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GPS-Based Frequency Reference Pt.2: By JIM ROWE Last month, we published the circuit for our new GPS-Based Frequency Reference and described how it works. This month, we show you how to build and adjust it. B uilding the GPS-Based Frequency Reference is quite straightforward, since all the parts are mounted on two PC boards: a main board coded 04103071 (143 x 123mm) and a smaller display board coded 04103072 (145 x 58mm). All wiring between the two boards is via a short 16-way ribbon cable, fitted with an IDC line socket at each end (to link CON6 & CON9). Everything fits snugly inside an ABS plastic instrument case measuring 158 x 155 x 65mm, the display board mounting vertically at the front. As you can see from the diagrams and photos, the main board has a small rectangular extension at front right for the 10MHz and 1MHz output connectors (CON1 and CON2), while the display board has a matching rectangular cutout to fit around these connectors. 38  Silicon Chip In addition, the display board has a small cutout at upper left, to provide clearance for the interconnecting cable between the two boards. When the case is assembled, output connectors CON1 and CON2 are accessible via the front panel, while the remaining connectors are all accessible via the rear panel. The LCD and status LEDs are also at the front, along with the three main control pushbutton switches (S1-S3). The GPS receiver initialisation button (S4) is operated via a small access hole in the front panel, along with a similar access hole for adjusting the display contrast (via trimpot VR2). Main board assembly Fig.6 shows the parts layout on the main board. Begin by installing the wire links, then install PC stakes at test points TP1-TP3 and the adjacent TPG and GND terminals. Follow these parts with the resistors, diodes and the MKT and ceramic capacitors. Table 1 shows the resistor colour codes but you should also check them using a DMM, as some colours can be difficult to read. Note that the 10kW and 20kW resistors in the resistor ladder DAC (just to the left of IC12) are mounted in inverted-V fashion, to fit them all in. Table 2 shows the capacitor codes. Take care to ensure that the diodes are all correctly oriented and be sure to use the correct type at each location. Next, fit the IC sockets if you’re using them (they’re recommended for this project). The IDC header pin connectors CON6 & CON7 can then go in, followed by BNC connectors CON1CON4, power input connector CON5 and RCA connector CON8. The finned heatsink for regulator REG1 is next on the list. Make sure it’s seated all the way down on the PC board before soldering its mounting pins to the board pads. siliconchip.com.au Regulator REG1 is mounted vertically against the heatsink and is attached to it using an M3 x 6mm machine screw (this goes into a tapped hole in the heatsink). Apply a thin smear of heatsink compound to both the back of the regulator and the heatsink surface before screwing them together, to ensure a good thermal bond. Tighten the mounting screw firmly, then solder the regulator’s leads to its board pads. The remaining smaller parts can now all be installed. These include trimpot VR1, trimcap VC2 and quartz crystals X1 and X2. CON4 CON3 CON8 ERROR PULSE (INVERTED) 1Hz GPS 12V DC IN + 1k 68Ω 33k 680Ω 100nF IC13 LM358 CON7 (GPSRX) 1M 10 µF VC1 15pF NPO 180Ω 22pF 1M NPO IC3 74HC04 + IC7 74HC4046 100nF 10k 4.7pF NPO VC2 3-10pF 100nF TP3 50kHz IC6 74HC73 100Ω IC5 74HC160 100nF PIC16F628A 100Ω 100nF IC1 + 4.7 µF IC4 74HC160 MAIN BOARD Fig.6(a): follow this parts layout diagram when building the main PC board and refer also to the detail drawing (Fig.7) when installing the parts for the mini oven (under the film canister). The Garmin GPS 15L mounts on spacers above ICs 9 & 12 (see Fig.8) CON1 10MHz OUT CON2 1MHz OUT Fig.6(b): assemble the display PC board as shown here. Switches S1-S3 must be mounted with their flat sides as shown, while LEDs 1-3 should sit 11mm above the board surface. Note also that the 10mF capacitor must be mounted flat against the board (see photo). siliconchip.com.au 180Ω 74HC374 IC12 7002 C 100nF 1nF 3.3k 16 15 4148 33k 33pF BB119 35mm FILM CANISTER (SHORTENED) 33pF 10MHz X2 100nF TO DISPLAY PCB 2 1 1 2 9 10 1k 4148 April 2007  39 GND TP1 6.8k D4 D2 FINNED TO-220 IC10 LM335Z OXDSPG HEATSINK 17030140 Q1 A DRAOB BD136 X1 CON6 100pF D3 CABLE TIES TO HOLD DOWN CANISTER 2.0k 2.2nF 4148 IC14 74HC04 5k IC11 74HC14 1k 100nF VR1 IC2 LM311 TPG TP2 7805 REG1 3.3k + ANTENNA CONN 100pF 47k 10 µF 100nF 33Ω FINNED TO-220 HEATSINK 100nF 100Ω 100Ω 100nF IC8 74HC161 20k D6 D7 2.2k 4.7k GARMIN GPS15L Rx 20k 20k 20k 20k 20k 20k 20k 20k 10k 10k 10k 10k 10k 10k 10k D5 1000 µF 10 µF 10 µF + 1N4004 D1 1M 74HC161 1M 1M IC9 CON5 Making the mini oven The first step in making the oven is to fit the second finned heatsink. Before doing this however, it needs to have a chamfer cut along both inner edges of the two centre