[time-nuts] Newbie questions

Bob Camp kb8tq at n1k.org
Wed Jan 27 12:43:12 UTC 2016


Ok, so let me answer the questions you *should* have asked:

(They are in no particular order. Number 3 probably should come first)

1) Is the gear I have enough to do this project? 

No, you will need some sort of frequency  / time standard. An atomic
clock of some sort is pretty much a minimum. You probably also need
a working GPSDO (or set of them) for comparison as well. You will also 
need a working / modern precision counter that will give you data down 
in the < 100 ps range. 

2) How will this ultimately be built? 

At the very least, you will be building this with surface mount devices. 
If it’s a scratch build, you will be dealing with fine pitch parts. That gets
you into a whole bunch of gear. It also gets you into a very real “is this 
fun or not” sort of question. 

3) What *is* the goal? 

"I’m going to make dinner” is the start of a process. It’s not enough of a 
goal to accomplish the task. Starting the task with a general objective is fine.
It does need to be refined a bit before you go much further.  

Is this what most of us would call a GPSDO (self contained box) or is it
something with a PC in the middle of it? 

Is this an OCXO based “precision” device or is it something more simple?

Is a pure software solution good enough?

Each of those decisions (and that’s by no means a full list) will send you off 
in a very different direction. 

4) How long is this likely to take?

Best guess based on the others who have done the same thing - several 

5) How much is this likely to cost?

If done the way others have done it, several thousand dollars up to 
quite a bit more than that. 

6) How much research is involved?

Quite a bit. The information you need is scattered all over the place. Figure
that you likely will read at least several hundred papers. There is a whole 
statistical language that is unique to these gizmos. This is *not* a follow a 
set recipe sort of project. 

Lots of fun !!!


