external 10 MHz reference phase noise requirements ?

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  • Updated 3 years ago
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( I am way-out-of-my-league technically in this area so if I ask some really dumb questions please make allowances .... )

As I write this there is .... somewhere out over the vast Pacific Ocean ... a little package with my name on it ... containing a 10 MHz Piezo oven-ized crystal oscillator ('OCXO' - Model 2920136) that I purchased a few days ago on eBay.

My intent is to build-up a secondary frequency standard using the Piezo OCXO with input from articles that I've been able to locate (some I remembered from years ago) and also from exposure I had to this type of device long ago when in the Air Force (I was stationed at a base overseas and, in the next aisle from my duty station, there was a rack with a Sulzer double-ovenized crystal frequency standard).

My question has to do with the phase noise requirements that are
applicable  to the FLEX-6x00 external 10 MHz frequency reference :

  - What are they ?

  - What do they need to be so that the phase noise characteristics
     of the radio are not compromised ?

My understanding of how the radios use the external reference (based on Steve's & Tim's comments in other news-threads) is that on power-up the radios do a search for a frequency reference : first checking for a 10 MHz external reference, then (if there wasn't an external reference) looking to see if the GPS internal reference is installed and finally (if neither of the previous were found) the radio looks to its on-board TCXO (temperature compensated crystal oscillator).

Now my presumption is that the (very impressive) phase noise specs, which FLEX gives to the 6x00 radios, is based on the use of the on-board TCX0.

Does the external 10 MHz reference directly 'drive' the radio or instead is there a heavily-filtered PLL that is 'locked' to the external reference ? It occurs to me that a very 'slow-tracking' PLL would serve to filter out phase noise coming in on the external 10 MHz reference .... but I'm only guessing that such a filtering action is applied.

Information please.

- Paul, WB5AGF
  Garland, Texas
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Paul - WB5AGF

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Posted 3 years ago

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Steve - N5AC, VP Engineering / CTO

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Official Response
Hey, Paul, here's the scoop.  Since I know you're an EE, I'm going to use that to my advantage ;-)  The primary oscillator in the FLEX-6500/6700 uses what is commonly referred to as a "jitter cleaner".  This is a TCXO in a PLL with a very small loop bandwidth (think 10Hz range).  The phase noise characteristics of the crystal (in the TCXO) are where the radio derives it's phase noise characteristics.  The only place where the jitter cleaner is susceptible to reference noise is inside of the loop bandwidth.  So, if the phase noise characteristics of your reference are worse that the radio inside of 10-20Hz, your reference will dominate.  But, this is really not an issue, generally, as the phase noise characteristics we care most about are in the 500Hz-100kHz range anyway.