[time-nuts] Newbie questions

wb6bnq wb6bnq at cox.net
Wed Jan 27 08:02:45 UTC 2016

Hi Mike,

The element that you are missing is the impedance.  When you look at the 
common formula it refers to a ratio of power or voltage and the 
impedance is left out with the understanding that the impedance is equal 
for each power or voltage in the ratio.  The actual formula (for power) 
is DB = 10 X log10 ( E1^2 / R ) / ( E2^2 / R ).  Hint   Power P = E^2 / R.

In the RF world that impedance is 50 Ohms and ZERO DBm(illiwatts) is ONE 
milliwatt into 50 Ohms which is 0.223606797749979 Vrms.

In the audio world the reference impedance is 600 Ohms.  So ZERO DBu is 
One milliwatt into 600 Ohms which is 0.7745966692414834 Vrms.  More 
commonly referred to as 0.775 Vrms.

As for your GPS questions, I will leave that up to others to answer.


timenut at metachaos.net wrote:

>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.
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