FAQ: 24/02/2009

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Q: Why is there an offset between some frequencies entered in the CRO and
   the final loced frequency?
   
A: Due to internal pre-divider, not all frequencies can be set exactly.
   Only even and depending on the frequency, modulo 2,4 and 8 frequencies
   are possible. Therefore an offset can occure. However this offset does
   not influence the precision of fx.

   The formula is: 

                     ( (Finput + X) )           fx = real locked frequency
      fx = (integer) ( ------------ ) * (2 * x) Finput = CRO frequency input
                     (    2 * X     )           X = see table below

   Table:
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         < 20MHz: X = 1 . Offset max 1Hz.
                  Example: Finput=16.000057 MHz  -> fx=16.000058 MHz               
                           Finput=10.000001 MHz  -> fx=10.000002 MHz               
                           Finput=10.000000 MHz  -> fx=10.000000 MHz               
                           Finput= 9.999999 MHz  -> fx=10.000000 MHz               
                           Finput= 9.999998 MHz  -> fx= 9.999998 MHz               

   20MHz < 40MHz: X = 2 . Offset max 2Hz.
                  Example: Finput=35.123458 MHz  -> fx=35.123460 MHz
                           Finput=35.123457 MHz  -> fx=35.123456 MHz               
                           Finput=25.000000 MHz  -> fx=25.000000 MHz               

   40MHz < 80MHz: X = 4 . Offset max 4Hz.
                  Example: Finput=70.123459 MHz  -> fx=70.123456 MHz
                           Finput=60.525000 MHz  -> fx=60.525000 MHz               

         > 80MHz: X = 8 . Offset max 8Hz.
                  Example: Finput=108.009896 MHz -> fx=108.009904 MHz
                           Finput=108.009895 MHz -> fx=108.009888 MHz               
                           Finput=123.444640 MHz -> fx=123.444640 MHz               
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   Bear in mind, that the real frequency (fx) must be entered in the 
   CRO-Online Tool. 


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Q: The RF and 1PPS check says "ok", but I get only the "off-sync"
   message.
   
A: Check the 1PPS signal. It must be a low pulse between 80ms and 120ms ,
   TTL level. 100ms is perfect. A longer or shorter low pulse will cause
   wrong operation.

 ------__------//------__-----//
      100ms           100ms
      |----1 sec------|

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Q: What is the p-factor exactly and what is it used for?

A: The p-factor (Precision Factor) is an important system constant. It 
   can be chosen by the user before he gets the characteristics of the 
   oscillator. After that, the value should NOT be touched, except the 
   user intends to get a new characteristics curve.

   In case for oscillators with lower quality (poor stability etc) the
   p-factor causes an adjustment of the necessary dynamic area of the
   phase counter to the real available dynamic area. The maximum of 
   precision which can be achieved at all, is divided by the p-factor. 
   That means as lower p-factor is, as higher the reachable precision.
   
   For good OCXOs like the HP10811, a p-factor of 1 can be used. For 
   good home brewed OCXOs (like DF9LN OCXO with excellent components) a value
   between 5 and 10 can be achieved. Don't try to use a better p-factor
   as the oscillator is good for! 

   Bear in mind: Set the p-factor, take characteristic, close the loop
   and forget the p-factor (except you need it for online-analysis
   PC-program) 

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Q: How can I check if my OCXO can be used together with the CRO?

A: Provided the oscillator has been wormed up (at least 1 day, if a new
   one is used much more longer) the CRO should be used in LOOP OPEN
   mode. Setup all you need and connect the oscillator to the CRO. Start
   the ONLINE-Analysis PC program and let it run. Observe the graph which
   can be seen after a while. Try to adjust the oscillator that it delivers
   a line as horizontal as possible. Set the scaling of the PC program as
   small as possible.
 
   The graph of the oscillator should not ...

         ... change the direction suddenly
         ... show jumps (occasional jumps caused by interference are ok)
         

   The oscillator should ...

         ... stay in direction at least for 100 sec (better longer) 

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Q: How can I pull out the PLCC devices without damaging the sockets?

A: Do never use an other instrument than a special instrument which is made
   for pull out EPLDs. Do not use a screw-driver etc. ,you will damage the socket.
   A broken socket will cause wrong operation of the CRO. It is almost impossible
   to substitute a socket!