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

[Magnus Danielson]

On 10/03/13 20:30, 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.

1575.42 - 2*76.8*10.23 = 4.092 MHz

This is the L1 C/A code receiver.

> 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.

1227.6 - 2 * 59.2*10.23 = 16.368 MHz

This is the L2 P code receiver.

> 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.

On L1 and L2 will the P(Y) code be encoded on the I signal,
and the C/A code is encoded in the Q signal of L1. Knowing that, what 
you see is logical.

> 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.

The doppler will kill you, so you need to recover message, resolve time 
etc as this is the way you compensate for things. That DAC probably 
trained the 10 MHz for your needs.

I took the numbers above and used the knowledge that 10.23 MHz is the 
basic rate of GPS, with C/A running at a tenth of that rate.

Do have a look into ICD-GPS-200 as all the key info I gave you comes out 
of that.

Cheers,
Magnus