[time-nuts] Effects of noise on EFC line?

Alexander Pummer alexpcs at ieee.org
Tue Aug 5 16:03:46 EDT 2014


and use not to large capacity, because above the serial resonance 
frequency [trace inductance, own inductance] it will be inductive!
73
Alex
KJ6UHN

On 8/5/2014 12:16 PM, Jim Harman wrote:
> I was able to quiet things down a lot by putting a 100 ohm resistor in
> series with the 74HC output. If these guys drive more than a few inches of
> wire, they ring like crazy. Also make sure you pay close attention to the
> bypassing of the 5V supply. Make your bypass cap lead lengths as short as
> possible.
>
>
> On Tue, Aug 5, 2014 at 2:36 PM, Bob Stewart <bob at evoria.net> wrote:
>
>> I've run into a noise problem on the EFC line of my GPSDO engine at the
>> frequency of the oscillator.  I've traced the source down to the 74HCT365
>> I'm using to output the 1(or 5)MHz and 10MHz signals.  When I pull it, the
>> EFC quietens down a lot.  I'm seeing about 50mv of 10MHz noise at the
>> output of the op-amp that feeds the EFC voltage divider at the OCXO.  The
>> voltage divider is corrected by the VRef from the OCXO with a simple
>> circuit using temp-co'ed resistors.  On the 0.1uf cap at the OCXO's EFC
>> pin, I'm seeing about 5mv of 10MHz signal.
>>
>>
>> I've considered switching the HCT out for a 74LS365, assuming my drive
>> levels are compatible.  Unfortunately, I don't have one "in stock", and
>> "I'm way out of my pay grade", as they say.  I've also thought about
>> putting a 100uh inductor in series with the EFC line.  I wonder if I'll
>> have to isolate the 74xx365 chip's VCC through an inductor?  Any thoughts?
>>
>>
>> I'm also wondering what the impact of this level of on-frequency noise
>> will be?  Is the impact somewhat mitigated, since it's at oscillator
>> frequency?  I don't have anything better than an HP 8558B to look at the
>> output of the board.
>>
>> I'm not quite ready to generally share my schematic with the list, but I
>> can make individual exceptions.
>>
>>
>> Bob - AE6RV
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>
>




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