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GPS-Based Frequency Reference Pt.1: By JIM ROWE Need a source of very accurate 10MHz and 1MHz signals for calibrating frequency counters, radio receivers and signal generators? Here’s just what you need: a frequency reference which is linked to the Global Positioning System (GPS) satellites, to take advantage of their highly accurate on-board caesium-beam “atomic clocks”. N OT TOO MANY decades ago, the only way most people could generate reasonably accurate frequency signals was by using a quartz crystal oscillator. Following this, it became possible to achieve slightly better accuracy by heterodyning a local quartz oscillator with an HF radio signal from one of the standard frequency and time stations, such as WWV in the USA or VNG in Australia. By about 1980, even higher accuracy could be obtained by locking a local quartz crystal oscillator with the horizontal sync signals from one of the national TV networks. That’s because the networks used a master timing clock that was locked to an “atomic 64  Silicon Chip clock” based on either a caesium beam or rubidium vapour oscillator. The GPS system The Global Positioning System (GPS) became operational around 1990 and is run by the US Department of Defense. By using this system as a reference, it’s possible to generate reference frequencies with extremely high accuracy – even better than using the previously listed methods. That’s because each of the 22-odd GPS satellites orbiting the Earth has two caesium beam atomic clocks on board. These are necessary to generate the very accurate frequency and time signals needed for accurate position- ing. And since there are always at least four GPS satellites “in view” at any time from any point on the Earth, this means that there’s always access to an “ensemble” of about eight caesium beam clocks to serve as a frequency reference – provided you have the right GPS receiving equipment, that is. The only problem was that until a couple of years ago, GPS receivers were quite expensive. However, costs have fallen quite dramatically since then – so much so that handheld and mobile GPS navigators are now everyday consumer items. In fact, low-end navigators with colour LCD screens are now down to around $400. Small wonder they’re becoming so popular! siliconchip.com.au The unit is housed in a plastic case and provides accurate 10MHz and 1MHz reference frequencies via front-panel BNC sockets. A range of data can also be displayed on the LCD, including UTC time and date and the receiving antenna’s latitude, longitude and height above mean sea level (see panel). with good-quality signals, though. This means mounting a small active GPS antenna in a clear area outside, as high as possible so that it can get an unobstructed “view” of the sky in order to receive the satellite signals. The antenna is connected to the antenna input of the receiver using a suitable length of good quality 50W coaxial cable. This delivers the amplified 1.575GHz GPS signals to the receiver and also feeds the active antenna with DC power (provided by the receiver). In our case, we chose a Garmin GA 29F flush-mount active antenna, which costs about $90. This was mounted on a plastic junction box and fitted to the top of the author’s TV antenna mast (see photo). We also tested a D-3856 antenna made by Australian firm RF Industries, which also worked well. This unit is available from Dick Smith Electronics and Tandy outlets for just $69. Taken together, the GPS receiver module and an active antenna will set you back about $200. The rest of the parts will probably be around the $150 mark, so you should be able to build the whole shebang for about $350. This is just a fraction of the price you’ll pay for a commercially available GPS-based frequency reference. How it works As you might expect, inside each of these navigators is a complete GPS receiver module. However you don’t have to buy a navigator to get the receiver module, because they are also available separately for use in other equipment. And that’s just what we’ve done here – use one of these “bare bones” receiver modules as the heart of this project. Garmin GPS 15L The GPS module we chose to use is a Garmin GPS 15L, which is available from local distributors