[time-nuts] Did a member of time-nuts buy this?

Bob Camp kb8tq at n1k.org
Thu Dec 11 07:04:40 EST 2014


> On Dec 10, 2014, at 11:12 PM, Mike Monett <timenuts at binsamp.e4ward.com> wrote:
>>> It would appear the best approach is to simply start reading the
>>> posts at the beginning and save the interesting ones.
>> There are lots of interesting threads on Time Nuts ...
> Yes. It is very easy to get distracted.
>>>> First step is to be able to extract timing data from individual
>>>> sat's. Not all GPS modules do this correctly.
>>> Can you tell me some of the ones that do?
>> I have yet to see one for under $2K that does it correctly. I don't
>> have the cash to buy ones at those sort of prices. Some have
>> reported that the old Motorola UT's will do it. The samples I have
>> tried have not done very well. I may have not had them running right
>> - who knows.
> OK, that pretty much eliminates the NIST data.

or means that more research is needed on single sat boards. There are a *lot* of them out there. 

>>>> 5) Feed that into your control loop equation.
>>> There's another term I need to research!
>> It's not a simple control process, but it's not all that terrible
>> either. It just takes a bit of work to optimize. Figure a few months
>> to a few years for the optimization depending on what you have for
>> issues along the way.
>>> OK, so I figure out how to do this. How do I tell if this is
>>> making the gpsdo more accurate? In other words, how do I get the
>>> ADEV without having an H-Maser?
>> You get a Cs (or other atomic standard) or you compare several
>> different GPSDO's against each other. The preference would be for
>> groups of three so you can rule out ones that are not doing what
>> they should.
> I see the issues people have here getting their cs to work. Maybe the best
> approach is different gpsdos in a N-corner hat.

A Cs is normally a very limited time item. If you are doing the sort of long term tests that they really are useful for, you will wear one out pretty fast. They are not a good reference for short tau.

> I have already bought stuff in groups of 3 to prepare - gps receivers,
> rubidium and morion oscillators, a trimble, etc.

The ideal would be > 3 TBolts, > 3 same model Rb’s, > 3 same model OCXO’s and more than three groups overall. Each has their own ADEV curve  Comparing devices with vastly different ADEV is not the best way to go. 

>> You also need the measurement gear to resolve frequency down in the
>> 1x10^-13 range. A normal counter will not do that.
> Yes, I have a new invention for a DMTD that may work.

Well, make sure it works. Testing is at least as critical as any of this. 

>> Backing off a bit - why do this?
>> You can buy commercial GPSDO's on eBay.
> Yes, I also see the difficulties some have here trying to get them to run
> or even talk to them.

It’s roughly 100X easier to get an eBay GPSDO running in 8 hours than it is to design a GPSDO in under a year.

>> You can build published GPSDO designs that are known to work.
> Yes, I have seen most of them. I think I can do better.

That’s fine.

>> What is the objective here?
> I'm retired, kids are married, wife moved back to France, and I have no
> girlfriend or interest in getting one. I have a small pension so I can
> afford to buy parts and pcbs.
> I have been involved in precision instrumentation since 1960 and have a few
> patents and some new inventions I think could be applied to time and
> frequency applications.
> There are many new dacs, op amps, and bipolar microwave devices that offer
> much lower noise than current designs use. I think the performance can be
> improved in some areas with new components and design techniques, and I
> have  equipment and time to explore.

There are only two points that system noise really comes into the GPSDO design:

1) The TDC must have enough resolution

2) The DAC on the EFC must have noise below the OCXO

The TDC is limited by the basic resolution of the GPS system. It’s easy to build one that has far more resolution than needed / useful. 

The DAC issue is normally solved with a < $4 part. Unless you have an OCXO with a very wide EFC range, it’s noise is unlikely to be an issue. DAC resolution can be addressed to any desired level with two DAC’s (fine and coarse). (Once you get going, you only use the fine DAC).

The real “fun and games” revolves around the software used to implement the filtering / control loop between the GPS and the OCXO (or Rb, or TCXO, or MEMS, or VCXO, or Cs or …)

> As usual, I will have to build much of my own test equipment to measure the
> improvements, but I have been doing that for 5 decades so I'm used to the
> idea. Some of the items may be good enough to start a new company and bring
> to market. So I can call it R&D and get tax benefits.

I like the idea of tax benefits from doing this …

> Most of all, it's an interesting challenge. Lots to learn, many new ideas
> and concepts, and a ready source of information in the archives and online. 

Focus on the software. 


> What could be better?
>> Bob
> Thanks,
> Mike
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