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, m...@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@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@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.