Retronetworking » OCTOI - Osmocom Community TDM over IP

The spare Ericsson GPS receivers I have here are GPS03 (successor of GPS02), attached a MA-700G antenna and loked at the current consumption. The absolute peak was 161 mA during start-up. Once the GPS signal is locked (green led continuously on), it's 97..103mA @ 12V.

At the predicted worst-case loop resistance of 100m CAT5 of 20 Ohms, that would be a 2V voltage drop for the average power consumption, and 3.2V on peak.

Will do some voltage range tests next, and also insert a 20Ohm series resistor.

regarding capacitance, https://en.wikipedia.org/wiki/Category_5_cable states 52pF/m so about 5.2nF for 100m. To that we'd have to add the capacitance of the RS422 receiver input, but for sure it's just going to be some pF, so negligable.

• 22 ohms series resistance between PSU and GPS 03
• 100 ohms parallel to 4.7nF termination of 1PPS or NMEA-Tx pair
• current consumption increased from 100mA to 120mA
• scope traces of signal looked fine to me
• receiver operation down to about 10V PSU output voltage

• terminating the second transmitter (NMEA Tx) increased current consumption from 120mA to 148mA
• operation of receiver still down to 10V from the voltage source
• cold start also possible from 10V voltage source

So all in all, we can conclude it is safe to operate the GPS03 with the chosen antenna over 100m of CAT5.

some scope traces of the data on the "remote" end of the simulated CAT5:

Building the entire setup (22R on 12V rail, 100R || 4.7nF on each of the three differential pairs) on a small adapter board (mis-appropriating one of my RJ45 jumper box PCB) looks like this:

For some reason the waveform of the 1PPS now looks much more reasonable. Maybe I had a bad probe GND connection or something last time. It's a nice 50us square pulse. Rise time has not changed.

the over voltage protection devices for the RS485/422 signals (DCO SD2 MD) have meanwhile arrived. However, the part for 12V was missing from the shipment (despite listed on packing list). Have complained with the supplier.

in case anyone is curious, this is the internals of the "DCO SD2 MF HF 5":

If I got it right, the schematics look like this:

picture of the APU-sized RS422 OVP board that goes into the 1U enclosure:

Did some more testing with the following chain:

• GPS antenna
• GPS03
• 100m-rj45-resistance-and-capacitance-simulator
• rs422 OVP + 12V injection board
• USB-RS422 converter attached to my laptop / 1PPS to scope

This setup was working fine for hours. It's still missing the DIN-rail OVP next to the GPS03.

I later swapped the 100m-simulator agaist an actual spool of 100m CAT5. Surprisingly, the RS422 signals still look excellent and the NMEA is correctly decoded. However, I think something is wrong with the phantom power, as it looks like the GPS03 is crashing repeatedly. IT does get a fix, but then soon after the first valid standard NMEA sentences, it starts all over again.

new hub is fully deployed, GPS 1PPS arrives in the rack and icE1usb reports err=0 most of the time, i.e. the 2.048 MHz clock runs at 1PPS-disciplined 2048000 Hz.