[time-nuts] Need info on Trimble 4000S GPS Surveyor

[Ed Breya]

I recently acquired a junker Trimble 4000S GPS surveying unit. It's 
mid-1980s technology, so very big, but nice to salvage various RF and 
signal processing goodies from. I have no plan to get it working, and no 
need for the function - it's just for parts/subsection use. I found that 
this unit is way beyond obsolete, and found no support info on-line. I'm 
just wondering if anyone has or knows of info on it - even a block 
diagram would help, or anything to fill in some blanks.

I've saved the RF/PLL modules intact, and recorded the interconnections 
and supplies, so I can (eventually) figure out what everything does. 
This unit uses both the L1 and L2 carriers, and has eight canned 
function modules plus an OCXO to do the front-end work. Other than the 
power supplies, the entire interconnection appears to be just two IFs 
and two references going to the control/DSP board assembly (four huge 
boards), and a 16-bit parallel DAC drive from there to control the EFC 
line to the OCXO.

The L1 1575.42 MHz chain uses a 16.368 MHz VCXO locked to the 10 MHz 
reference, running an LO of some integer multiple that results in a 
reference around 38.4 MHz labeled "ECL 38.4 F0" on the main board, and 
an unlabeled signal IF called "TTL LIMITER." Internal markings "768" and 
"384" may indicate PLL IFs of 76.8 and 38.4 (76.8/2) MHz.

The L2 1227.6 MHz chain uses a 28.644 MHz VCXO locked to the 16.368 MHz 
reference, and LO that results in another unknown IF that runs through a 
similar TTL limiter. It appears that the LO is an integer multiple of 
the 28.644 MHz, with a PLL IF possibly around 59.2 MHz, marked "592 FO." 
Only the unknown signal IF from this section goes to the processing 
boards - no PLL IF seems to go beyond these modules. The unknown signal 
IF goes only to one of two apparently identical DSP boards, unlike the 
others that all go to the main board.

The L1 downconverter appears to use quadrature mixing, but I can't tell 
what happens after that - the I-Q signals go into a bunch of baseband 
circuitry. The L2 one also has a quadrature mixer, but only one output 
goes into its baseband circuits - the other is just terminated. As I 
understand, the L2 is always encrypted, so useless for data, but its 
carrier can be used to enhance overall accuracy - I recall studying that 
a few years ago, but forgot the details. So, maybe the L2 portion is 
only for carrier recovery of some sort.

So, here's what goes between the RF section and the rest:
RF to main board:
1. Main reference 16.368 MHz
2. L1 DSP reference approx 38.4 MHz
3. L1 IF unknown frequency

RF to one DSP board:
L2 IF (or recovered carrier?) unknown frequency

Main board to RF:
16 bits DAC EFC tuning for 10 MHz reference

Antenna to RF:
Input to diplexer, with +15 VDC feed for preamp

I'd appreciate any info or ideas on deciphering the rest of the way - 
maybe the modules will be useful for something as a system, rather than 
just parts. I'm especially interested in GPS carrier recovery techniques 
for frequency only - not time.

Ed