Fullscreen HTML5Med Res (??MB)HTML4

Improving stability & adding a serial port GPS-Based Frequency Reference: Circuit Mods & Additions The short term frequency stability of the GPS-Based Frequency Reference described in the March and April issues can be improved with some circuit changes, as described here. It’s also quite easy to add an RS-232C serial output, so the NMEA data stream from the GPS receiver module can be fed out to a PC for other purposes. By JIM ROWE A FEW DAYS after the March issue of SILICON CHIP had been published with the first article describing the GPS-Based Frequency Reference, an email arrived from New Zealand reader Dr Bruce Griffiths advising that the method used for cascading the synchronous frequency dividers IC4, IC5 and IC6 was not the best way. Correcting this would improve the divider’s reliability and hence the short-term frequency stability. After studying the data, it turned out that Dr Griffiths was correct. As a result, I performed some “surgery” on the prototype reference, changing the divider configuration over to one which should give more reliable synchronous division. Then I set it up again and let it run for a few days, while I monitored its operation. Sure enough the short-term stabil- Fig.1: adding a serial data output port to the GPSBased Frequency Reference is easy. Here’s how to do it. 84  Silicon Chip ity did seem to be better, although not dramatically so. The benefits were mainly in terms of reduced jitter in the “raw” PLL error correction voltage, before filtering. New PC board Now while it’s not too difficult to change the configuration on the original PC board by cutting tracks and soldering in short insulated wires underneath, I have revised the board design so that the job will be even easier if you get a board etched with the new pattern. These should be available from board manufacturers shortly. The revised divider circuit configuration is shown in Fig.2. The 10MHz clock signals coming from the crystal oscillator via buffer IC3c are now fed directly to the clock inputs (CP pins 2) of IC5 as well as IC4, and also to the clock inputs of IC6 (pins 1 & 5) via inverter IC3e. The inverter is needed because the flipflops in IC6 are negative edge triggered, while IC4 and IC5 are positive edge triggered. The TC (terminal count) output of IC4 now feeds into the CET (clock enable) input of IC5, to ensure more reliable synchronous count cascading, while the TC output of IC5 is also fed to the J and K inputs of IC6a and IC6b (pins 14, 3, 7 & 10) for the same reason. As a result, the complete synchronous division process down to 50kHz should now be significantly more reliable. Fig.3 shows how the revised main PC board is wired for this section of the circuit. The changes are all at the front righthand corner of the PC board, just behind CON1 and CON2. siliconchip.com.au Fig.3: here’s how to wire the revised PC board. All the changes are in the bottom righthand corner. Fig.2: the revised divider circuit configuration for the GPS-Based Frequency Reference. Wiring up the new divider configuration should be very easy if you use this diagram as a guide, instead of the corresponding section of the original overlay diagram. The same diagram should also help you if you’re maksiliconchip.com.au ing the change by operating on the original board. Want to add a serial port? A couple of other readers sent emails asking how easy it would be to add a serial data output port to the GPS-Based Frequency Reference, so that the NMEA data stream coming from the GPS receiver module could be fed out to a PC – for synchronising real-time clocks and other purposes. As it happens, adding such a port is very easy. All that’s needed is to mount a DB9M connector on the rear panel in a suitable spot (say above CON3 and CON4) and connect it to the main board via the simple inverting buffer circuit shown in Fig.1. As you can see, this involves only two resistors and one PN100 transistor, so it could be wired on a postage-stamp sized piece of matrix board and supported on the back of the DB9M connector by the wiring. There are only three connections to be made between this serial port circuit and the main PC board: one for the ground connection, one for the +5V line and the third for the buffered RS-232C GPS receiver’s data stream available from the output of IC14e (pin 10). This last signal is also conveniently available via the wire link on the top of the main board, just to the rear of IC8 and its 100nF supply bypass capacitor (one of the two links just to the front of CON7). So if you want to add a serial output port to the Frequency Reference to make the GPS data stream available for SC external use, that’s how to do it. May 2007  85