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Accuracy better than 100 parts per BILLION! Lab Quality Programmable GPS-synched FREQUENCY REFERENCE ... ... ... Part 2 by Tim Blythman and Nicholas Vinen Our new GPS Frequency Reference is really accurate, precise and flexible. It’s also compact and easy to use, thanks to its touchscreen interface. You can set the frequencies of its three programmable outputs over a wide range (1-100MHz) and you can save preferred frequencies to a set of four presets for each output, to make switching between them quick and easy. L ast month we described the circuit of our new Frequency Reference project and gave some details on how the software worked. We also explained its general concept and how it achieves such flexibility and accuracy in the frequencies that it can produce. This month, we have detailed assembly instructions and further information on how to use it, including all the various customisable settings. While the settings all have sensible defaults, allowing you to build it and start using it without any fiddling, you can tune the software parameters to suit your particular requirements. Construction is pretty straightforward, despite the use of 78 Silicon Chip mainly surface-mount components. There are just one or two that are slightly tricky but they are not that difficult, as long as you use the right tools and take your time to get it right. Later in this article, we describe how the voltage-controlled oscillator (VCO), which forms the heart of this Frequency Reference, can be manually adjusted. This can be handy if you have access to a high accuracy frequency meter or can’t access a GPS signal – for example, if you’re operating the unit in a basement or the middle of a steelreinforced building. Building the LCD BackPack The first step is to build the Micromite LCD BackPack. You can use the original 2.8-inch version (described in the February 2016 issue; siliconchip.com.au/ Article/9812) or the revised version from the May 2017 issue, Australia’s electronics magazine siliconchip.com.au Fig.3: use this diagram as a guide when building the Frequency Reference. The orientations of IC1-IC6, LED1, REG2 and TS1 are critical so take care to fit them the right way around, as shown. You only need to fit one of CON5 or CON6, not both. Note the approximate location of the bottle cap or similar cylinder which encloses the oven section of the board. which incorporates the Microbridge (siliconchip.com.au/Article/10652). We used the latter in our prototypes. Note though that you cannot use the software-controlled backlight option if you build the BackPack V2 as this uses pin 26, which we have had to use for a different purpose on the Frequency Reference board. So you need to omit Q1 and Q2 and fit VR1 instead. The backlight brightness is then adjusted using trimpot VR1, as it was on the original BackPack. Both versions of the BackPack are available as complete kits from the SILICON CHIP Online Shop and can be purchased with the chip pre-programmed to suit this project (Cat SC3321 or SC4237). We do not recommend that you use the Micromite Plus LCD BackPack as we have not tested it in this project. If you need assembly instructions for either kit, refer to the articles mentioned above. But once you have the parts, the assembly is pretty easy, as only about 20 components are involved and the position/polarities of most of these are printed on the PCB. Solder the components on the PCB where shown, being careful with the orientation of the IC(s), regulator and LED. The capacitors supplied in the kit will not be polarised types. Start assembly with the lowest profile components first and work your way up to the taller ones. Note that the 18-pin and 4-pin headers are mounted on the back of the board and these should be fitted last. You can then plug the screen into the provided header socket and attach it to the BackPack board using short machine screws and tapped spacers. Trim the solder joints on the top of the LCD with some sharp sidecutters, so they do not interfere with the lid when fitted later. PCB assembly Next, we’ll assemble the Frequency Reference PCB. Use the overlay diagram (Fig.3) as a guide to fitting the components. Before starting, check that you have all the components needed. If you have a kit of parts, don’t pull them all out yet as some are hard to distinguish from others, especially those which have no markings (eg, ceramic capacitors). We will refer to the orientation as though the board is sitting as shown in Fig.3, with the single BNC socket (CON3) at the left, and the two BNC sockets (CON2 and CON4) to the right. This orientation is convenient since most of the labels are right-side-up. It’s easiest to start with the fine-pitch ICs first as these are more difficult to solder once surrounding components have been fitted. So start by fitting IC2, the CDCE906 PLL IC. This part is only available in an SSOP SMD package with a 0.65mm lead pitch. It requires the most care to solder but it is not too difficult to do by hand if you are careful. The other components are much larger and have leads spaced