[time-nuts] Slight tune-up of rig

Magnus Danielson cfmd at bredband.net
Sat Jul 23 20:06:21 EDT 2005


Back from the summer house earlier today, I found that a certain package has
arrived. Needless to say, I found myself an excuse to go buy some food (package
delivery in stores give you this excuse for free). Anyway, it contained an
HP Z3801 with antenna. ;O)

I have been running my Rubidium for some time now, but I have had no UTC
traceability, no time (and no position for the house). I also have a few good
crystal oscillators, but only one of them is in continous operation (the other
two is a pair of HP 10811 as located in suitable HP instruments).

First up was doing the traditional RS-232 modification. Instead of doing the
dual soldering iron technique I used my hot-air gun, which is *much* quicker,
especially considering I already have it. ;O)
Add to that the need to wire up a suitable RS-232 cable.

OK, so how apply power? I found that my stock of old PC parts was a great place
to raid, and I cut of a HDD/FDD power-lead of a PSU, poped out the +5V and GND
wires (suitably red and black) and inserted them into pin 1 and 3 of the power-
jack. I took the lab-psu flipped it to 48V, released the current limiter and
hit the power-button. OK, so after some fiddeling I got power and all, great.

Having it communicate with the SatStat took little time once I had figured out
I had got the cable wrong and made a good one. Now came the soft problem, how
to make it track satelites. Forcing it to a position in my neighborhood rather
than the last position it had when used (over quarter of an earth away from its
new position) helped it to make sense out of the GPS almenac. However, my poor
antenna position (in the window) didn't help, so dropping out the window
(gently) and toss it up on the garage roof (with a larger part of the sky in
clear view) made the trick and it is now working its way throught the survey
and oscillator locking. It is taking more time than I anticipated, but it keeps
working at it, so I'm quite happy so far.

I will have to work some on the whole setup thought. The current position is
more convenient from the point of bringing up first time then operation. I need
to fix a propper PSU arrangement (backup-battery included), properly rig the
antenna (I want to close the window!) and find a good spot for the antenna
considering clear-sky view. Then I want to setup some good continous training
of the Rubidium and crystal clock I have, besides the nice and mr clean 10 MHz
I am getting in the back.

While inside the Z3801 I noticed a number of other features on the PCB that I
want to investigate further. Does anybody has the EPROM images to share?
Lacking an EPROM reader at home (well, working one, OK?) and wanting to stay
out of work on the last week of vacation... ;O)
Anyway, I noticed that a second 10 MHz output should not be hard, a pair of
transistors and passives and its all set. There is also place for additional
outputs, but I am assuming that the missing FPGA is a clue here. BTW, the
Xilinx FPGA 3042 is not a very powerfull one, it's only 144 CLBs (12x12) and
each CLB is 2 4-input LUTs (and more). Dividing down the 10 MHz to PPS, provide
control over it and also buffer the 10 MHz to the output should however be a
feildtrip even with that one (it is unbeleivably limited by todays measures).
The missing FPGA is probably for the additional 2,048 MHz and 1,544 MHz output
options of the HP58503 if I where to do an advanced guess. With 10 MHz one can
DDS out these frequencies.


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