Hi 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 years. 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 !!! Bob > On Jan 26, 2016, at 7:04 PM, [email protected] 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 > > > > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