further apart so, after this chip, it’s downhill all the way. Start by applying a thin layer of flux paste to the pads for IC2, then line up the chip with the pads, ensuring the pin 1 marking is to upper right. Using a fine-tipped soldering iron, tack solder one of the corner pins down and check that the all the IC pins line up in the centre of the PCB pads and that the IC is flat on the PCB. If you are happy with the location of IC2, carefully solder each pin. If you have used flux paste as recommended, simply touching the iron to the pin and pad at the same time should cause a small amount of solder to flow onto both. The stability of the reference may be improved by reducing the impedance of ground tracks on the PCB. This can be done by soldering a wire from a ground pad near VCO1 to the ground end of IC2’s bypass capacitor, then another wire from there to the ground pad of IC5’s bypass capacitor and also to the via near the GND terminal of CON1. siliconchip.com.au Australia’s electronics magazine November 2018  79 Here are two views of the completed PCB, along with our highly technical purpose-designed oven (in situ on the right). If it looks just like a milk bottle cap, then, ummmm . . . You will need to add a bit of extra solder to the iron from time to time. At this stage, if there are solder bridges between pins, don’t worry about them. The important thing is to make sure that all the pins are soldered properly. Patience, and keeping the tip clean of impurities like dark oxides will make this process easier. Once all the other pins have been done, go back and retouch the first pin. If you have some solder bridges (which are almost inevitable), apply some fresh flux and use solder braid (wick) to remove the excess. Check deep between the pins, as a single hidden bridge is enough to cause trouble. We’ve found taking a photo with a smartphone camera can allow us to zoom in and see bridges and other defects that aren’t immediately obvious to the naked eye. Next on the list are the USB sockets, which can be a bit fiddly but fortunately, you only need to install one of them. We chose the mini-USB socket as it is slightly larger and easier to handle but the micro-USB socket is now the more common type in use (especially on phones), so you can fit that if you prefer. Again, start by putting a little flux paste on the pads. Place the socket on the PCB and its pegs should drop into the provided holes in the PCB, making alignment easier. Solder the large mechanical pads first, making sure the socket is flat and flush with the board. Now carefully apply a little solder to each of the small leads to lock them in place. We only need the pins at either end for power but it’s probably a good idea to solder them all anyway. Be sure to check that the USB data pins are not bridged to the power pins, 80 Silicon Chip as this may cause problems if the GPS Frequency Reference is powered from the USB port on a computer. You can now fit IC1 and IC4-IC6, using a similar process as for IC2. These are considerably larger and easier to handle. Check that the pin 1 markings are correct. IC1, IC4 and IC6 have their dot facing upwards, while IC5 has its dot facing downwards. If there is no dot, you might find a bar on one end of the IC or even a bevel along one edge. In each case, pin 1 is close to the dot/bar/bevelled edge. Next on the list are REG1, REG2 and IC3, which are all within the oven outline. This is marked with a circle and there is also a corresponding copper pour on the PCB. While this should not present any difficulties, you might find that the extra mass of copper pulls heat away from the iron, so you may need to turn its temperature up slightly to compensate. If you are using the SOT-23 version of REG1 then it should be soldered first, as it is quite low. It will only fit one way, so tack one leg in place, check the alignment and then solder theother two leads and touch up the first pin. If you are using the TO-92 version of REG1, you can fit it lat- SMD Resistor Codes           1 1 3 1 2 1 6 1 1 4 8.2MΩ 10kΩ 4.7kΩ 2.7kΩ 2.0kΩ 1.1kΩ 510Ω 220Ω 51Ω 39Ω 825 103 472 272 202 112 511 221 511 390 or or or or or or or or or or 8204 1002 4701 2701 2001 1101 5100 2200 51R 39R Australia’s electronics magazine er, once all the SMDs are in place. IC3’s pin 1 goes towards the upper left corner while REG2’s pin 1 goes to the top right. Solder these components using the same technique as the other ICs. Now is a good time to solder VCO1. The pin 1 marking on this module is one of the smallest we have seen. If you cannot find it, then rotate your PCB so that CON3 is at the bottom. Then place the VCO on the board so that the writing on it is right-way-up. You might now see the small marking at the bottom left, matching the dot on the PCB. For smaller components like this, adding a small amount of solder to one pad before placing the component means that you don’t have to apply solder while trying to position the component. Use tweezers to hold the component flat and aligned while adjusting its position, then when you are happy, solder the other leads in place. The VCO’s pads are