for about $130. It’s quite a tiny device, measuring just 46 x 36 x 8.5mm and weighing in at only 14.1g. But don’t let the size fool siliconchip.com.au you because there’s a lot packed into it. Inside, there’s a complete 12-channel GPS receiver which can track and use up to 12 GPS satellites at once. And it can provide a swag of GPS-derived time, date, position and satellite status information in serial RS-232C text form – updated each second, no less. It also provides a one-pulse-persecond (1PPS) output, where the leading edges of the pulses are very accurately locked to the UTC-derived GPS timing system. It’s these pulses that we mainly use in the reference, to control the frequency of a local 10MHz crystal oscillator. For best performance, you do need to feed the Garmin GPS 15L receiver To get a handle on how it all works, refer now to the block dia-gram of Fig.1. Basically, the frequency of the 10MHz crystal oscillator (top, right of Fig.1) is controlled using a phase-locked loop (PLL). This PLL, in turn, uses the very accurate 1Hz pulses from the GPS receiver module as its reference. However, the PLL configuration is a bit more complicated than normal, so let’s look at this in greater detail. Basically, the reason for the added complexity is that it isn’t easy to control a 10MHz crystal oscillator using a reference frequency as low as 1Hz – at least not using a standard PLL. That’s because with a standard PLL configuration, the oscillator frequency must be divided by 10,000,000 (to get 1Hz), to be compared with the reference frequency in the phase comparator. However, such a high division factor involves a relatively long time delay and this adversely affects the error correction feedback, making it very March 2007  65 generates a “phase error” pulse, the width of which is directly equivalent to the timing difference. One of these phase error pulses is produced at the start of each 1Hz GPS pulse and they can vary in width from zero (when the two signals are exactly in step) up to a theoretical maximum of 20ms (when the two signals are one period of 50kHz out of step). In practice, we use the PLL’s feedback loop to maintain a fixed phase error of about 10ms (ie, halfway in the range). This gives the PLL the widest possible control range, to ensure reliable locking of the 10MHz crystal oscillator. Deriving the feedback voltage A small active GPS antenna is necessary to receive the GPS signals. The author used a Garmin GA 29F antenna. This was mounted on a plastic junction box and fitted to the top of an existing TV antenna mast. difficult to stabilise the PLL. To get around this problem, we divide the 10MHz oscillator output by a much smaller factor – only 200 times in fact. This is done in separate divide-by-10 and divide-by-20 stages using synchronous divider ICs, so that we end up with 50kHz pulses which have the timing of their leading edges (L-H transitions) very closely synchronised with the leading edges of every 200th pulse from the 10MHz oscillator. This means we have effectively transferred the phase of the 10MHz oscillator signal (averaged over 20ms) to the 50kHz signal at the output of the divide-by-20 divider. And it’s the phase of this signal which we feed into the second input of the phase comparator, where it’s compared with the leading edges of the 1Hz pulses from the GPS receiver module. The phase comparator does exactly what its name implies – it compares the leading edge of each 1Hz GPS pulse with the 50kHz pulse nearest to it and 66  Silicon Chip OK, so how do we use the varying phase error pulses from the comparator to produce an error correction feedback voltage for the 10MHz oscillator? Well, what we do is use the error pulses to control an AND gate which then passes pulses from a second crystal oscillator (running at about 10MHz) to an 8-bit binary counter. So as the error pulse width varies, it allows a varying number of these “about-10MHz” pulses to reach the counter. For example, if the phase error pulses are 8.0ms wide, 80 pulses will be gated through to the counter. And if the pulses are 11ms wide, 110 pulses will be fed through, and so on. So at the start of each 1Hz GPS pulse, a burst of “about-10MHz” pulses will be fed to the counter, the number of pulses in the burst being