[time-nuts] LTE-Lite Plans

Bob Camp kb8tq at n1k.org
Tue Nov 25 20:40:49 EST 2014


If you decide to run the circuit from +5V, get the 74ACT04 instead of the 74AC04. It will trigger better on the 3.3V output from the LTE. 

The 74AC(T)04 will not in any way impact the phase noise or ADEV coming out of the LTE, if a reasonable supply is used…

With a decent PCB layout and SMT parts, the isolation can be *very* good if multiple gate packages are used. 


> On Nov 25, 2014, at 8:28 PM, S. Jackson via time-nuts <time-nuts at febo.com> wrote:
> Guys,
> I never expected such an intense discussion about using and buffering  the 
> outputs from the LTE-Lite board since the actual circuit to use can  be 
> quite simple.
> To address these questions, I drew up a simple schematic that uses a DIP-14 
> 74AC04 gate, six resistors, and two caps. Everyone who can solder should 
> be able  to build this simple circuit as a dead-bug type build on a 
> copper-clad  board.
> This circuit will buffer all three outputs (1PPS, TCXO RF, and Synthesixed  
> RF) of the LTE-Lite eval board with CMOS 3.0V levels that can drive 50 Ohms 
> terminations. For simplicity I grab the 3.0V power from the DIP-14 TCXO on 
> pin  14 of that part on the eval board, even though I would strongly 
> suggest to use a  separate low noise 3.3V or 5V power supply to power the 74AC04 
> chip.
> You can add 100nF caps in series to the two RF signals before they feed  
> into the coax output connectors for less power consumption and removing DC for 
> instruments that don't like DC inputs.
> Using a single IC for the three signals will result in crosstalk between  
> the signals, but it should be clear from the schematics how one could break  
> up the signals by using three independent ICs to minimize crosstalk. 
> We use this circuit in a small box here using SMT components, and it works  
> really well.
> Excuse my horrible writing, using keyboards has made my fingers  numb..
> Hope that helps,
> Said
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