much larger than necessary, to make it easier for you to get the iron in contact with them despite the tiny size of the device. Ensure VCO1 is symmetrical about the pads so that each one makes good contact. Carefully apply more solder if necessary but avoid getting any near the top of the VCO, as it may stick to the metal can and cause problems later. Passive components The passives should be fitted next. The capacitors are not usually marked, so only take them out of the package one value at a time. Fortunately, they are not polarised. Fit them where shown in Fig.3. Follow with the resistors. There are several different values but forsiliconchip.com.au The Frequency Reference PCB “hangs” underneath the Micromite BackPack PCB, as shown here. The BackPack PCB also holds the bottle cap “oven” in place. If you mount it differently, the cap will need securing to the PCB via the holes provided. tunately, they are marked with codes indicating their values if you get them mixed up. Fit them in the same manner as the capacitors and again, refer to the overlay diagram to see which goes where. The final items are three 1.1k resistors. You may have noticed that we had four 1.1kresistors in our original parts list but there are only three on the board. We found that the resistor on IC2’s Y4 output was limiting the swing on the 40MHz signal going back to the Micromite, so its frequency wasn’t being measured accurately. Thus, we removed this resistor from the final design. Through-hole components Now is a good time to fit Q1, TS1, LED1 and (if you are using the throughhole version) REG1. Ensure LED1 is inserted with its longer anode lead through the pad marked “A” on the PCB. The orientation of the TO-92 package devices is shown in Fig.3 but you may need to bend their leads out (eg, using small pliers) to fit the pads provided. Now you can solder the headers in place. This includes CON1, GPS1, LK1, JP1 and JP2. These are all fitted on the same side of the board as the other components and, except, for CON1, they are standard pin headers. CON1 consists of two female header sockets, one with 18 pins and one with four pins. You can cut these down from longer sockets if necessary. When fitting the GPS header, be careful to ensure it is perfectly vertical since otherwise, it may be difficult to plug the GPS connecting wires into it later. To make sure they will fit, it’s best to plug the BackPack into the Frequency Reference board after soldering one or two pins on GPS1, so that you can check that the header clears the board above. To help line the CON1 sockets up correctly, you can plug them into the siliconchip.com.au corresponding headers on the Micromite LCD BackPack first and then insert them into the pads on this PCB and solder the pins in place. The final items to fit are the three BNC sockets, CON2-CON4. The large posts require a decent amount of solder to hold them in place (and heat to make those solder joints). If you’re building the unit into a larger box than specified, you could run some shielded cable out to chassis-mounted sockets. Setting up the BackPack If you haven’t used a PIC pre-programmed with the software for this project, you will need to set up the LCD screen and touchscreen. You can do this by connecting a USB/serial adaptor to the 4-pin header and plug it into your computer, then open up a terminal program, select the correct COM port and set the baud rate to 38,400. Reset the Micromite and you should receive a greeting banner in the console. If you don’t, check the serial wiring, COM port, baud rate, power supply and that you have assembled the PCB correctly. Assuming you do get the greeting, you can set up the display and touch controller by issuing the following commands: OPTION LCDPANEL ILI9341, L, 2, 23, 6 OPTION TOUCH 7, 15 GUI CALIBRATE You then need to use a sharp object (but not too sharp!) like a toothpick to press on the middle of the targets which appear on the screen. Once you’ve done that, you’re ready to load the BASIC software for this project. Loading the software Now that the two PCBs have been assembled plug them together but leave all the jumpers off for now. The next step is to load the BASIC software onto the microcontroller. If you have a PIC chip in your BackAustralia’s electronics magazine Pack that was pre-programmed with the GPS Frequency Reference software then you can skip right to the testing stage. We suggest that you then use the MMEdit software to upload the BASIC program and the following instructions assume you will be using this method. If you are familiar with using the Microbridge to upload HEX files directly to the chip then you can do that instead. Open MMEdit and load the BASIC file for this project, which is available from the SILICON CHIP website. Connect the Micromite to your PC via the USB socket on the BackPack itself (not the one on the GPS Frequency Reference PCB). Under the Connect menu, select New and find your Serial port number, then select it. Set the baud rate to the rate your Micromite is set up for (the default is 38,400).. Under the Advanced menu, ensure that the “Auto Crunch on Load” option