fins, to clear the small flange around the bottom of the crystal can – see Fig.7. Basically, you need to remove enough material so that the outer fins rest on the top of the PC board. A small rotary “hobby grinder” can be used to make these chamfers or you could use a small dental burr or milling cutter. After cutting the chamfers, apply a small smear of heatsink compound to both sides of the crystal can and to both sides of the centre slot in the heatsink. This ensures a good thermal bond between the two when the heatsink is fitted. It’s now just a matter of slipping the heatsink into position over the crystal and soldering its mounting pins to the board pads. Mini-oven heater transistor Q1 is in a TO-126 package. As shown, it’s fas- ERROR VOLTS This is the fully-assembled main PC board but with both the Garmin GPS 15L receiver and the oven cover (ie, the film canister) removed so that the components under them are visible. Make sure that all polarised parts are correctly oriented. tened to the end of the oven heatsink, again using an M3 x 6mm machine screw. Smear both the transistor and heatsink mating surfaces with heatsink compound before slipping the transistor into position. Take care with the orientation of Q1 – its metal surface goes towards the heatsink. Don’t forget to solder its leads to the board after tightening its mounting screw. The LM335Z temperature sensor (IC10) is next on the list – see Figs.6 & 7. It’s in a plastic T0-92 package and slips easily into place between the heatsink fins. Before doing this though, give it a generous coating on both sides with heatsink compound. That done, slide it down between the heatsink fins so that its body sits about 6mm above the PC board before soldering its leads to the board. Assuming you’ve already fitted varicap VC1, its 15pF series capacitor and the 47kW isolating resistor, the inside of the mini oven is now complete. All that remains is to fit its outer casing. This casing is made from a 33mmdiameter plastic film canister (you can get one from a photo processing store) and lined with expanded polystyrene foam sheet about 3mm thick. It’s built as follows: (1) Shorten the canister to about 32mm long, using a pair of scissors or a sharp knife. (2) Cut a 31mm diameter disc from the expanded polystyrene foam sheet and push it right down to the bottom of the canister. (3) Cut another piece of the foam into a 28 x 70mm strip and make a series of shallow cuts across the strip on one side, so that it can be rolled lengthwise Fig.7: this detail drawing and the photo at left show how the mini-oven is built. Not shown here is heater transistor Q1 which is fastened to the back of the heatsink. 40  Silicon Chip siliconchip.com.au A 16-way IDC cable (see Fig.9) is used to connect the display board to the main board. Take care to ensure that the three LEDs are correctly oriented and that their bodies sit 11mm above the PC board. The 10mF capacitor must be mounted with its body flat against the PC board as shown. into a tubular shape. Fit this inside the canister to form the wall lining. Having lined the canister, the next step is to “up-end” it and lower it down over the mini-oven components on the PC board. Note, however, that you may have to cut a small “pocket” in one side of the foam liner to clear the 15pF capacitor. Finally, a long plastic cable tie (or two shorter cable ties in series) can be threaded through the adjacent 3mm holes in the PC board and tightened to Table 2: Capacitor Codes hold the canister down. The mini-oven assembly is now complete. Value 100nF 2.2nF 1nF 100pF 33pF 22pF 15pF 4.7pF Installing the ICs The next step in the assembly is to install all the ICs. If you’ve previously installed IC sockets, then it’s just a matter of plugging the ICs in, taking care to ensure they are all correctly oriented. Be sure to use the correct device at each location. Note that most of the ICs are CMOS devices and are easily damaged by mF code 0.1mF .0022mF .001mF NA NA NA NA NA EIA Code    IEC Code   104 100n    222      2n2   102   1n0   101 100p    33   33p    22   22p    15   15p      4.7   4p7 Table 1: Resistor Colour Codes o o o o o o o o o o o o o o o o o o o o siliconchip.com.au No. 5 1 1 2 1 9 10 1 1 2 1 1 3 1 3 2 4 1 1 Value 1MW 68kW 47kW 33kW 22kW 20kW 10kW 6.8kW 4.7kW 3.3kW 2.2kW 2kW 1kW 680W 330W 180W 100W 68W 33W 4-Band Code (1%) brown black green brown blue grey orange brown yellow violet orange brown orange orange orange brown red red orange brown red black orange brown brown black orange brown blue grey red brown yellow violet red brown orange orange red brown red red red brown red black red brown brown black red brown blue grey brown brown orange orange brown brown brown grey brown brown brown black brown brown blue grey black brown orange orange black brown 5-Band Code (1%) brown black black yellow brown blue grey black red brown yellow violet black red brown orange orange black red brown red red black red brown red black black red