directly proportional to the phase error. The counter is actually reset at the end of each 1Hz GPS pulse, so it counts up from zero each time. At the output of the counter we also have an 8-bit latch and a simple digital-to-analog converter (DAC) using a resistor ladder network. After the end of each phase error pulse, the latest error-proportional pulse count is transferred into the latch, replacing the previous count. As a result, the output of the DAC is a DC voltage which varies in level each second, according to the phase error. So the phase error has been converted into a varying DC error voltage. Get the idea? When there’s a fixed phase error of say 10ms, the counter will have a count of 100 each time and the DAC will have an output voltage of almost exactly 1.953V. This voltage will vary up or down in steps of 19.53mV, as the phase error pulses vary in width and the number of “about-10MHz” pulses fed to the counter varies up or down. Each of the “about-10MHz” pulses fed to the counter corresponds to a phase error step of close to 100ns, so our phase error-to-DC error voltage conversion circuit has a conversion gain of 19.53mV/100ns or just under 2mV for every 10ns change in phase error. Why two 10MHz oscillators? By now, you are probably wondering why we go to the trouble of using a second 10MHz crystal oscillator to provide the 100ns pulses for the phase error counter. Why not just use the output of the main temperaturecontrolled 10MHz oscillator, at upper right? We use a second 10MHz oscillator because this inevitably drifts in phase compared with the main oscillator and this introduces a small amount of “dither” into the phase error counting operation. The random noise introduced into the DAC’s output voltage as a result of this dither allows the PLL’s error correction to have a significantly higher resolution than if we used pulses from the main 10MHz oscillator. The reason for this is quite straightforward. If we had used the pulses from the main oscillator, the fact that they would be locked to the 50kHz pulses (and hence the phase error pulses as well) would mean that the DC error voltage could only ever change in 19.53mV increments. This corresponds to 100ns changes in phase error. However, the dither introduced by using the second oscillator means that the average error voltage will change in somewhat smaller increments. And that means that we can maintain the main oscillator’s phase locking to much closer than 100ns. As shown in Fig.1, the DC phase error voltage from the DAC is fed through a buffer to a low-pass filter stage based on capacitor C1 and resistors R1 & R2. The filtered error correction voltage is then used to control the capacitance of a varicap diode, to fine-control the frequency and phase of the main 10MHz oscillator. This unusual type of PLL system is very effective when it comes to phase-locking a 10MHz oscillator to the GPS 1Hz pulses but it does have a limitation. Because it divides down siliconchip.com.au Fig.1: the GPS-Based Frequency Reference uses a phase-locked loop (PLL) to control the frequency of a 10MHz crystal oscillator (top, right). This PLL in turn is referenced to the very accurate 1Hz pulses from the GPS receiver module. A PIC microcontroller decodes the GPS data, interprets the switches and drives the display module. the oscillator frequency by only 200 times instead of 10,000,000, it’s just as effective at phase-locking an oscillator at a frequency of 9.999800MHz or 9.999600MHz, or 10.000200MHz or 10.000400MHz. In other words, it’s capable at phase-locking at frequencies that are separated from 10.000000MHz by exactly 200Hz or multiples of that frequency difference. This means that when you are setting up the frequency reference, it’s very important to adjust the free-running frequency of the main crystal oscillator to within 100Hz of 10.000000MHz. If you don’t, the PLL may lock it to 9.999800MHz or 10.000200MHz instead of the correct frequency! Making use of the data OK, that’s how the main part of the GPS Frequency Reference works. The only part we haven’t discussed yet is the section down in the lower left of the block diagram. This section is functionally quite separate from the main section. Its purpose is to make use of the stream of useful data that