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

Brooke Clarke brooke at pacific.net
Mon Mar 11 16:41:45 EDT 2013

Hi Ed:

I have a little info at:

Have Fun,

Brooke Clarke

Ed Breya wrote:
> 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
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