Discussion:
Need info: Spectrum Instruments brand GPSDO
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ed breya
2018-07-27 18:24:35 UTC
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I recently picked up some old cell base station units, and retrieved two
of what appear to be this unit, from Spectrum Instruments:

http://www.spectruminstruments.net/products/custom2/custom2.html

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.

Ed



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Thomas S. Knutsen
2018-07-27 18:39:41 UTC
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That looks like a re-packaged Spectrum TM-4M: Loading Image...
With the internal lead battery and routing to different front
connectors removed, it looks quite alike.

If so, they can do either their custum binary protocoll or NMEA. I
have been meaning to suggest for Mark Sims that it would be a
apreciated addition to Lady Heather to add the binary protocoll.
I should have a manual around, if the TM-4 manual can't be found on
the internet.

BR.
Thomas.
Post by ed breya
I recently picked up some old cell base station units, and retrieved two
http://www.spectruminstruments.net/products/custom2/custom2.html
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.
Ed
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--
With Best regards, Thomas S. Knutsen.

Please avoid sending me Word or PowerPoint attachments.

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ed breya
2018-07-28 23:48:44 UTC
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Thanks Thomas, for the link to the TM-4M. It appears the TM-4M is a much
newer and fancier unit than these things. There may be some operational
and HW similarities, but the ones I have are about twenty years old, and
are clearly bigger (large single-board) than the dimensions of the newer
types.

Ed

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ed breya
2018-07-28 23:43:24 UTC
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I have it temporarily rigged now with both units apparently operational
and locked. I changed the antenna pigtail leads on the GPS RX units from
TNC to SMA for more convenient HW experiments. I disconnected the DC
feed from one and loaded it with a dummy resistor, so only one provides
antenna power, while the other thinks it does. The two inputs are fed
via a Wilkinson type splitter, which passes the DC to the antenna port.
There is no extra amplification or isolation yet.

I won't be able to tell really just how well they are working until I
can talk to the RX units via RS-232, but at least they seem OK so far.

Ed

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