[time-nuts] LTE-Lite Plans

Said Jackson saidjack at aol.com
Tue Nov 25 22:37:17 EST 2014


Its not the 1PPS that would be suffering, its the 10MHz that will have all the 1Hz and its harmonics making the PN graph look ugly..

Agree with you that the regulators cost zip these days and using individual buffer ICs and regs is the best way to go.


Sent From iPhone

> On Nov 25, 2014, at 18:45, Bob Camp <kb8tq at n1k.org> wrote:
> Hi
> One simple point:
> Do you *need* ultra low phase noise on your 1 pps output or is real good ADEV all you are after? 
> If you need good phase noise .. exactly what are you doing ???
> So… tack a 78L05 onto your bulk power and run the pps output “empire” off of that supply. Maybe wire the 1 pps stuff on it’s own little chunk of PCB material. 
> Save the fancy low noise regulator(s) for the 10 MHz “empire”. ( If you get a good one, 78L05 might do just fine there as well).
> What’s the massive cost impact of this radical approach? 
> Well the inverter chips are < $0.20 each from several outfits.
> The 78L05 is also < $0.20. 
> The resistors and caps should be on your bench already. If not plan on another $0.30 for the bunch. 
> So you have added (at most) $0.70 to the cost of the circuit by doing this. 
> Skip the order of fries with lunch and it’s paid for.
> The above does not include the cost of connectors, enclosure, power or switches. All of that will be part of any design you do. Enclosures and power are going to be lower with this circuit than just about anything else you could do. No hogging pockets out of a 1 foot cube of aluminum required …..
> Bob
>> On Nov 25, 2014, at 9:14 PM, S. Jackson via time-nuts <time-nuts at febo.com> wrote:
>> 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.
>> _______________________________________________
>> 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