[time-nuts] FE-5680A sweep range setting

[Paul Berger]

On 3/01/14 11:35 AM, Magnus Danielson wrote:
> On 24/02/14 19:14, Paul Berger wrote:
>> A couple further observations, to make life easier when adjusting C245
>> you can unplug the top card and move it aside, the CVXO will lock
>> without it, but you will not have the 10MHz to monitor, but you can
>> monitor the CVXO output instead.  On the 6 pin connector you will see a
>> 1 near the outer long edge of the second card, the CVXO output appears
>> on pin 5 of this connector and on mine, it locks at 50.25505808 MHz
>> according to my 5335A.   I also noted that when the 5680A is well warmed
>> up it sweeps through a much smaller range.
>
> In that case it would be good if the "mid" voltage would be 
> established, so that once lock have been achieved, further trimming 
> can be done until the VCXO steering is in the middle of the range. 
> That would make sure there is plenty of margin to either end, rather 
> than sitting on the edge and just barely lock.
>
> Cheers,
> Magnus
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After some careful looking around, I determined that this C245 that we 
are adjusting is part of an LC tank in one leg of the crystal. The one 
that I am having trouble with definitely has a fault in the VCXO 
oscillator it seems to want to start up up at about 50% of the desired 
frequency with a very small noisey signal, but if I touch almost 
anything around the crystal it snap into the right frequency range, with 
a nice strong, clean sine wave  and locks rather quickly.  I am finding 
it hard to trace out the circuit as this little card has components on 
both sides of it, and to make matters worse seems to be multi layer as 
well.  I have set it aside for now but will go back to it later.

I thought it was a little odd that these do not have a block of 
insulation over the bulb, does this matter?  The other thing that is 
different about  the two that I just got, from my first one is, these 
seem to be using an AD9830A synthesizer chip to generate the 10MHz 
output signal.  There is a PIC beside which likely controls it and the 
PIC is connected to a RS-232 level converter chip however the external 
side of the RS232 chip is currently not connected to anything, but that 
can be quickly remedied.

Paul.