brown brown black black red brown blue grey black brown brown yellow violet black brown brown orange orange black brown brown red red black brown brown red black black brown brown brown black black brown brown blue grey black black brown orange orange black black brown brown grey black black brown brown black black black brown blue grey black gold brown orange orange black gold brown April 2007  41 Fig.8 (above): the mounting details for the Garmin GPS 15L receiver module. Fig.9 (right): this diagram how to fit the 10-way IDC line socket to the Garmin GPS 15L’s cable. It also shows how to make the 16-way IDC ribbon cable. electrostatic discharge. It’s really just a matter of taking a couple of precautions: (1) avoid touching the IC pins; and (2) earth yourself while you’re removing them from their packaging and plugging them in (eg, by periodically touching an earthed metal object or by using a wrist strap). Installing the GPS module Fig.8 shows the mounting details for the Garmin GPS 15L receiver module. This mounts above the main board, behind the mini-oven assembly and above IC9, IC12, the resistors in the ladder DAC and sundry other parts. As shown in Fig.8, the module is mounted on three M3 x 15mm tapped spacers and secured using three M2 x 25mm machine screws, together with six M2 nuts, six M2 flat washers and six M2 lockwashers. Note that the GPS 15L module has a very small female MCX connector for the active antenna lead on one of the Fig.10: the LCD module is secured to the display board using M2 x 10mm screws, nuts & flat washers. 42  Silicon Chip longer sides and an ultra-miniature 8-way SIL “flex” connector on one end for all other connections. The module is mounted over the main PC board with its antenna connector facing towards the front and the flex connector end on the right (near CON7). Once the receiver module has been mounted, shorten all eight wires on the special interconnecting cable supplied with it (ie, with the tiny 8-way flex connector at one end) to about 60mm long. Don’t bare their ends though, because they need to be fitted to a 10-way IDC line socket to mate with CON7. Although IDC sockets are intended for use with ribbon cable, they can also be used with separate light-duty hookup wires of the type used to make the receiver module’s cable. The idea is to partly assemble the socket first and then feed the end of each wire through from one side, passing it over the teeth of its connector pin and out the other side. Fig.9 shows where each wire goes on the connector. Once all eight wires have been fitted, the two halves of the connector are squeezed together firmly in a small vyce, to make the insulation displacement connections. Finally, the top part of the socket can be fitted if you wish and a small cable tie or two used to keep the wires together. The completed cable can now be connected between the GPS module’s connector and CON7. wire links (four under the LCD module), then install the resistors, trimpot VR2 and the transistors Q2-Q4. Follow these with CON9, the 14-way (7 x 2) pin header for the LCD module, switches S1-S4 and the 10mF electrolytic capacitor. The latter must lie flat against the PC board – see photo. Take care when installing switches S1-S3. Each switch must be seated all the way down on the PC board with its flat side to the left. The next step is to fit the three LEDs (LED1-LED3). These must be installed with their bodies exactly 11mm above the board, so that they later protrude through matching holes in the front panel. A cardboard spacer cut to 11mm is the easiest way to do this – just push each LED down onto the spacer and solder its leads. All that’s left now is the LCD module. Fig.10 shows the mounting details. Install the four M2 x 10mm screws first and secure them using M2 nuts. That done, place an M2 flat washer on top of each nut, then mount the LCD module is position, making sure it mates correctly with the header pins. The module can now be secured in position using four M2 washers, four lockwashers and four M2 nuts. That done, the header pins can be carefully soldered to the pads on the top of the LCD module. Display board assembly You now need to make up a small ribbon cable assembly to connect the two PC boards together. This is made Fig.6(b) shows the display board assembly. Begin by installing the nine Interconnecting cable siliconchip.com.au Fig.11: these diagrams show the drilling details for the front and rear panels. using a 95mm length of 16-way IDC ribbon cable, fitted with a 16-way IDC line socket at each end – see Fig.9. Note that the two sockets both face in the same direction. Note also that you can’t fit the usual top cover to the socket at the display board end, because there isn’t enough space for it to clear the front panel. In fact, you may even need to file about 0.5mm from the top of the line socket to provide enough clearance. You now need to prepare the front and rear panels of the case by drilling and cutting the