is selected. This is necessary as the program will not fit into the flash memory without being “crunched”. Press the button to upload the code and when it finishes, type OPTION AUTORUN ON into the console which appears and press Enter. This sets the program to run next time the unit is powered up. Wiring up the GPS module There is not much spare room in the specified enclosure for the GPS module and anyway, you will probably get better results by mounting it externally, as we have on our prototype. Alternatively, you could use a module with an external antenna connector and mount a socket just above the USB power socket on the case. Because the GPS header (GPS1) is so close to the BackPack board above it, we recommend that you use slim DuPont-style headers to make the connections. November 2018  81 The pins are labelled as follows: V+ (module power supply), R (goes to Rx/RxD on the module), T (goes to Tx/TxD on the module), P (goes to 1PPS output on the module), G (GND) and E (enable – connected to V+). If you are not using the recommended module then your module may not have an enable pin, or it may require a different voltage. You will need to use a module with a 1PPS output and TTL serial interface. Testing Close the console and unplug the USB cable from the Micromite. Insert a jumper on the LK1 header. This will connect the VCO output to the Micromite’s pin 12, and also ensure that the console does not start up and interfere. Plug a powered USB cable into the USB socket on the GPS Frequency Reference PCB and observe LED1. It should fade on and off for a few seconds. At this stage, everything should be working and the splash screen should now be shown. To follow the status of the startup, press the “Status” button. The six lines at the bottom of the Status screen are the important ones to watch, as the top lines are mostly information taken from the GPS module’s NMEA data. You may not see all items go to “OK” in the startup page straight away, particularly the GPS related items, as the GPS module usually takes some time to achieve a satellite fix. If you are using the VK2828 GPS module, you will know when it has a fix, as the green LED on it will start flashing. The “Temp Sensor” line should read “OK” and the temperature should be rising or near the setpoint. That means the oven is working correctly. If “Temp Sensor” shows “Not ready” then TS1 is not wired correctly. If “Temp Sensor” shows “OK” but the temperature is not rising, there is a problem with Q1, the 2.7kresistor or DAC IC6. If the LED was fading initially then the DAC is probably working. There are three lines which indicate the status of the GPS module. The first one to check is “GPS Receiver”. If that does not show “OK” then no data is being received and you should check the GPS module’s wiring. 82 Silicon Chip The “GPS 1PPS” and “GPS Locked” status lines will typically be the last ones to show “OK”, as they depend on the GPS module having a good satellite fix. If you are testing indoors, you may find they flick between “OK” and “Not ready”. The “PLL unit” and “VCO output” lines are only updated at startup, so will not change if left for a while. If “PLL unit” does not show “OK” then the Micromite cannot communicate with IC2. This may be due to problems with the I2C bus. “VCO output” shows OK when the Micromite detects a ~40MHz signal. That means that the PLL and VCO are working to some extent. If there is no “VCO output” then check that the VCO chip is soldered to the PCB correctly. As the 40MHz signal to the Micromite is also fed through the PLL (IC2), you should confirm that there aren’t any problems with IC2 as well, eg, solder bridges between pins or bad solder joints. Another test that you may like to do if you have an oscilloscope or frequency counter is to check that there is an output from each of the BNC sockets (or the JP2 header). Assuming that JP2 is set to the “BC” position, all of CON2, CON3 and CON4 should be producing a 40MHz signal. If this is the case, then it is time to complete assembly. Finishing the oven While you would have seen the temperature of the oven increasing on the status page, and the unit is effectively functional, we can add some insulation to the oven to improve its ability to hold heat. This helps to ensure that the temperature inside the oven is uniform, so that the temperature measured by TS1 more closely reflects the temperature of the other components inside the oven. We’ve sized the oven to be roughly the same diameter as a bottle cap from a two-litre milk bottle. We’ve found that most of them also have a foam insert which provides extra insulation The height of our cap was precisely 12mm, which matches the tapped spacers between the two boards. Unfortunately, due to the components on the Micromite BackPack PCB, the Australia’s electronics magazine available space is reduced slightly, and the rim of the bottle cap will probably need to be trimmed. It’s a good idea to give the lid a thorough clean with soap and hot water to ensure there is no milk residue. We can imagine nothing worse than a GPS Frequency Reference that smells like mouldy