[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
correctly..
bye,
Said
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.
Thanks
Mark Spencer
On 2014-11-25, at 5:28 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
> <CMOS_buffer_for_LTE-Lite-Eval.JPG>
> _______________________________________________
> time-nuts mailing list -- time-nuts at febo.com
> To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
> and follow the instructions there.
More information about the time-nuts
mailing list