various holes. These are all shown in the panel cutting diagram – see Fig.11. The 12.5mm dia­ meter hole in the upper centre of the rear panel is used for mounting a BNC female-female panel adaptor. This is used to bring out the GPS receiver module’s antenna lead. Once the panels have been drilled, they can be dressed by attaching the front panel artworks (the relevant file can be downloaded from the SILICON CHIP website and printed out on a siliconchip.com.au colour printer). These artworks are attached using double-sided adhesive tape. Once attached, they can be protected by covering them with clear selfadhesive film (eg, wide sticky tape). Case assembly Now for the final assembly. The first step is to loosely fit the front and rear panels to the main board. That’s done by removing the nuts and lockwashers from BNC connectors CON1-CON4, then fitting the panels in place over these connectors and refitting the nuts and lockwashers. Don’t tighten the nuts at this stage though. Instead, leave them loose so that the panels can be adjusted. Having attached the panels, you can now lower the entire assembly into the bottom half of the case, sliding the front and rear panels into their matching case slots as you go. Similarly, the display PC board slides into the third board slot from the front. The main board is then secured to the integral moulded support pillars using the four small self-tapping screws supplied with the case. The next step is to fit the cable that connects the GPS receiver module to CON7 on the main board. That done, fit the 16-way IDC cable between and CON6 on the main board and CON9 on the display board. Construction can now be completed by fitting the BNC-BNC adaptor to the rear panel and connecting the internal MCX-BNC antenna cable between this adapter and the GPS receiver module. That done, tighten the nuts on the front and rear panel BNC connectors. Setup & adjustment Before doing anything else, you need to install your active GPS antenna. This must be mounted outside and as high as possible, so that it gets an unobstructed “view” of the sky. A good position should be on the top of your TV antenna mast but you may decide on somewhere else because of the need to keep the cable length as short as possible. April 2007  43 What The PIC Firmware Does The main part of this project is the hardware circuitry which effectively locks the phase of the main 10MHz crystal oscillator to the very accurate 1Hz pulses from the GPS receiver module, as explained in the text. However, since the GPS receiver module also provides strings of useful GPS-derived data every second, along with the 1Hz pulses, we use a PIC micro to “catch” these strings of data and allow selected data items to be viewed on the LCD. The GPS data stream is sent in ASCII sentences at 4800bps, or 480 characters per second. The main part of the firmware program in the PIC simply scans front panel pushbuttons S1-S4 and if none of the buttons is pressed, it simply waits until a character arrives from the GPS receiver and is “caught” by the hardware USART module in the PIC. When this happens, the PIC then jumps into an interrupt servicing routine and after making sure there This view shows the completed assembly ready for installation in the case, again with the oven cover and the Garmin GPS 15L module removed. 44  Silicon Chip weren’t any errors, it reads the received character from the USART and then inspects it to see if it has any special significance – such as the start or end of a sentence. If it isn’t one of these special characters, it simply saves the character in the next available address in a buffer area in its data RAM. However, if the character is a “start of sentence” character, it doesn’t save it. Instead it simply resets the PIC’s “pointer” to the RAM buffer, so that following characters in the sentence will be saved from the start of the buffer. On the other hand, if the character is an “end of sentence” character, it jumps to a separate part of the interrupt routine which analyses or “parses” the sentence in the RAM buffer to identify which kind of a sentence it is. It then saves the wanted data in that sentence into specific RAM addresses where they can be displayed later. As well as scanning the push­ buttons, the main part of the project simply displays some of this received GPS information on the LCD – ie, the UTC time and date, plus the GPX receiver’s fix status and the PLL lock status. However, if you press S1, S2 or S3, the program switches to one of three alternative display modes, which allow some of the other GPS information to be displayed – the latitude and longitude, the antenna height above mean sea level, the number of GPS satellites currently in view and so on. Each of these alternative display modes only lasts for about 20 seconds, after which the program switches back to the main time and date display. Finally, press switch S4, the program displays a message to advise that it is sending initialisation