Power

The power connector is a standard 50 pin DE connector. By removing all the PCBs and tracing some tracks it was deduced that the unit needs a 28V DC power supply with the positive lead connected to pin 40 and the negative lead connected to pin 41.

Internal view (GPS RX is mounted on lid)

GPS receiver

The GPS receiver is a UT+. This unit is better in performance than any of the Tanner GPS and is very good for accurate 1PPS production, especially as it has the RAIM algorithm in the chip and has fixed position mode. This may be interfaced  electrically in the same way as the Tanner $15 GT+ receiver. However unlike the GT+ the unit cannot produce NMEA sentences. It can only produce and be communicated to in Motorola Binary protocol. More information is contained in the I/O commands and Controller commands of the manual (both zipped .pdf files).  The easiest way to do this is using a program such as TAC32, which even helps in deciding what Binary command does what.

When power is first applied to the unit, after a short pause the following is sent to the GPS unit by the large microprocessor chip on the large PCB:-

-> @@Aw Time Correction Mode GPS
-> @@Ay 1pps offset set to 0.000 microseconds
-> @@Az 1pps cable delay set to 0 nanoseconds
-> @@At Position Hold Mode DISABLED
-> @@Ag Elevation limit set to 10 degrees

While this is a good starting point to running a lock box, more commands need to be sent to the GPS unit (ie doing a survey and putting it into Position hold mode). Using TAC32 to force the GPS to do a survey and go into position hold did not change the EFC control voltage, showing locking was not happening. After examining the circuit it appears that the process is triggered by external commands to the unit, the format of which is unknown. However it is probably not an RS232 signal as no applicable interface chips could be found

10MHz oscillator

The unit contains a Datum 105242-001 10MHz Oscillator.

The unit is powered directly from the +28V supply. The oven pin is at +5V until the oven has warmed up when it drops to ~0.2V. The Electronic Frequency control (EFC) pin allows the frequency of the unit to be pulled by an external voltage:-

+5V     Offset +12Hz

+2.5V Offset 0Hz (ie 10.000000MHz)

0V    Offset -12Hz

This gives a sensitivity of +4.8 x 10-⁷ parts per volt. This makes it suitable for use in the Shera lock box as I demonstrated over the weekend

This oscillator was obviously meant to be locked from GPS judging by the A to D converter, divider and PLL chips close to the oscillator

Other pieces

The smaller PCB has a 7474 driving a 15MHz bandpass filter driving 2 mod amps driving a mini circuit 15MHz low pass filter which feeds to the edge connector. This circuitry can be activated by earthing the centre pin of the ZV2106 FET (the TO5 can device) through 22k. Interestingly the socket or components for feeding a signal appear to be missing (unless it gets them from somewhere else via the multi layer board) although no signal is spontaneously provided tot he edge connector when the chain is activated. I found out what it was doing by sweeping the circuit with a network analyser.

There is a square gold coloured DC bias T in the box. This blocks the 5V from the GPS receiver going to the antenna (as would normally happen) instead feeding a 5V fused supply fed through the coax from the large PCB to the antenna. This presumably is to stop engineers damaging the GPS board by accidents with the antenna. For normal use the GPS antenna can be connected directly to the PCB with the appropriate cable.

Conclusions

In the absence of any information on how to control the box externally, it looks as though, if nothing else it will provide a good quality 10MHz EFC oscillator and GPS suitable for use in a GPS locked 10MHz source. However I would be interested in hearing from anyone who knows any more about the operation of the box

WW2R 26/6/03