cheese! If the lid is a snug fit between the Micromite BackPack PCB and the GPS Frequency Reference PCB, it can simply be sandwiched in place. Otherwise, holes are provided on the PCB for cable ties to hold it in place. Alternatively, you could use a small amount of neutral-cure silicone sealant around the rim to seal it and stop it from moving around. The underside (ie, non-component side) of the PCB should ideally be insulated as well. You could either use a foam insert from another milk bottle (held in place by the same cable ties) or merely apply some foam-backed double sided tape to the back of the PCB. PCB jumper settings The jumper on JP1 selects whether the GPS module receives 3.3V or 5V. Most modules will run off 3.3V, including the VK2828U7G5LF but if you are not sure, check the module’s data sheet. Fit a jumper shunt between the pins labelled B and C on JP2 if you want a programmable frequency on CON2. Alternatively, fit the shunt between the pins labelled 1 and B for a (disciplined) 1PPS (1Hz) output from CON2. The pins labelled “G” are connected to ground so you can run a shielded cable from the pairs of pins at either end to a chassis connector if you want to make both of these signals available externally. A shunt is placed on LK1 for normal operation but this prevents programming the Micromite chip, so remove it if you need to reprogram the chip. CON7 is for debugging the software so you can safely ignore it unless you plan to modify the software. Putting it all together Now power down the GPS Frequency Reference and detach the Reference PCB from the BackPack. If the BackPack display is attached by screws, remove them to allow the front panel to be fitted. siliconchip.com.au Assuming everything is apart, start by attaching the LCD to the laser-cut acrylic UB3 lid panel, using M3 machine screws at the front and tapped spacers at the back. Insert 1mm Nylon washers between the lid and LCD to provide clearance for the solder joints. Use 20mm-long machine screws on the bottom left, bottom right and top right holes. This is with the touch panel orientated so that its flex connector is on the right, along with the 14-pin header. For the top left machine screw, use one of the shorter ones initially fitted to the LCD BackPack, again with a tapped spacer on the back. This is necessary because a fourth long screw would interfere with the GPS header. The Micromite BackPack PCB can now be inserted over the three long machine screws shafts and can be loosely secured with a short machine screw into the single tapped spacer. Now feed the tapped spacers over the three remaining screw shafts. Ensure everything is tight and lines up. In particular, check that the LCD’s screen is flush with (or slightly behind) the lid panel. Finally, attach the GPS Frequency Reference PCB to the back of the Mi- cromite BackPack PCB using the three remaining short machine screws from the original BackPack kit. Putting it in the box The enclosure specified is a standard UB3 Jiffy box. You will need to make cutouts at two ends for the BNC and USB sockets; see Fig.4 for details. You only need to make one of the cutouts for the mini-USB and micro-USB socket, depending on what you fitted to the board. We used a stepped drill to make the BNC socket holes although you could use a standard drill and then enlarge them to size with a tapered reamer. We made the vertical slot for CON3 using a hacksaw, cutting straight down from the top of the side of the box. The holes for the USB sockets can be started with a small drill bit and completed with a file. If you are feeling lazy, or don’t enjoy cutting square holes, you could make (slightly larger) round holes for the USB sockets. You may find that you have to make the hole larger than shown in the diagrams if the shroud on your USB plug is unusually large. The final step is to carefully thread CON2 and CON4 into the holes in the right-hand side of the case and then lower CON3 down into its slot. Check that all the holes line up and that a USB cable will plug in. Then attach the acrylic lid to the Jiffy box using the supplied screws (or longer ones, if the ones that came with your box are too short) and fit the nuts and washers to the BNC sockets. Using it As you are reading the following instructions, you may wish to refer back to the first article on this project in last month’s issue, as it included images showing many of the screens described below. Once power is applied via the USB socket, the start screen will show for three seconds, after which the main screen appears. Press the STATUS button to check that everything is working as expected. The BASIC program is quite busy processing data, so sometimes it is necessary to press on the buttons for more than a brief ‘tap’. The Temperature line shows the current oven temperature and setpoint, followed by the oven heat controller DAC output, where zero is off and 4095 is full power. After the unit has tuned itself and the oven temperature has reached its set point, it will provide a high degree of accuracy. Fig.4: cutting and drilling templates for the UB3 Jiffy box. You will only need