[time-nuts] Need info: Spectrum Instruments brand GPSDO

ed breya eb at telight.com
Fri Jul 27 14:24:35 EDT 2018

I recently picked up some old cell base station units, and retrieved two 
of what appear to be this unit, from Spectrum Instruments:


I have asked them if any info is available, but no response yet. If 
anyone knows anything about this unit, I'd appreciate hearing about it.

Speaking of redundant references, I can picture two identical units 
operating in the "exact" same environment and GPS view, but redundancy 
is not my purpose here. I have tested both units separately, and they 
seem to work and provide GPS and ref lock indications. I've got them 
both mounted in one chassis, with a common PS (15-24 VDC). The plan is 
to feed them both with the same input from one GPS active antenna, and 
then experiment to see how they behave separately and versus each other.

I assume I can eventually at least talk to the the Motorola GPS RX 
modules, but I don't know about the rest of the custom GPSDO brains. 
There are two DB-9 RS-232 (probably) ports on each unit - apparently one 
for the GPS, and one for the rest.

One thing I'm wondering is how much interaction/interference may occur 
between the front-ends. I'll be building a module with DC to feed the 
antenna, a preamp stage to boost the signal a little and split it 
between the RXs, and provision to fool them both into thinking their 
usual antenna DC load is present. The post-split RF path lengths will be 
closely matched. I can add extra isolation if needed, but I'd like to 
see what happens.

I'd expect that their 1 PPS and 10 MHz should ultimately be in phase. I 
don't know if these units do any kind of sawtooth correction, but I 
guess I'll find out eventually. The 1 PPS signals are not available on 
the panels, but I know they're in there.

As-are, the GPS modules' internal clocks (19.095 MHz marked on XTLs) are 
independent. Other experiments could involve forcing them to be the same.


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