emerges from the GPS receiver module siliconchip.com.au each second, along with (but separate from) those accurate 1Hz pulses. This data is delivered as ASCII text and appears at the module’s RS-232C serial output port. It’s in the form of coded data “sentences”, sent at a rate of 4800bps (bits per second) using a sentence format known as NMEA1083. This format was first standardised by the US National Marine Electronics Association (NMEA) for information exchange between marine navigation equipment. As shown in Fig.1, we use a programmed PIC16F628A microcontroller to “catch” and analyse this serial data. The decoded data is then feed it to an LCD module. Pushbutton switches S1-S3 are included to allow you to display some of the more esoteric information for a short time, as required. Normally, the display simply shows the current UTC time and date (updated each second), plus the GPS fix and PLL locking status. The fourth switch (S4) forces the PIC micro to send an initialisation code command to the GPS receiver module, to initialise it correctly if it ever becomes “confused” (the GPS receiver also contains a microcontroller, of course). In fact, the receiver module has an RS-232C serial input as well as the output, provided for this very purpose. However, because this initialisation is rarely required, S4 is not readily accessible like S1-S3. Instead, it must be accessed through a small hole in the front panel of the project, using a small screwdriver or probe tip. Circuit details Now that you have a basic understanding of the way the GPS-Based Frequency Reference works, we should be able to work quickly through the main circuit, to clarify the fine details. Fig.2 shows the main circuit while Fig.3 shows the associated display circuit with its LCD module. The two connect via a 16-way header cable. In operation, the Garmin GPS 15L receiver module (lower left of Fig.2) is fed via an external active antenna. The resulting GPS-locked 1Hz pulses are on the grey wire of its 8-way output cable and this goes to pin 5 of a 10way IDC line socket that mates with CON7. The 1Hz pulses are then fed March 2007  67 +11.4V + – 12V DC INPUT D5 D1 1N4004 A K D6 A D7 K A VR1 GND 3 x 1N4004 1000 F 16V 3.3k +5V OUT IN K A CON5 REG1 7805 K 10 F 2.0k CON6 +5V TO DISPLAY BOARD 10 9 680 1 3 2 2 3 18 4 1 11 13 13 12 15 11 16 10 5 17 14 D4 Vdd RA4 MCLR RA3 RA1 IC1 IN PIC16F628A RB7 RB6 RB3 RB5 RB0 RB4 RB2 RA0 14 2 13 12 7 7 2 IC14c ~10MHz 5 5 4 180 1M RS-232C DATA 3 ~10MHz 9 10 33pF X2 10MHz 33pF 7 2 10k 6 GPS 1Hz 3 2 1 WHT 1 14 2 100pF CET PE Vdd CP D3 D2 Q3 CP Q3 D2 Q1 D1 Q0 11 IC9 12 Q2 D3 74HC161 D0 MR Vss 1 8 13 14 11 IC8 12 Q2 74HC161 Q1 D1 D0 TC 15 CEP TC 15 CEP Q0 MR Vss 1 8 IC11c 5 16 CET 16 Vdd PE 13 14 RESET COUNTERS 6 ORG IC11a 4,6 BLK 100 4 +5V 9,10 GRN CON3 5 GPS 1Hz PULSES CON7 6 6 8 9 100nF 10 11 5 IC10 LM335Z TEMP SENSOR ADJ 100nF IC14f IC14a IC14e GRY Q1 BD136 HEATER +5V 10 GARMIN GPS-15L RECEIVER MODULE C TP2 IC14: 74HC04 7 IC14d YEL 1 + 8 9 BLU E B – 1 RED 6.8k 7  ERROR PULSE 8 EXTERNAL ACTIVE GPS ANTENNA 4 1nF 5 68 9 5 7 2 3.3k 6 RB1 Vss 100nF 10MHz FROM IC3a CLK 16 RA2 8 6 IC2 LM311 3 4.7 F A 4 100nF 33k 2.2nF 2.2k TP1 K 14 33 33k 5k D2 1N4148 1k IC11d 8 K 9 IC11: 74HC14  ERROR PULSE PHASE ERROR PULSE WIDTH COUNTERS A IC11b 3 4 7805 SC 2007 GPS-BASED FREQUENCY REFERENCE through Schmitt inverters IC11a and IC11b which act as buffer stages. The resulting 5V p-p pulses from IC11b are then fed directly to pin 14 of IC7, which is the phase comparator. The 10MHz crystal oscillator that’s 68  Silicon Chip MAIN BOARD phase locked to the GPS pulses is based on inverter IC3f and crystal X1, plus varicap diode VC1 and several low-value capacitors. Its 10MHz output is fed via inverting buffer stage IC3b to CON1 and also via IC3c to di- OUT GND IN vider stage IC4. This stage divides the signal by 10 and provides two 1MHz outputs, at pins 12 & 15. The pin 12 output is then fed via inverter IC3d to CON2, to provide the 1MHz output signal at BNC connector CON2. siliconchip.com.au BD136 LM335Z IC3f 13 ADJ – C 1 IC3b 3 5 CAPACITOR VALUE MAY NEED TO BE CHANGED TO SUIT CRYSTAL – SEE TEXT E 2 12 + B IC3a 10MHz TO IC1 1M 180 X1 10MHz 7 IC3: 74HC04 IC3c 6 +5V 100nF VC2 3-10pF 4.7pF NPO 22pF NPO 15pF NPO TEMPERATURE STABILISED ENCLOSURE 2 IC4 