Here Brian Kirby, N4FMN describes the modifications that allow TAC32 software to monitor the Motorola Oncore GPS engine used in the HP Z3801A. A side benefit of using the GPS engine signals is a useful front panel one PPS indicator that does not require hardware pulse stretching.

This project was designed to allow monitoring of the Motorola Oncore VP receiver that is used in the HP Z3801A GPS Disciplined Oscillator. I have experience with the Motorola Oncore VP GPS receivers and I wanted to monitor the GPS jitter on the 1 PPS signal, to see how much reduction was performed by the HP Smart Clock Technology. I also wanted to know about the GPS "sentences" the HP Z3801A uses.

The program I use to monitor the receiver is TAC-32 by CNS. You can download an evaluation copy from them that will work for 30 days. If you are a member of TAPR, you can obtain the program from them at the discounted member rate. There are other programs that can also decode the Motorola binary strings, such as WinOncore and OutlookGPS.

TAC-32 can decode the Motorola binary signals to ASCII. This allows you to examine the results. Note that in this implementation, TAC-32 will only be operated in a receive only mode and cannot control the GPS receiver.

This is a typical receiver ID message:

The circuit uses a Maxim MAX232 integrated circuit for signal/level translation. The Motorola Oncore VP receiver uses TTL signal levels. In order to use this data with a computer, the data must be converted to RS-232. Use shielded mini-coax for the data and 1 PPS interconnects. In my first attempt, I used unshielded wire, and I had the MAX232 circuit board installed between the Z3801A power supply and the main board. I had intermittent results and I believe that the power supply switching regulators induce noise into the MAX232 circuit. My wiring connections were on the top of the GPS receiver.

In this final design, I use RG-174 coax for the receive data and 1 PPS connections. It is connected to the underside of the main board and held securely using hot melt glue. I mounted the MAX232 circuit board near the front of the Z3801A between the front panel LED board and the 10811 oscillator. I mounted a DB-9F connector on the rear panel that connects the MAX232 circuit to a computer.

The second part of the circuit allows for a GPS 1 PPS indicator, which is missing from the Z3801A. I cut the traces to one of the front panel LED indicators and wired the MAX232 circuit to the LED. I choose the second LED, labeled "enable" as we have not found any use for it, and it is not used in the GPSCon software. I also added another 1 PPS output, which is a TTL level from the smart clock circuitry in the Z3801A. I mounted a BNC connector to the rear of the chassis. This pulse is very narrow, typically 20 to 50 microseconds.

Some folks are concerned that connections to the GPS receiver may cause degradation of the Smart Clock function. There is a concern and the point is well taken. I believe that satisfactory results may be obtained if shielded interconnects are used and if you use techniques that will not change the characteristics of the 1PPS circuits within the smart clock.

The Motorola Oncore VP manual shows that the receiver uses inverted TTL logic. Maxim does not specify the input impedance of the TTL to RS232 transmitter, but it appears to be a high impedance, so loading of the 1 PPS circuits should be minimal. The MAX232 transmitter is TTL and CMOS compatible. You can download a data sheet on the MAX232 from Maxim if you desire to examine the circuit.

Since modification of the receiver, it has ran a couple of weeks, and I have not seen any degradation of the smart clock function. I have two Z3801A's and I have compared it to the unmodified unit and against a Efratom FRK rubidium oscillator.

© 2003-2007 by Brian Kirby, N4FMN