to make a rectangular cutout for one of the USB sockets, according to what has been fitted. If required with an external GPS, the slot to allow CON3 to be lowered into its hole could instead be used to feed out GPS antenna wiring, or you could make a dedicated hole or mount a GPS antenna socket above the USB connector hole. siliconchip.com.au Australia’s electronics magazine November 2018  83 If your workbench area typically gets above the 35°C we have set for the oven, you may need to make the setpoint a bit higher, so that the oven has a consistent temperature in all conditions. See the Settings section below for details on how to do that. The “GPS 1PPS” and “GPS Locked” status lines need to show “OK” before oscillator disciplining occurs but once the unit has got past the start screen, it is effectively operational, although it will not yet be operating with full accuracy or precision. The tuning algorithm waits until it has received 1000 1Hz pulses with GPS lock, then calculates the average oscillator frequency (as seen on the top line) and adjusts the VCO control voltage to bring it closer to 40MHz. Given that 1000 pulses take about 17 minutes and it takes some time for the GPS receiver to get a satellite fix, it should begin tuning itself within about half an hour of power-on. You can explore the features of the unit before it has fully tuned itself; the initial tolerance on the VCO is 2ppm, which makes it a useful tool straight away. Pressing the “BACK” button to go back to the main menu, you can jump straight into any of the adjustment pages for CON2-CON4 to set their output frequencies. Note that these labels are adjacent to their respective BNC sockets, which helps you to remember which is which. Once you’ve entered one of the output setup screens, the “SEEK F” button allows a frequency to be entered on a keypad and the unit will find the nearest frequency that it can synthesise to what you enter. It will show the frequency, the various PLL dividers and even the PLL’s internal frequency to allow you to decide if that particular combination is suitable. Pressing “OK” will then update the PLL parameters to those shown and the new frequency will be immediately available from that output socket. Press “CANCEL” to go back to the output setup screen without changing the output frequency. Manually setting up the PLLs The “ADVANCED” page permits manual selection of the N, M and P dividers in the PLL, except for CON4, 84 Silicon Chip Fitting the assembly to the UB3 case is a little tricky – but it can be done! After drilling/filing the required case holes, you need to introduce the boards to the holes for CON2 and CON4 at a quite steep angle, as shown here. If your holes are accurately drilled, the board should slip into place quite easily . . . where only the P value can be changed; the N and M values are fixed because this PLL is shared with the output that provides VCO feedback to the Micromite. As explained last month, the incoming 40MHz signal is multiplied by N and divided by M to give the PLL frequency and then divided by P to give the output frequency. While the PLL is supposed to operate between 80MHz and 300MHz, we found that it worked outside this range (perhaps with more jitter). The PLL frequency is displayed near the top of the page, and if it would be out of range, it is displayed in yellow. In this case, you should verify that the output frequency is accurate and stable. If the resultant output frequency is above 99999999Hz, it is displayed in red. Although such frequencies can be set, they appear to be very unstable and may cause the PLL to stop functioning. In any case, the output buffers will not work well above 100MHz, so we do not recommend that you use such frequencies. The only conditions that are enforced when you enter the PLL configuration manually are that N is between 1 and 4095, M is between 1 and 511, P is between 1 and 127 and that N is greater than M. Like with the other configuration screen, once you have set parameters that you are happy with, press the “OK” button to update the output Australia’s electronics magazine frequency or the “CANCEL” button to return to the previous screen without making any changes. Using frequency presets The CON2-CON4 setup pages also show four preset frequencies. They are initially 80MHz, 40MHz, 20MHz and 10MHz (all using a PLL frequency of 160MHz). The frequency of the output can be changed to any of the presets by pressing that button briefly. Or, to change one of the presets, set the output to the desired frequency and then hold down the preset button for more than one second. There is also the option to copy presets between the outputs by using the “PRESETS” page, which can be accessed via the “SETTINGS” button on the main page. The preset page has two buttons at the top to allow you to scroll between the various connector presets and output value settings. Their current values are displayed below. Further down, there is a “COPY” button and a “PASTE” button, followed by the current ‘clipboard’ values. Pressing the “COPY” button copies the currently selected preset or output value to