform-shaping mechanism (e.g. a filter) to minimize out-of-band noise.

    But then again I'm by no means a radio specialist :rolleyes:
     
  7. Syncopator

    Syncopator

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    Jan 12, 2017
    Not all of them. Well known exceptions are c.w. transmitters which switch the carrier on and off, and radar transmissions.
     
  8. davenn

    davenn Moderator

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    Yup, that is why significant filtering is required :)
     
  9. Arouse1973

    Arouse1973 Adam

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    What makes you think a 2.4 GHz computer is running at 2.4 GHz? The speed of a computer is an equivalent speed as if it was running on 2.4GHz. The clock speed will be a lot less. A computer clock is not a rf transmitter and falls under the EMC directive for IT equipment and not the RED directive.
    Adam
     
    Last edited by a moderator: Feb 10, 2017
  10. BobK

    BobK

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    The clock going into the processor is a lot lower, but it is multiplied internally by a phase locked loop (PLL) and it is the internal clock frequency of the processor chip. I doubt that there is much radiated at that frequency since the chip itself is not much like an antenna.

    Bob
     
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  11. Qualcomm_inside!

    Qualcomm_inside!

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    Jan 27, 2017
    How does this PLL multiplier work?
    Is the base frequency the same as the system bus / DRAM speed?

    So when a processor is rated for _ Mhz or GHz that means that only the core logic of the CPU can ' oscillate ' or pulsate ' at that frequency?
     
  12. Qualcomm_inside!

    Qualcomm_inside!

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    Jan 27, 2017
    By system bus I mean the base clock of the DRAM and the front-side bus to the chipset,
    ( if present ). This is also where I get confused, if the base clock of the memory bus is say 100MHz bit you've got DDR or some other multiplexing scheme, does that really mean that the system bus is signaling twice that speed or better? How does this tie in with a processor internal clock?
     
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Often these can be varied independently.

    The one where there is a relationship is where you can set the clock multiplier. That sets the ratio between the internal clock and whatever it is a multiple of.

    With memory, wait states can have more impact than actual clock speed.

    It's been a while since processors and memory have been slow enough that I've bothered.
     
  14. BobK

    BobK

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    A PLL multiplier works like this.

    You have an input frequency, generally from a crystal oscillator.
    You also have a voltage controlled oscillator that operates at a much high frequency.
    And you have a digital divider.
    The phase locked loop locks the high frequency of the voltage controlled oscillator onto a multiple of the lower frequency by comparing the lower frequency to the divided down frequency.

    So, say you have a 10 MHz crystal oscillator. To get a 1GHz clock, you run the voltage controlled oscillator through logic that divides by 100 (1GHz / 10Mzh = 100). Then you lock that divided frequency onto the 10 MHz oscillator. Once they lock, the voltage controlled oscillator must be running at 100 times the 10MHz clock, or at 1GHz.

    The different clocks for processor, DRAM, standard busses etc are either divided down from the highest frequency or multiplied from the lower frequency by separate PLLs.

    Bob
     
  15. Harald Kapp

    Harald Kapp Moderator Moderator

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    DDR = double data rate.
    Whereas a typical clock scheme uses one edge of the clock to perform an operation (e.g. transfer of data), the DDR technique uses both edges of the clock, thus transferring twice the amount of data compared to a single clock edge transfer.
    When you have different clocks in a system, the transfer of data between different clock domains (areas of the circuit that use the same clock) needs to be synchronized (see this Wikipedia article, chapter communication). Synchronizing different clock domains is a science by itself as the interface between these domains is subject to effects like hazards, races, metastable staes etc. (see e.g. Wikipedia for explanations) which need to be controlled tightly.
     
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