commands to the GPS receiver (and does just that). It then switches back to the main display again. The receiver end of the antenna is fitted with a BNC plug, to mate with the “outside” section of the rear panel BNC adaptor. Be sure to fit this plug without introducing any short circuits, because this cable carries DC power up to the active antenna (via the GPS receiver module), as well as carrying the GPS signals down to the receiver. A short circuit could damage the GPS module. Once the antenna is in place, apply power via the DC input socket (CON5). LED2 (PLL Lock) on the front panel should begin glowing almost immediately and you should also be able to measure +5V on the wire link just to the right of IDC header CON6 (relative to the TPG ground pin to the left of REG1). The LCD should also spring to life, although it will probably be showing mainly zeroes for the first 10-20 seconds. After this time, the GPS receiver module should have found a “fix” and the display should change to show the current UTC time and date, plus a “1” in the upper righthand siliconchip.com.au The main PC board is secured to integral spacers on the base of the case, while the display board slides into one of the case slots. Note that the front and rear panels must be attached to the BNC sockets on the main board before mount ing it in the case. corner to show the fix status. LED3 on the front panel should also begin to blink once per second, showing the GPS 1Hz pulses, while LED1 should also begin glowing continuously to show the fix status. LED2 may now either be off or it may begin to flash, because the PLL may not be able to lock the phase of the 10MHz crystal oscillator with the 1Hz GPS pulses as yet. The Garmin GPS 15L receiver module is mounted on spacers attached to the main board (see Fig.8). It sits above ICs 9 & 12 and is connected to the main board via the 10-way IDC line socket. Adjusting the mini-oven The next step is to check the status of the mini oven’s temperature control. First, measure the voltage at TP1 relasiliconchip.com.au April 2007  45 This view shows the fully-assembled unit, with both the Garmin receiver and the oven cover in place. Note the internal antenna connection from the Garmin GPS 15L receiver’s socket to the BNC-to-BNC adapter on the rear panel (see also picture on facing page). Monitoring Its Performance If you’re using your frequency reference in a normal workshop/home lab environment, there’s probably no need to monitor its performance any further than glancing at its front panel displays from time to time – to confirm that its GPS fix and PLL lock status are both OK. However, if you need to monitor its performance in more detail, this can be done fairly easily using the DC error voltage fed out via CON8 on the rear panel. There is a direct relationship between this error voltage and the instantaneous phase error in the frequency reference’s PLL. In fact, each 19.53mV of this error voltage corresponds to 100ns of phase error, so if you have the PLL stabilised at an average phase error of 10ms, the error voltage will have an average value of 1.953V. And as the phase error jitters up and down in 100ns increments, the instantaneous error voltage will similarly vary up and down in 19.53mV increments. This means that if you monitor the DC error voltage continuously using a DMM and link the DMM to a PC running a data-logging program, you can record the frequency reference’s PLL performance over a suitable period of time. You can then plot the mean value and standard deviation of its phase lock error. This will give you a much better idea of its medium and long-term accuracy, as well as the short-term error tolerance. 46  Silicon Chip tive to ground pin TPG; this should measure very close to +3.15V. You should find a similar voltage on TP2 (within a couple of tens of millivolts). This is the voltage across temperature sensor IC10 and reflects the temperature inside the mini oven (3.15V = 315K = 42°C). If the voltage on TP2 is outside the range 3.14-3.16V, try adjusting trimpot VR1 in one direction or the other until the voltage drifts back inside this range. Don’t adjust the trimpot setting in large jumps though, because the temperature changes quite slowly following each adjustment. Adjusting the PLL When you are satisfied that the voltage at TP2 is stabilising inside the correct range, you are ready to turn your attention to setting up the 10MHz crystal oscillator and the PLL. For this, siliconchip.com.au The rear panel carries BNC sockets for the antenna (top, centre) and for the GPS 1Hz and phase error pulse outputs (bottom left). It also carries an RCA socket for the phase error voltage and provides access to the DC power socket. you will need to use an oscilloscope and a frequency counter. The input of the scope should be connected to CON4 on the rear panel of the frequency reference, where it will be able to monitor the PLL’s phase error pulses (inverted). By contrast, the counter’s