Question: How do I wire the serial port of the controller to the DB-9 connector on my computer?
Your DB-9 should be wired: DB-9 U10 function ----- --- -------- pin 2 pin 5 data: controller to PC pin 5 ground ground return The above is all you need if you just want to monitor the controller output as discussed in the QST article (footnote 10). You can also make a cable arrangement that will permit you to communicate between your PC and your GPS receiver. You can then use the TAC32 software to initialize "position hold" mode in your receiver and monitor its operation. If you are using a Motorola OnCore UT+ or VP model the following description may be helpful.
To set up the UT+ (or VP) using a PC program, you can make a separate DB-9 connector that uses the spare sections of the MAX233 as inverter and level shifter to change the (inverted) TTL signals from the UT+ into real RS-232. This provides functionality similar to the TAC-2 board from TAPR. You will need to make 2 harnesses. One connects the controller board to the PC, the other connects the controller board to the UT+. The wiring is as follows: DB-9 U10 UT+ 2x5 header function ---- --- -------------- -------- . 1 ext. BBRAM battery backup (+3.6v) . 2 +5v supply to UT+ (can come from controller) . 3 gnd (ditto) . 4 n.c. . 5 n.c. 1 6 1 pps output (TTL) . 7 gnd . 1 8 TX data to PC (inverted TTL) 2 18 TX data to PC (RS-232) 3 4 RX data from PC (RS-232) . 3 9 RX data from PC (inverted TTL) 5 gnd 10 serial port ground return
Note that the RX and TX signals are true RS-232 since they are converted by
the MAX-233 chip. The 1 PPS pulse to the DCD input pin of the PC serial
is TTL, however this works fine for most PCs and, in any case, TAC32 will
work even without the 1 PPS input.
The pin layout on the UT+ 2x5 header as viewed from the top
side of the board (pins 1 and 2 are at the end nearest to the
edge of the UT+ board).
__________
| 1 2 |
| 3 4 |
| 5 6 |
| 7 8 |
| 9 10 |
----------
As noted above, the +5 v supply for the UT+ can come from the
regulated 5 v supply bus on the controller. The A&A PC board
provides a header to make this connection to the UT+ header.
To monitor the operation of the controller attach the simple DB-9 cable described at the top of this note to your PC. To set up the GPS receiver attach the DB-9 from the dual-harness arrangement to your PC. If you want to be fancier make a single cable with a switch that combines both functions.
Question: What can I do with the serial port data?
The first thing to do is capture the data into a file. There are many programs that will do this and almost any program that is designed to make your computer act like a terminal for connection to a modem should work. The baud rate of the serial port is 9600 and the parity, etc are 8N1.
With a data file in hand you can start by making a plot, either of the phase difference or the DAC voltage. A PC program for parsing the serial port data and converting it to tab-delimited format for input to a spreadsheet has been written by Gary Sanders. It can be downloaded from this website.
I have usually used Mathematica to analyze data. In addition to plotting, I can readily compute the rms jitter, fit average aging and day/night variation parameters, perform additional smoothing to see the effect of long-term filtering, etc. Most of these operations can be done using the build-in functions and packages that are standard in Mathematica. Sometimes a few tricks need to be used to let Mathematica efficiently handle the very data sets that the controller can generate after a few days of data collection. I also have some routines for computing Allan variances from the data. The Mathematica code to perform these operations is available for those who are interested.
Question: I just bought a truckload of Quartztown model 17YZR-4P2-S crystal oscillators on eBay. Can I use these with the controller?
Probably not. The controller was designed to produce a frequency standard that would compare in accuracy with the GPS atomic clocks themselves. The stability of the VCXO is important in achieving this goal since we must rely heavily on it to provide short term accuracy. This generally means that the VCXO must reside in a temperature-regulated oven and that the quartz crystal unit be fabricated with stability, not low cost, in mind. If you are going to the effort of building this frequency standard it is reasonable to try to find a good quality VCXO. A surplus ovenized oscillator module (e.g. the HP 10811) is a good choice. The surplus HP 5328A counter with the high stability option contains an HP 10811 module (check to make sure it has not been removed) and these are often available for about $100.
Question: I have just bought a carton of Super Skyrider model 17YZR-4P2-S GPS receivers on eBay. Can I use these with the controller?
Probably not. Unless a GPS receiver is especially designed otherwise, the 1 pps timing pulse is likely to have a large time jitter due to quantization inherent in the receiver CPU clock. A jitter of 1 microsecond in not uncommon. If you are going to the effort of building this frequency standard it is reasonable to get a GPS receiver designed for timing applications such as the Motorola VP or UT+ which have a timing jitter of the order of 10 nanoseconds (when averaged over 30 seconds). See "Construction and Setup Notes" for information on getting the UT+.