[time-nuts] Clarification from a new member

Richard W. Solomon w1ksz at earthlink.net
Mon Feb 4 12:19:06 EST 2008


Try measuring the lock voltage on the VCXO. Use a high impedance meter as you
can easily load down the circuit. If the lock voltage varies with stability loss
than your GPS receiver may be faulty, or your GPS signal is not strong enough
(antenna placement). You can monitor signal strength with freeware such as
VisualGPS. They are simple circuits but can be a bugger to de-bug !!

73, Dick, W1KSZ

-----Original Message-----
>From: Dennis Tillman <dennis at ridesoft.com>
>Sent: Feb 3, 2008 3:52 PM
>To: 'Discussion of precise time and frequency measurement' <time-nuts at febo.com>
>Subject: [time-nuts] Clarification from a new member
>
>In my introductory email to the group, I mentioned that I was disappointed
>in the GPS receiver I built from James Miller's design. 
>
>I must clarify what I said with apologies to James for any misunderstanding
>I might have created about his design! 
>
>The receiver has an intermittent problem and I have not been able to track
>down the cause. The measurement of time to this level of precision is new to
>me. As of now, I have few pieces of test equipment suited to the task. What
>I do have, I either built, or bought broken and repaired. The problems I am
>experiencing with the GPS receiver are almost impossible to track down
>because they are intermittent, they occur infrequently, and they involve
>measurements to a level of precision I am not well equipped to make. 
>
>At this point, I do not know what is causing these brief losses of
>stability. They last for less than a few minutes as near as I can tell. My
>next step, which is to try to monitor the output voltage of the PLL and
>correlate it with the loss of stability, requires a way to log voltages.
>Unfortunately the two GPIB Digital Multimeters I own both broke before I
>could do this. Until I know what part of my GPS receiver is causing the
>trouble I should not speculate on the cause. 
>
>What I can say is that I followed James design to the letter. It uses the
>Rockwell Jupiter TU30-D140 Receiver, James' PLL PCB, the IsoTemp OCXO134-10,
>and a Symmetricom 58532 GPS L1 Reference antenna mounted on my roof and
>connected to the receiver through low loss cable. The entire circuit is
>professionally mounted into a Tektronix TM500 Prototyping Plug-in that is
>powered by a TM501 power supply that is, in turn, plugged into an
>Uninterruptible Power Supply.
>
>One feature I wish this design had is an indication of when the output is
>locked to the GPS satellites. I must admit, I could not figure out a simple
>way to do this. I have since then come across a circuit that purports to do
>this. However, until I solve the intermittent problem I have been reluctant
>to add this circuit to the design.
>
>Dennis
>
>------------------------------------------------------------------------
>> * Two years ago I built my own GPS frequency standard based on the 
>> design by James Miller but it has been a big disappointment. I started 
>> to suspect it was not reliable so I wrote a LabView program to compare 
>> it to the reference outputs of two ArgoSystems AS210 systems I found 
>> on eBay and restored. The LabView program confirmed my suspicions that 
>> the GPS receiver was briefly loosing its stability for brief periods 
>> several times a week.
> 
>
>
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