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Re: [softrock40] Re: Can SDR radios using a Si570 be locked to an external frequency standard?

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  • Alan
    ... Subject: Re: [softrock40] Re: Can SDR radios using a Si570 be locked to an external frequency standard? ... John, Thanks, that s clarified things. I was
    Message 1 of 20 , Aug 27, 2012
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      ----- Original Message -----
      Subject: Re: [softrock40] Re: Can SDR radios using a Si570 be locked to an external frequency standard?

      >> But what is 7ppm?


      Thanks, that's clarified things. I was confused, not unusual.

      I was thinking temperature coefficient, in ppm per °C.
      But the Si570 parameter quoted is temperature stability in ppm over the whole range, -40 to +85C. The data sheet I looked at gave
      that range as "Test Condition".
      So 7 ppm over that range is quite a high spec.

      73 Alan G4ZFQ

      >> 7Hz in 1MHz per °C?
      >> My test here of a 50ppm <http://groups.yahoo.com/group/softrock40/attachments/folder/919848385/item/1519111622/view>
      >> shows 2Hz/°C at 10MHz, OK, over a very small range but that would indicate it may well be better than 7ppm over a much bigger
      >> range.
      >> (Unless I am wrong about ppm)
      >> And this is the sort of drift others have noticed.
      >> So what is the difference between the grades? Reminds me of the early LVDS Si570s I bought that were graded "C" but seem to
      >> operate
      >> to the "A" frequency specification. Then they disabled the dividers on the C versions..
      >> 73 Alan G4ZFQ
      > Alan,
      > Parts Per Million. 1MHz is 1,000,000 Hz so 7ppm at 1MHz is 7Hz.
      > 7ppm at 145MHz is 1.015kHz. 7ppm at 432MHz is 3.024kHz
      > For quartz crystals ppm can be used to specify two important parameters, temperature stability over a given temperature range and
      > adjustment tolerance at a stated temperature.
      > The Si570 internal quartz crystal determines the temperature stability. For quartz crystals temperature stability is usually
      > quoted as a number of ppm over a given temperature range say -10 deg C to +50 deg C. The crystal angle is selected to have
      > minimum temperature coefficient over the specified range. It usually means the crystal will not move in frequency more than a
      > given number of ppm when operated at any temperature within the specified range. So say an 8.000000MHz crystal specified to have
      > a temperature stability of 5ppm over -10 deg C to +50 deg C should not change frequency more than 40Hz when operated at any
      > temperature over the range relative to the calibration temperature, usually +25 deg C. For crystals this is a fairly tight
      > temperature spec. Silicon Labs Si570 datasheet only gives a ppm temperature stability figure without giving a temperature range
      > other than giving the operating temperature range of the chip.
      > Whether this relates to chip safe operating temperature range or temperature stability or both is not clear.
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