[time-nuts] 10MHz LTE-Lite
saidjack at aol.com
Sat Nov 22 12:44:28 EST 2014
Hi Paul, Jim, David,
Let me address all your emails:
Glad you got your boards. $50 in overseas additional charges from your post office sucks!
Some hints for experimenting from what I have learned:
You definitely want to build a 50 Ohms buffer for the 10MHz boards and the synthesized outputs on all boards; on the 20MHz boards on the Tcxo output it's optional.
The biggest problem is building a suitable 3.3V or 5V power supply. I built a buffer using two NC7SZ04 chips receiving the input in parallel with a 1M terminating resistor to ground. Then using a 100 Ohms series resistor on both outputs to get ~55 Ohms equivalent impedance, and combining the two R's to drive the 50 Ohms inputs through a 100nF cap for DC blocking. You can use a 74AC04 just as well. I tried a standard high quality 3-pin regulator and got very bad AM noise modulation due to the large noise on the rail. Then I used a very low noise LDO from LT and that solved the problem and the output is now very clean and drives 50 Ohms inputs with ease.
you can grab the very low noise 3.0V rail output from the eval board to power the buffers (see the schematics in the user manual) but loading that creates a bit of heat on the LTE-Lite which will affect stability a tiny bit.
On the power consumption, you can see we go through a linear regulator to get 3.3V from 5V USB/EXT power. This is very inefficient. For best power consumption you want to use a very high efficiency buck regulator to generate the 3.3V from your battery. This means you loose the USB interface though as that chip runs from 5V and has an internal LDO.
On the zero Ohms R2/R3 resistors, check the schematics - these allow you to power the DIP-14 Tcxo from either the digital 3.3V rail, or the low-noise 3.0V rail (default). The software will auto-detect if you attach a 10MHz or 20MHz Tcxo, no configuration is needed.
On drafts, yes that is the number one cause of phase drifts. We put the board into an ESD bag, and put some thin ESD padding material on top. That prevents drafts, and following the EFC curve you can see the unit still reacts slowly to the AC or heaters kicking on. That's normal, and that's why we discipline to GPS.. In our setup the units have typically less than 20ns standard deviation on the RF and 1PPS outputs. The raw gps 1PPS output on the header is even better on average, but has the sawooth error on it. The sawtooth error correction value is in the PSTI NMEA message for those that want to use the raw gps 1PPS output and correct the sawtooth externally. This chip has a very high rate internal system frequency that results in very low residual sawtooth error.
On the auto survey process - this is disabled when using 3D mobile mode by shorting pins 1 and 3 on the 3-pin header as described in the "read me first". But be aware that changing that header with power applied results in flash memory corruption, and thus a very bad day. That's why we did not solder the header - to avoid any accidents..
Sent from my iPad
On Nov 22, 2014, at 8:42, paul swed <paulswedb at gmail.com> wrote:
> Ok give it a week for the magic to wear off.Then its time to hack.
> I am sort of headed into that mode.
> The system draws what I would technically call squat for power. Hmm wonder
> how thats measured VA watts??
> Locks pretty darn fast and recovers pretty fast. But you do always go
> through the survey. Not really a negative.
> It does produce a relatively stable output. But you can see it slip as
> compared to the likes of the Z3801 or 3811. This tends to be due to drafts.
> Its quite sensitive.
> So following best practices two thick socks are on top of it now.
> So to the hacking/curiosity.
> I will group my interests in several areas (No particular sequence);
> - A good enough reference to replace my cheap-y $25 Telco RB reference.
> - Adding the various buffers to have useful signals.
> - Trying to keep the support power consumption down to match the LTE.
> - The curiosity of adding a oven 10 Mhz oscillator. I have a PTI and
> - Battery back up a real question given the lockup time of the unit.
> The buffering and dividing will come first and may have to be 74 HC or HCT
> to get the 20 to 10 Mhz. Its unfortunate because 74 AC would allow
> everything to run on 3.3V. May just wait and order some AC chips and do it
> Then LPF the output 10 Mhz to a sine wave and hit a transistor buffer.
> The buffer will be the biggest power pig of all. They always are.
> I always seem to need various ticks. The 1 PPS will be adapted to RS232 and
> RS 485 using buffers/converters. Simple 1 chip wonders.
> The output only data feed could also be RS232 and there is a spare
> transmitter in the max chip I would use. These can be wired on a board or
> for almost nothing ordered from ebay these days with shipping delays.
> The oven is really a curiosity. I have a 20 MHz unit. I think that by
> changing the 3 zero ohm Rs on the system I can shift to 10 Mhz and directly
> replace the 20 Mhz TCO. If thats not true then the typical oven has to be
> doubled to 20 and then converted to a clean 3V digital signal. This thread
> already has some hints on the EFC voltage.
> Lastly Battery backup. By the time I get to here I will have decided if its
> even worth the effort. Batteries are a pain in the .... But is nice in that
> the system just runs. Having an oven absolutely takes a 1-2 watt solution
> to a 30 watt total solution. Essentially what my RB consumes today when you
> look at the wasted energy in the transformers and such. I am using an HP
> battery system that drove RB and CS references circa 1980. So not very
> efficient. But sure does work.
> Someplace sooner then later a box for it all. Drafts do upset the TCXO. It
> may need to be a highly customized temporary box. These boxes are available
> at most supermarkets. Ask for cardboard.
> So there you have it my 10 cents worth of musings on the direction I am
> On Sat, Nov 22, 2014 at 10:37 AM, Jim Sanford <wb4gcs at wb4gcs.org> wrote:
>> Just ordered a second 10 MHz board for my rover station....
>> wb4gcs at amsat.org
>> On 11/20/2014 3:32 PM, S. Jackson via time-nuts wrote:
>>> Hello everyone,
>>> after what must have been the longest thread in T-nuts history its
>>> all quiet today. I am going to take advantage of that and announce some
>>> good news:
>>> Its a miracle: the 10MHz DIP-14 TCXOs for the LTE-Lite came in weeks
>>> of schedule from the factory! And they work very well.
>>> We will thus be shipping out the 10MHz LTE-Lite eval boards in the next
>>> couple of working days. There are still a number left for sale on Ebay
>>> (search for "LTE Lite GPSDO"), so if you were hesitant to get one due to
>>> the long
>>> lead-time, then now is your chance.
>>> Also, after being in time nuts hands for almost a week I am surprised
>>> there are very few mails, questions, or comments about the 20MHz boards,
>>> and we
>>> have received almost no feedback on Ebay :( I hope that is a good sign.
>>> time-nuts mailing list -- time-nuts at febo.com
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>>> and follow the instructions there.
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