74HC160 CP 8 K 100nF 9 7 10 16 PE CEP CET Vdd 12 9 Q2 MR 1 10MHz 47k CON1 10MHz OUT 100 4 Vss TC D0 D1 D2 D3 VC1 BB119 3 4 5 14 CON2 100 8 1MHz OUT IC3d 15 6 A 100nF 20 18 17 14 13 8 7 4 Vdd D7 Q7 D6 Q6 D5 Q5 D4 Q4 IC12 74HC374 D3 Q3 Q2 D2 D1 Q1 19 16 15 12 9 6 5 20k 20k 10k 20k 10k 20k 10k 20k 10k 20k 10k 2 IC13a 4.7k 1 ERROR VOLTS CON8 +5V 9 7 10 16 PE CEP CET Vdd MR 2 IC5 15 CP TC 74HC160 8 Vss D0 D1 D2 D3 3 4 5 6 11 IC13: LM358 1k IC3e 5 6 10 F IC13b 7 4 +5V IC6: 74HC73 14 10MHz (INV) 1 3 J CLK R IC6a K 1M 20k Q 7 12 5 Q 13 10 1k A 11 R J IC6b Q 100nF 50kHz 9 TP3 CLK K 8 Q 50kHz +5V +5V IC11e 4 6 11 LADDER DAC K 100kHz 10 2 1M 10k 100nF 1 1M LATCH ENABLE 10 8 3 10k 20k 3 D0 CP Q0 2 Vss OE 1 11 10 20k 10 F 100nF 1MHz 10MHz +5V 5 3 D3 1N4148 Cin 14 Sin 100pF GPS 1Hz PULSES 16 Vdd INH Vss 10 F 100nF IC7 74HC4046 PC3o 15 8  ERROR PULSE PHASE COMPARATOR  ERROR PULSE 1N4004 A K By contrast, the 1MHz pulse output from pin 15 is fed to a second divideby-10 stage based on IC5 (ie, to the CET input at pin 10). The resulting 100kHz pulse output from pin 15 of IC5 is then fed to the J and K inputs of siliconchip.com.au 1N4148 A K IC11f BB119 A 12 13 K flipflops IC6a and IC6b. Note that the 10MHz output from IC3c is used to clock IC5, IC6a & IC6b, the latter two stages via inverter IC3e. This ensures that the counter and divider outputs are correctly synchronised. 7 100 CON4  ERROR PULSE (INV) Fig.2 (above): the complete circuit for the GPS-Based Frequency Reference minus the display circuitry (LCD & LED indicators). The PLL-controlled 10MHz oscillator is built into a small temperature-controlled oven to ensure stability, with power transistor Q1 acting as the oven heater. March 2007  69 Fig.3: the display circuit interfaces to the PIC microcontroller (IC1) in the main circuit via IDC connector CON6. It includes the LCD module, three LED indicators (LED1-LED3), switching transistors Q2-Q4 and four pushbutton switches (S1-S4). IC6a & IC6b are both are wired for divide-by-2 operation. The 50kHz pulses from the Q output (pin 12) of IC6a are fed to the Cin input (pin 3) of phase comparator IC7, for comparison with the 1Hz GPS pulses on pin 14 (Sin). Note that these 50kHz pulses have their rising edges closely aligned with the rising edge of every 200th pulse from the 10MHz oscillator. The phase error pulses emerge from pin 15 of IC7 and are fed directly to the clock gating inputs of 4-bit synchronous counters IC8 and IC9 (74HC161), 70  Silicon Chip which together form the 8-bit phase error pulse width counter. This is done because the AND gate shown in Fig.1 is actually inside the two counter chips, rather than being a separate device. The “about-10MHz” clock oscillator used by the error counter is based on crystal X2 and inverter stage IC14c. Its output is buffered by IC14a & IC14f and fed to the clock inputs (pin 2) of the two counters. The eight output bits from the two counters are then fed to the data inputs of IC12, the octal latch. Its outputs are used to drive the resistive-ladder DAC (digital-to-analog converter). In practice, this counter-latch-DAC sub-circuit is arranged so that it performs a new count of the phase error pulse width at the start of every 1Hz pulse from the GPS receiver module. The sequence is as follows: on the falling edge of each 1Hz pulse (100ms after the start), the counters are reset by a very short pulse on their MR-bar pins (pin 1). These short reset pulses are derived from the 1Hz pulses at the output of IC11a. The 1Hz pulses are differentiated using a 100pF capacitor and 1kW resistor and fed to the MR-bar pins of IC8 and IC9 via IC11c. The two counters begin counting when the phase error pulse from IC7 arrives at their CEP pins (7). This allows them to count the “about-10MHz” pulses which are fed to their CP (pin 2) inputs via buffer stages IC14a and IC14f. Counting continues until the end of the phase error pulse and then stops. Another very short pulse, this time derived from the falling edge of the phase error pulse signal and applied via IC11e to pin 11 of IC12, then transfers the count into IC12’s latches, replacing the previous count. As a result, the DC output voltage from the DAC changes in response to the new count. The counters are then reset again at the end of the 1Hz GPS