[time-nuts] LTE-Lite Plans

SAIDJACK at aol.com SAIDJACK at aol.com
Tue Nov 25 21:14:22 EST 2014

Hi Mark, Bob,
two comments:
* I forgot to mention that feeding the 1PPS signal through the IC inverts  
the signal of course, so the falling edge becomes the active edge. Use the 
two  inverters in series rather than parallel to avoid that problem, at the 
cost of  lower drive capability and higher Tpd.
* On the interaction between the three signals: the worst is when the 1PPS  
signal hits and drives 3V into the 100 Ohms  equivalent termination (30mA).
At that point the power supply will sag, causing AM modulation to appear on 
 the RF signals. The result is humps in the ADEV plot at 1Hz, 2Hz, 3Hz, etc 
etc  all the way up to a couple of KHz. This is why separate power supplies 
and  driver IC's are recommended (a separate LDO for the RF signals and one 
just for  the 1PPS would solve this 1PPS crosstalk). This is one reason why 
I don't like  DC 50 Ohms terminations and love open-ended coax cables.
In fact Tom V.B. some years ago reported here that he could measure  the 
1PPS LED current (!!!) from one of his GPSDOs as it fed THROUGH THE AC POWER  
LINE into another unit.. Albeit at levels of xE-014 or lower if I remember  
In a message dated 11/25/2014 17:51:37 Pacific Standard Time,  
mark at alignedsolutions.com writes:

Thanks  Said.   Strangely enough I was just about to ask the group for  
comments re the practicality of using inverters in parallel with resistors as  
a simple means of buffering 1 pps signals.

I'll give this a  try.


Mark Spencer

On 2014-11-25, at 5:28 PM, "S.  Jackson via time-nuts" <time-nuts at febo.com> 

>  Guys,
> I never expected such an intense discussion about using  and buffering  
> 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 
> 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 
>  RF) of the LTE-Lite eval board with CMOS 3.0V levels that can drive 50 
> terminations. For simplicity I grab the 3.0V power from the DIP-14  TCXO 
> 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  
> 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 
> really well.
> Excuse  my horrible writing, using keyboards has made my fingers  numb..
> Hope that helps,
> Said
>  <CMOS_buffer_for_LTE-Lite-Eval.JPG>
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