[time-nuts] Newbie questions

timenut at metachaos.net timenut at metachaos.net
Wed Jan 27 00:04:10 UTC 2016


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

   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.


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