[hpsdr] Disabling 10 MHz clock on Penelope

[David McQuate]

Hi Alberto,
  When there's no 10 MHz reference clock, the phase-locked loop built 
around each of the 122.88 MHz
VCOs have no reference signal, so those VCOs just free-run, at some 
frequency ("of their choice").  Since their
tuning range is small, their unlocked frequency isn't terribly far from 
the exact value, but it will be different
for each VCO.  Thus, if you stay away from band edges, and don't care 
exactly what frequency you're listening
to or transmitting on, you don't need any 10 MHz clock.  (That's hardly 
"high performance".)

Dave
wa8ywq

Alberto I2PHD wrote:
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> Phil Harman wrote:
>> Hi Alberto,
>>  
>> Perhaps I can explain.  Both Mercury and Penelope have 122.88MHz 
>> master clocks.  These are used to derive the various signals needed 
>> by their respective FPGAs.
>>  
>> Since we generally want to transmit and receive on the same frequency 
>> we can't rely on both of these clocks being on exactly the same 
>> frequency.
>> So we use a 10MHz reference that both these clocks are locked to.  
>> The 10MHz can come from Mercury, Penelope or Atlas ( via Excalibur 
>> perhaps).
>>  
>> If you use a high accuracy external 10MHz reference - say a GPS 
>> locked reference, then your Tx and Rx frequencies also be very 
>> accurate. 
>> We have reports of folks having an accuracy  of  2 mHz  ( 2 milli Hz!).
>>  
>> If you don't select any 10MHz reference then you may get lucky and 
>> find your two 122.88MHz master clocks are close enough not to be a 
>> problem.
>> So if you have a nice accurate clock and you remove your 10MHz 
>> reference then, as you found, you may only move a few Hz.
>>  
>> In my shack today it reached 34C - I can tell you my clock moved a 
>> lot more than 2Hz without a 10MHz reference connected!
>>  
>> 73's Phil....VK6APH
> Hi Phil,
>
>   that makes a lot of sense, still I am puzzled, as I don't have any 
> external reference connected to the Atlas bus, it has only Ozy, 
> Mercury and Penelope.
> So my question still is, when I disable both Mercury and Penelope as 
> sources of the 10 MHz clock, where does the Atlas bus pick up the 
> clock that is then
> used to receive ?  Mind, I haven't used Penelope for transmitting, 
> yet. I just noticed that, when receiving, I have three possibilities 
> for the source of that
> clock, Mercury, Penelope or none of the two. Each choice corresponds 
> to a slightly different frequency.... who generates the 10 MHz clock 
> when neither
> of the two cards is selected ??
>
> 73  Alberto  I2PHD
>  
>
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