[time-nuts] A few questions about Tboltmon

Magnus Danielson magnus at rubidium.dyndns.org
Thu Aug 13 20:00:30 EDT 2015


On 08/08/2015 07:05 PM, Bob Camp wrote:
> Hi
> A factory reset will not brick the unit.
> Either:
> 1) Your TBolt is blown
> 2) The cable has an issue
> 3) The antenna has an issue.

I've seen them all over the years, so neither is necessarily the most 
likely. I'd also add:
4) Power-issue
5) Serial cable problem

For this case 1-3 should be your culprit.

Oh, do remember that engineers invent the most complex scenarios of what 
the failure mode is, but fail to identify the simple ones such as power, 
cables and connectors failing.

> For troubleshooting this sort of thing, multiples of each are a
> handy thing to have. Baring that:
> 0) Put a DVM on the coax and see if you have bias to the antenna
> 1)  Hook up a TDR to the cable and ring it out both with a load and a short on the end.
> 2) Put the antenna on a *very good* spectrum analyzer and look at what is coming out.
> 3) Grab a signal generator that will simulate a GPS constellation and drive the TBolt with that.
> Since nobody (other than Magnus) ever has the sort of gear for 1-3, and it’s all pricey stuff

TDR is a nifty tool for this sort of thing so 1) is nice, but it won't 
really help you and you will have to know what to expect from a 
unpowered LNA. Spectrum analyzer will not directly help you since the 
satellite signal spectrum is below the noise-floor, but you *might* see 
the amplified noise as shaped by the LNA pre-filtering, which is 
hopefully SAW filtered, so 2 is doable but tricky to interpret for the 
novice. If you have a constellation simulator lying around, it will help 
you to see if the receiver is working at all, but even I don't have that...

Having a VNA helps, and the nifty TinyVNA for instance will be quite 
useful. Similar to the TDR, it sends a signal up the wire and analyze 
the response, but in frequency plane rather than time-plane. Again, some 
experience is required but this is a good time to learn.

> the simple answer is:
> 1) The antenna is probably the cheapest part of the setup. I’d swap it out first.
> 2) The cable is cheap but a pain to run, is it #2 or #3.
> 3) Hook up another timing receiver to the cable. There are lots of them out there in the $100 to $150 range.
> TBolts do die. My experience is that roughly 1 or 2 in 50 show up with a fatal issue. Another 1 or 2 show up
> with a (correctable) minor problem. I have had one drop dead after running for a while.

I would grab an antenna, toss it out a window and see if I get anything. 
It is always handy to have additional antennas and cables around, and 
for checking things to be operational, only modest requirements in type 
and position is needed.

Similarly, having another GPS receiver to see if I get any form of 
signal is a great tool.

Just taking the time to do quick and dirty tests helps. I've found that 
I made stupid mistakes, so just doing a round of quick reality checks 
have been important hints to find errors.


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