pulse, ready for the next sequence. The varying DC error voltage from the DAC is fed first through buffer stage IC13a and then to a low-pass loop filter which is formed using a 1kW resistor (R1 in Fig.1), a 10mF capacitor (C1) and three 1MW resistors (R2). From there, the filtered error voltage is then fed through IC13b to become the automatic phase correction (APC) voltage. This APC voltage is applied to varicap diode VC1 which varies its capacitance accordingly. As previously stated, VC1 forms part of the 10MHz crystal oscillator circuit and its capacitance variations bring the oscillator into phase lock. Trimmer capacitor VC2 and its parallel 4.7pF capacitor are used to initially adjust the oscillator so that its free-running frequency is within 100Hz of 10MHz – ensuring that the PLL locks correctly to this frequency. Temperature stabilisation OK, so that’s the basic PLL section of the GPS-Based Frequency Reference circuit. By now, though, you’re probsiliconchip.com.au Fig.4: here are the two outputs provided by the Garmin GPS 15L receiver module. The upper trace (yellow) shows one of the extremely accurate 1Hz pulses, while the lower (purple) trace shows the start of the RS-232C data stream giving UTC time and date, latitude and longitude, etc. Note that the frequency reading on the bottom line should read exactly 1.000000Hz; the actual reading shows the scope’s measurement error. ably wondering about the function of comparator IC2, transistor Q1 and the LM335Z temperature sensor (IC10). What are they for? These parts are used to achieve temperature stabilisation of the main 10MHz oscillator crystal (X1), varicap diode VC1 and its series 15pF capacitor. In practice, these components are housed in a “mini oven” to keep the temperature constant. This oven includes a small TO-220 heatsink to which is attached the crystal, the LM335Z temperature sensor and a power transistor (Q1). It’s basically an insulated enclosure made from a cut-down 35mm film canister which is lined inside using expanded polystyrene. The construction of this mini oven will be described next month. All you need to know for now is that IC10 (LM335Z) is mounted inside the enclosure to sense the internal temperature. Basically, the voltage across IC10 is directly proportional to its temperature (in Kelvins) and this voltage is applied to the non-inverting input of comparator IC2. IC2’s inverting input is fed with a reference voltage of close to 3.15V, derived from a voltage divider (2kW & 3.3kW) across the regulated 5V supply rail. As a result, IC2’s pin 7 output switches high when the temperature sensor’s voltage rises siliconchip.com.au Fig.5: shown here are the leading edge of the GPS 1Hz pulses from the receiver module (upper yellow trace), and the inverted error pulse from the Frequency Reference’s phase detector (lower purple trace), when the PLL is locked with a fixed phase error of 11.54us. The jitter visible on the trailing edge of the error pulse is normal and is caused by noise, GPS propagation variations and so on (see text). slightly above 3.15V and switches low when the sensor’s voltage falls somewhat below this level (depending on the hysteresis applied to the comparator). IC2 is used to control power trans­ istor Q1, which is used here purely as a heater. This transistor is attached to the finned heatsink which forms the frame of the mini oven, so when it conducts it generates heat to increase the temperature. As a result of the feedback provided by IC10, the temperature inside the mini oven is maintained Specification Summary (1) This unit is a low-cost frequency and time reference based on a Garmin GPS 15L receiver module. It is able to control the frequency of a local 10MHz crystal oscillator by reference to the very accurate 1pps (1Hz) pulses broadcast by GPS satellites (referenced back to UTC as maintained by the USNO). This allows the frequency of the local 10MHz oscillator to be controlled to within about 0.2Hz averaged over a 30-second period and even more tightly when averaged over a longer period such as 30 minutes. (2) The built-in 10MHz reference crystal is housed in a small temperature stabilised enclosure or “mini oven”. Buffered 10MHz and 1MHz outputs are provided for external use. Buffered outputs are also provided