the clipboard and pressing “PASTE” copies the clipboard value back to the preset or output value. A “BACK” button is provided to return to the main page. The software will give an error message if you try to copy any setting to siliconchip.com.au . . . and then it’s simply a matter of lowering it all into place so that CON3 and the USB socket mate with their holes on the left-hand end. There is no need for any screws holding the board from underneath – the screws which hold the front panel in place hold the whole assembly snug and secure. CON4 which is not compatible, ie, it does not have N=4 and M=1. Additional settings On the “SETTINGS” page, there are also options to adjust the oven “TEMPERATURE” control loop and the “VCO TRIM” settings. Under the “TEMPERATURE” menu, there are options for Setpoint, Gain and Offset. The Setpoint is the target temperature of the oven, and as we mentioned earlier, it should be higher than the highest expected ambient temperature where the unit is being used. The default is 35°C, which is suitable either for colder regions or buildings with air conditioning. The Gain and Offset values are used to change the behaviour of the control loop. It uses simple proportional control and the default values of 1000 for Gain and 3000 for Offset work well. It’s unlikely that you would need to change them unless your transistor Q1 has a wildly different gain from the components that we used in our prototypes. Both values are in DAC step units (out of 4095) with Offset being the DAC output level when the target temperature is reached and Gain being the change in DAC output level for a 1°C error. If you find that the oven temperature is oscillating wildly, the Gain value should be reduced. A small amount of drift (under 1°C) is to be expected siliconchip.com.au and is not a cause for concern. If you find that the oven temperature is consistently too high or too low, adjust the Offset value. Allow the unit to settle for about 10 minutes, then check the current DAC output (the number in brackets on the STATUS page) and enter this value as the Offset. You may need to repeat this a few times to get an ideal value. If you change these values, press the “SAVE” button to store the changes (shown in yellow) or the “BACK” button to go back to the settings screen without making any changes. Adjusting the VCO control loop The final settings page is for adjusting the VCO control parameters, which include a “Gain” value, a “C Value” (control value) and the “Update s”. The “Update s” value is the number of 1PPS pulses that are counted before an adjustment is made to the VCO. The default is 1000 but this can be extended to provide further precision, as more 1PPS pulses will be sampled. The C Value is the current VCO control DAC value (0-16,777,215). This is the value that is changed by the disciplining routine after the correct number of 1PPS pulses have been received. As such, you should see the value change as this occurs. The default value is chosen to be at the midpoint of the VCO’s pulling range. Australia’s electronics magazine The Gain value sets the number of DAC steps by which the C Value is changed per Hertz of error, and has been calculated as follows. The VCO has a pulling range of 0.5 to 2.5V, corresponding to a frequency change of 10ppm (from -5ppm to +5ppm around nominal). The DAC’s voltage reference is nominally 2.5V, so the span of the 2.0V pulling range corresponds to 13,421,772 DAC steps. With a 40MHz nominal frequency, the 10ppm range of variation corresponds to 400Hz. Dividing 13,421,772 by 400 gives 33,554 DAC steps per Hertz, which is our calculated Gain value. Another way to look at this is that each DAC step corresponds to a change of around 30µHz in the VCO output, which gives very fine control. This is all designed to ensure that the GPS Frequency Reference converges as quickly as possible on the first round of disciplining; given that this process is repeated, the unit is also able to adjust for drift and other factors automatically. Once again, use the “SAVE” button to commit any changes to flash memory. Manual VCO calibration If you have an accurate frequency counter, you can use this to adjust the VCO manually, using the calculations above. If you want to disable automatic adjustment, you can either remove the GPS module or set the VCO “Gain” value to zero. The “C Value” will then remain constant. To manually trim the VCO, allow the oven temperature to stabilise and set one of the outputs to 40MHz (they are set to this by default in the initial firmware settings). Check the frequency using a precision frequency meter and note the offset in Hertz. Take this offset, and multiply it by the 33,554 value we calculated earlier, and add (if the current frequency is too low) or subtract (if the frequency is too high) it from the current “C Value”. If there is a small residual error, you can repeat the adjustment to tweak it further. Conclusion That completes the construction and set-up of the GPS Frequency Reference. We are sure that you will find it useful; we certainly plan to make good use of our prototype. SC November 2018  85