input should be connected to CON1 on the front panel, where it can measure the 10MHz output signal. Before you start the setting up, see what frequency reading you are getting on the counter. It should already be quite close to 10.000000MHz, although the exact reading will depend on the calibration of the counter’s own timebase. Now look at the pulse waveform on the scope. What you should see is a negative-going rectangular pulse of 5V peak-to-peak, with a width somewhere between 0ms and 20ms. It may not be fixed in width, though – in fact, if the siliconchip.com.au PLL isn’t in lock as yet, it may be cyclically varying up or down in width within the 0-20ms range. At this stage, try adjusting trimcap VC2, which you’ll find just to the front right of the mini oven. Adjust it using a small insulated alignment tool and change its setting by only a very small amount in one direction or the other. As you do, watch the pulse waveform on the scope. If it was cycling back and forth in width, this cycling will slow down if you’re adjusting the trimmer capacitor in the right direction. Conversely, if it speeds up, turn VC2 back the other way until it does slow down. If it wasn’t cycling to begin with but does so when you adjust VC2, the same applies – turn it back the other way. The objective is to carefully adjust VC2 until the error pulse width stops cycling and remains fairly steady at a width of about 10ms. This setting corresponds to the PLL being locked close to the centre of its lock range. By the way, don’t be worried if the pulse width still varies up and down randomly in steps of 100ns (0.1ms). This is normal and is due to propagation jitter on the GPS signals, noise, dither in the PLL as a result of drift in the “about-10MHz” clock oscillator, and so on. Once you have achieved this stable pulse setting, check the reading on the frequency counter. It should now be reading very close to 10.000000MHz. If you get a reading very close to this, any error you see is almost certainly due to the calibration of the counter’s timebase. The only proviso here is if the counter reading is stable but very close to a frequency that’s 200Hz away from 10.000000MHz (ie, 9.999800MHz or April 2007  47 Fig.12: these full-size artworks can be copied and used to make the front and rear panels, or you can download the relevant file from the SILICON CHIP website and print it out on a colour printer. 10.000200MHz). In this case, it means that the PLL is locking quite nicely but to one of those other frequencies. So if you do get a reading very close to these “200Hz-away” frequencies, you’ll need to try adjusting VC2 again until the PLL locks at the correct frequency. If you can’t achieve this by adjusting VC2, you will have to replace the 4.7pF NPO capacitor located just behind VC2 with a lower or higher value – depending on which frequency your PLL had been locking at. For example, if it was locking at 9.000800MHz and VC2 couldn’t bring it up to 10.000000MHz, replace the 4.7pF capacitor with a 2.2pF capacitor. Alternatively, if it was locking at 10.000200MHz and VC2 couldn’t bring it down to 10.000000MHz, use a 6.8pF capacitor. When your scope shows a reasonably stable phase error pulse (with a width close to 10ms) and the counter 48  Silicon Chip displays a reading that’s very close to 10.000000MHz, your GPS-Based Frequency Reference should be set up and ready for use. LED1 (GPS FIX) and LED2 (PLL LOCK) should now both be glowing steadily, while LED3 should continue to blink reassuringly once per second. Similarly the LCD should normally show UTC time and GPS fix status (Fx1) on the top line and UTC date and PLL lock status (PLL: L) on the lower line. Additional information Additional GPS information is available on the LCD for about 20 seconds if you press one of the three frontpanel buttons. For example, pressing S1 (LOCATION) will display the exact latitude and longitude of your external GPS antenna, while pressing S2 (ANTENNA) will display the antenna’s height in metres above mean sea level plus the number of GPS satellites currently in view. Pressing S3 (SAT INFO) displays the identification number of the main four satellites in current view, plus the signal-to-noise ratio of their signals in dB – giving you a good idea of the current GPS “fix” quality. If your LCD readout isn’t very clear, try adjusting its contrast control pot using a small screwdriver through the hole in the lower centre of the front panel. This should give you an easyto-read display. Normally, you shouldn’t need to initialise the GPS receiver module using switch S4 (accessible via the second small hole in the front panel). However, by all means try doing this if you are unable to set up your frequency reference as described above. At the very most, this initialising should only be necessary once, because the GPS module normally saves its configuration data in non-volatile flash memory, where it’s read whenSC ever the power is applied. siliconchip.com.au