for the 1Hz GPS pulses and the phase error signals from the internal phase-locked loop (PLL) used to control the 10MHz oscillator. The error signals allow the user to log instantaneous phase error in the PLL, if this is desired for traceability. (3) The unit provides a continuously updated display on an LCD module, showing UTC time and date, GPS fix and PLL lock status information. It also allows optional short-term display of receiving antenna latitude, longitude and height above mean sea level, plus the number of satellites in current view and their reception quality. (5) The complete reference operates from 12V DC, which can be from a battery or a mains power supply. Average current drain is approximately 340mA, while peak current drain is about 420mA. March 2007  71 A Few Facts About GPS The GPS satellite network is controlled and operated by the US Department of Defense (US DOD). Currently there are between 22 and 24 GPS satellites orbiting the Earth at a height of 20,200km, in six fixed planes angled at 55° to the equator. Each satellite orbits the Earth in 11 hours 58 minutes – ie, about twice each day. This means that at least four satellites are within “view” of a given GPS receiver at almost any time, wherever it is located (providing it has a clear sky view). The GPS satellites broadcast pseudo-random spread spectrum digital code signals on two UHF frequencies: 1575.42MHz (known as “L1”) and 1227.6MHz (“L2”). There are two different code signals broadcast: the “coarse acquisition” or C/A code, broadcast on L1 only, and the “precision” or P code broadcast on both L1 and L2. Most commercial GPS navigation receivers process only the L1 signal. Each GPS satellite carries either caesium-beam or rubidium vapour “atomic clock” oscillators, or a combination of both. These are “steered” from US DOD ground stations and are referenced back to How Accurate Is It? What kind of frequency accuracy can you get from this DIY GPS reference? Well, the 10MHz output is accurate to within 0.2Hz, averaged over a 30-second period. It’s even more accurate when averaged over a longer period, such as 30 minutes or an hour. The accuracy of the 1MHz output is the same in relative terms, since it’s derived from the 10MHz output by frequency division. So it’s quite reasonable to describe the nominal frequency accuracy as within two parts in 108 – considerably better than a free running crystal oscillator, and good enough for most frequency calibration purposes. Coordinated Universal Time (UTC), as maintained by the US Naval Observatory (USNO) – itself kept within 100ns of UTC as maintained by the US NIST. This ensures they provide an accurate reference for both the carrier frequencies and the code signals from each satellite. Although the GPS network was designed mainly for accurate terrestrial navigation, the high frequency and time accuracy of the signals from the satellites has made them very useful as a reference source for frequency and time calibration. at very close to 42°C (315K) – within about ±1°, in fact. The exact temperature can be adjusted over a small range using trimpot VR1. RS-232C data The RS-232C data from the GPS receiver module emerges on the yellow lead and is connected (via the IDC line socket) to pin 2 of CON7. From there, it’s fed through inverting buffer IC14e to the RB1 input (pin 7) of PIC microcontroller IC1, which is used to process the serial data. Similarly, the RS-232C serial input for the GPS receiver module is its blue lead and this goes to pin 1 of CON7. As a result, initialisation commands from the micro’s serial output (RB2, pin 8) are fed to the module via inverting buffer IC14d. The phase error pulse from IC7 is also fed to the RB3 input (pin 9) of IC1, so that the micro is able to monitor whether or not the PLL is maintaining lock. Display circuit The rear panel carries BNC sockets for the antenna and for the GPS 1Hz and phase error pulse outputs, plus an RCA socket for the phase error voltage. It also provides access to the DC power socket. 