[hpsdr] 10 MHz clock source

[Phil Harman]

Hi Alberto, 

Let me see if I can shed some light on what is happening.  If you use Mercury and Penelope you need a way to phase lock the 122.88MHz clocks on each board.  There are a number of ways you can do this:

1. Use the 10MHz TCXO on Mercury 
2. Use the 10MHz TCXO on Penelope
3. Use an external 10MHz reference on Atlas C16 e.g. Excalibur

The TCXO's on Mercury and Penelope are not intended to be highly accurate - just good enough to lock the two 122.88MHz clocks together. If you would like higher accuracy then use an external reference. 

The 122.88MHz clocks on Mercury and Penelope are divided down to 80kHz as is the 10MHz reference from Penelope or Mercury or Atlas.  The two 80kHz signals are presented to an OR gate which is used as a phase detector in a PLL. 

So if you select Atlas as your 10MHz source, and there is no 10MHz signal on it, then the output of the OR gate phase detector will be a 80kHz square wave.  After this passes through the PLL filter the output is a DC voltage at  3.3/2 volts.  This voltage is fed to the 122.88MHz VCXO which sets it to the mid point of its frequency range.

If you are lucky, and have a good 122.88MHz clock, then this could be very close to 122.88MHz - closer than if you phase lock it to the relatively low accuracy 10MHz TCXOs. 

Hence you may find your frequency accuracy is actually better with no 10MHz clock on Atlas rather than using the 10MHz TCXO on Mercury/Penelope.

If you want higher accuracy then use an external 10MHz reference.  If  you use Penelope and Mercury, and select Atlas as your 10MHz reference, then you MUST have an actual signal on it or - depending on how good you luck is -  you will have a frequency difference between Tx and Rx. 

No mystery - just simple electronics!

73's Phil...VK6APH



  ----- Original Message ----- 
  From: Alberto I2PHD 
  To: Keith 
  Cc: hpsdr at lists.openhpsdr.org 
  Sent: Monday, June 08, 2009 6:51 PM
  Subject: Re: [hpsdr] 10 MHz clock source


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  Keith wrote: 
Hello Alberto
  I noticed the frequency shift when working PSK31 before the latest updates
and was told that on Mercury if I shorted across R37 it would get it much
closer. I tried it and the frequency is very close now 10-15hz. I believe
there is an eco out for the change it has to do with the leading edge of the
clock pulse on the C1 bus of Atlas. Hope this helps 
73 de N3ICK 
  Hello Keith,

    thanks for your answer. Will have a look at that EC.
  But still don't understand where the 10 MHz clock comes from, when Atlas is selected as source...

  73  Alberto  I2PHD




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