> On Jan 26, 2016, at 7:04 PM, timenut at metachaos.net wrote:
> Hi,
> I am a newbie to this list. I have downloaded the archives and read about
> 5,000 of the past messages. I plan on building my own GPSDO, probably using a
> LEA-6T (but LEA-7T or LEA-M8T would be good if I can find one affordably). I
> have a MTI 260 on order (although it could wind up being a 261 since they all
> appear to ship one or the other randomly).
> Currently, my resources include a DMM (well, a couple) and soldering /
> desoldering stations and quite a few tools. I also have an oscilloscope that I
> am currently repairing - a 400Mhz Tektronix 2465BCT analog scope. I am waiting
> on the final parts from Mouser. Once that is done I need to get it calibrated.
> All of that will probably take me another month. I also need to finish fixing
> my cassette deck - and then to finish writing a special recording program to
> use raw device drivers to get around the fact that Windows is not real time. I
> interrupted that project to work on the scope.
> In the meantime, I am reading the time-nuts messages (and lots of other
> things) to gather information and ideas about how I am going to do this and
> generally to learn more.
> So, I have some questions. Let me tell you a bit about me, so that you know
> the context and my limitations. I am a retired programmer. I wrote just about
> everything including device drivers, operating systems, utilities, various AI
> programs, telephone systems, compilers, encryption, web applications and much
> more. If I need to throw 50,000 LOC at a project, no problem. I have used many
> languages including quite a few different assembly languages (I have also
> written an assembler). I consider myself a mathematician / programmer,
> although I haven't really needed Calculus or Differential Equations for
> decades, so I am pretty rusty in that area. I do more work in formal logic
> than higher mathematics. But, I THINK like a mathematician. Formalism and
> abstraction come naturally to me.
> During my career I also helped to debug hardware during S-100 days. I have
> sporadically messed with electronics off and on, informally, with no education
> in the area. Now that I am retired (and have more time, but less money - it IS
> a zero sum game!), I am trying to learn more about electronics and start doing
> hardware projects. I have never been into model building or anything similar,
> so my construction skills are lacking. I understand a lot of things in theory,
> but practice still eludes me. For example, knowing a part exists or
> determining which of 10,000 apparently identical parts is the "right" choice.
> It can hours or even days to find the "right" connector. In many cases, the
> names or descriptions are completely meaningless. That all appears to be an
> experience related issue, so I will (hopefully) overcome that in time.
> I have no problem with soldering / desoldering, but I haven't designed or
> built my own PCB yet. I have designed / redesigned some minor circuits,
> especially on the power supply side. I can follow schematics reasonably well,
> but I am not comfortable with Eagle or other PCB layout programs. Every time
> I have tried one of those programs, half of the parts I needed were not
> available. I have started using TinyCAD which is much easier to use. So, I
> have a lot to learn. But, that is basically what I do, all day, every day. I'm
> the type of person that gets bored easily and quickly. As #5 said "more input,
> more input"!                                           6.02059991327962
> Paradoxically, I have no interest in time. As in time of day, day of week,
> etc.. I have never had a job where I got to work on time. My philosophy has
> always been "go to bed when sleepy, get up when not". I was notorious in high
> school for only showing up on test day. But, I am interested in being able to
> timestamp events accurately and in measuring time (and other things). I am
> also interested in how a very accurate frequency source can be used in
> other applications and test instruments. That brings me to my desire to build
> a GPSDO and my questions.
> I understand the logarithmic scaling used for voltage and power. I even
> understand why voltage uses a multiplier of 20 and power a multiplier of 10.
> It makes sense when working with a wide range of values. However, my DMM, my
> scope and generally schematics work directly with current, voltage and watts.
> So, I am constantly seeing statements like an output is 7 dBm or 13 dBm. If I
> knew the actual value for 0 dBm then the basic equations would resolve the
> values. However, I have not found a consistent answer for that. When I have
> attempted to work values backwards from various statements, again I don't get
> a consistent value (probably because those statements were approximations and
> not exact values). I always see statements that an increase of 6dBm doubles
> the value. It is used so often that most people forget it is an approximation.
> It is 6.02059991... and sometimes, it may make a difference. Worse, the zero
> value appears to be different for different applications. In some it appears
> to be completely arbitrary. So this leads to two questions...
>   1. What is the zero value for voltage and watts using logarithmic scaling
>      (at least as used here)? Is there actually a consistent underlying value
>      across all applications?
>   2. Why use it for specifying voltage or power in a limited range? Why not
>      just say that the output is 1.0v rms or 0.7v, or that it uses 50mW? There
>      does not appear to be any actual advantage to using a logarithmic scale
>      for a small range of values - and 1mV to 1kV IS a small range.
>      Especially when you have to convert the logarithmic value to a "real"
>      value to actually do anything with it.
> I have also been researching GPS antennas. From what I can see there are two
> basic types - the flat puck and the helical. I have not seen anything to
> distinguish the two types based on performance or usage or to indicate that
> one or the other might be better for GPS timing. However, I have seen "GPS
> Timing Reference Antennas" advertised. Most or all of those appear to be
> helical. But, I have not seen anything that specifies the difference between 
> an active GPS antenna and an active GPS Timing Reference Antenna.
>   1. What is the difference between a "normal" GPS antenna and a GPS Timing
>      Reference antenna? What features are of interest?
>   2. Is there anything extra needed besides a GPS antenna to enable the use
>      of WAAS or other services? Apparently the ubolt receivers can make use
>      of some of that, but it is not clear what is needed to provide that
>      information to them, or if they just pick it up automatically using a
>      standard GPS antenna.
> Also, from what I have read, using carrier phase for timing is potentially
> more accurate by a couple orders of magnitude. Are there any GPS timing
> receivers available that use carrier phase? Or use both L1 and L2 for
> increased accuracy? I see that the ubolt receivers can report some carrier
> phase information, but that doesn't appear to translate to increased accuracy.
> And the LEA M8T use dual channels, but don't appear to mix GPS and GLASNOS to
> improve accuracy. Do any receivers do that? I suspect that building a GPS
> receiver is probably more complex than can be easily handled by an amateur so
> I am most likely restricted by available receivers.
> I have also read, more than once, statements in this forum that something or
> another could be had for some low, low price so why build it yourself? I think
> that there are several reasons, including but not limited to the following.
>   1. It is an interesting project.
>   2. It is an educational project.
>   3. You may have some ideas about how things could be done differently or
>      better.
>   4. You may want some combination of features that is not commercially
>      available or perhaps is not affordable even with a generous budget.
>   5. Many people on limited budgets are not limited by total cost, but rather
>      by incremental cost. So, someone may not be able to afford several hundred
>      dollars for a pre-built system. But, they may be able to afford $50 here
>      and there. So, building it themselves is the only practical option.
> Mike
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