72  Silicon Chip The display circuit (Fig.3) interfaces to the main circuit via connector CON9 and includes the 2-line x 16-character LCD module – which is directly driven by microcontroller IC1 – plus its contrast control VR2. In addition, there are the four control switches (S1-S4) plus three status indicator LEDs (LED1-LED3), in turn driven by transistor switches Q2-Q4. Note that Q2 & Q3 (and thus LED1 & LED2) are controlled by the micro itself (via RA1 & RA2), whereas Q4 (LED3) is siliconchip.com.au GPS Frequency Reference: Parts List 1 ABS instrument case, 158 x 155 x 65mm 1 Garmin GPS 15L GPS receiver module 1 external active GPS antenna to suit – see text 1 PC board, code 04103071, 142 x 123mm 1 PC board, code 04103072, 144 x 58mm 1 16x2 LCD display module, Jaycar QP-5516 or QP-5515 2 T0-220 heatsink, PC-mount (Jaycar HH-8516) 3 SPST PC-mount snap-action pushbutton switches (black) 1 SPST PC-mount mini pushbutton switch 2 10MHz quartz crystals, HC-49U package 4 PC-mount BNC sockets (CON1CON4) 1 PC-mount 2.5mm concentric DC socket (CON5) 2 16-pin IDC line sockets 2 PC-mount 16-pin IDC header plugs (CON6, CON9) 1 10-pin IDC line socket 1 PC-mount 10-pin IDC header plug (CON7) 1 PC-mount RCA socket (CON8) 1 Panel-mount BNC-BNC malemale adapter 2 8-pin IC sockets, machined clip type 4 14-pin IC sockets, machined clip type 5 16-pin IC sockets, machined clip type 1 18-pin IC socket, machined clip type 1 20-pin IC socket, machined clip type 3 M3 x 15mm tapped spacers 2 M3 x 6mm machine screws, round head 7 M3 machine nuts 3 M2 x 25mm machine screws, round head 4 M2 x 12mm machine screws, round head 7 M2 machine nuts 7 M2 flat washers and star lockwashers 1 7x2 length of DIL pin header strip 7 1mm PC board terminal pins 1 35mm film canister, 34mm dia. x 34mm long 2 cable ties 1 5kW horizontal mini trimpot (VR1) 1 10kW horizontal mini trimpot (VR2) driven by the 1Hz pulses from the GPS module via IC11b. The microcontroller also scans the switches. As stated, S1S3 are pressed to display specialised data on the LCD, while S4 initialises the GPS receiver module. tary voltage drop to reduce the power dissipation in 3-terminal regulator REG1, which delivers a regulated +5V rail to power most of the circuitry. The only sections driven directly from the unregulated +11.4V input are comparator IC2 and heater transistor Q1 in the mini oven. Power supply Power for the circuit is derived from an external 12V DC supply (eg, a plugpack rated at 500mA or more). This is applied via power connector CON5 and diode D1 which provides reverse polarity protection. Diodes D5-D7 provide a supplemensiliconchip.com.au Semiconductors 1 PIC16F628A microcontroller programmed with GPSFrqRF.hex (IC1) 1 LM311 comparator (IC2) 2 74HC04 hex inverters (IC3,IC14) 2 74HC160 synchronous decade counters (IC4,IC5) 1 74HC73 dual flipflop (IC6) 1 74HC4046 phase comparator (IC7) 2 74HC161 synchronous 4-bit counters (IC8,IC9) 1 LM335Z temperature sensor (IC10) 1 74HC14 hex Schmitt trigger (IC11) Other signals That’s about it for the circuit description, except to note that various useful signals (in addition to the main 10MHz and 1MHz outputs) are brought out of the frequency reference to allow 1 74HC374 octal D-type flipflop (IC12) 1 LM358 dual op amp (IC13) 1 7805 +5V regulator (REG1) 1 BD136 PNP power transistor (Q1) 3 PN100 NPN transistors (Q2-Q4) 1 5mm green LED (LED1) 1 5mm red LED (LED2) 1 5mm orange/yellow LED (LED3) 4 1N4004 diodes (D1,D5-D7) 3 1N4148 signal diodes (D2-D4) 1 BB119 varicap diode (VC1) Capacitors 1 1000mF 16V RB electrolytic 4 10mF 16V RB electrolytic 1 10mF 25V tantalum 1 4.7mF 25V tantalum 11 100nF multilayer monolithic ceramic 1 2.2nF MKT metallised polyester 1 1nF MKT metallised polyester 2 100pF NPO ceramic 2 33pF NPO ceramic 1 22pF NPO ceramic 1 15pF NPO ceramic 1 4.7pF NPO ceramic 1 3-10pF N470 trimcap (white) Resistors (0.25W, 1%) 5 1MW 1 2.2kW 1 68kW 1 2kW 1 47kW 3 1kW 2 33kW 1 680W 1 22kW 3 330W 9 20kW 2 180W 10 10kW 4 100W 1 6.8kW 1 68W 1 4.7kW 1 33W 2 3.3kW its operation to be monitored. First, the very accurate 1Hz GPS pulses are brought out via IC11d and CON3. Second, an inverted version of the phase error pulse from IC7 is brought out via IC11f and CON4. And finally, the unfiltered DC error voltage from IC13a is brought out via CON8. Either of the last two signals can be used for logging the reference’s operation. That’s all we have space for this month. Next month, we’ll show you how to build it and describe the setting SC up and adjustment procedures. March 2007  73