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By JIM ROWE Deluxe GPS 1pps timebase for frequency counters Were you interested in the precision GPS timebase featured in the February 2013 issue? That was the “no frills” version. Here we present a Deluxe GPS 1pps Timebase which also suits our recently described 12-Digit Frequency Counter. It not only provides the same near-atomic-clock-accuracy 1pps pulses for the counter’s timebase but also extracts the NMEA 0183 data stream from the GPS satellites for processing on your PC. M EASUREMENT ACCURACY is the prime reason for building either the original no-frills version or this new Deluxe GPS 1pps Timebase. Either of them represents the simplest and most economical way to match the accuracy to the resolution of the 12-Digit High-Resolution Frequency Counter described in the December 2012 and January 2013 issues of SILICON CHIP. By using a GPS 1pps timebase, the counter can achieve a measurement accuracy approaching ±1 part in 1011. That’s up in atomic clock territory. Our February no-frills design comprised little more than a cheap GPS receiver module with the all-important 1pps output, plus a handful of components to provide the module with 60  Silicon Chip power and to buffer the output pulses. Despite its simplicity, this first GPS 1pps Timebase works extremely well. But while it was under development, we also had the intention of describing this deluxe version which would also have the NMEA 0183 stream of navigation data. This data is provided by virtually all low-cost GPS modules, along with the 1pps pulses but separated from them. So that’s the basis of new Deluxe GPS 1pps Timebase described here. The NMEA data is fed out to a USB socket and it’s relatively easy to analyse this data stream and extract the current UTC (Universal Time Co-ordinated) and date, along with such things as longitude, latitude, altitude and the number of GPS satellites in view. In addition, the PC can display the signalto-noise ratio (SNR) of the signals from the satellites and even the quality of the “fix” that the GPS module is currently able to achieve using them. This helps to confirm the accuracy and reliability of the 1pps pulses as a timebase. GPS clock driver Back in the June 2009 issue of SILICHIP, we described a GPS Clock Driver module. This took the NMEA 0183 data stream from a low-cost GPS receiver module and made it available for driving our May 2009 6-Digit GPS Clock. Alternatively, it could be fed to a PC via a “legacy” serial port. There were a number of freeware and shareware software applications available at the time which could be used CON siliconchip.com.au Par t s Lis t The parts are all installed on a small PCB which is then mounted on the lid of a UB3 jiffy box. The lid then acts as the base of the completed unit shown at left. to analyse the data stream and display much of the useful information. So one way of improving the February 2013 GPS 1pps Timebase would be to simply “bolt on” the relevant parts of the June 2009 clock driver circuit, to make the NMEA 0183 data stream from the GPS receiver module available (as well as the 1pps pulses). This would allow the GPS 1pps Timebase unit to drive the May 2009 clock or the serial port of a PC, as well as the timebase of the 12-Digit Frequency Counter. The problem with this approach is that most of today’s PCs don’t provide an RS232 serial port; they only have USB ports. So our deluxe unit features a USB port as well as an RS232 port, so it can be connected to a wide range of computers and laptops. This makes it easy to monitor the receiver’s “fix” status by running a freeware application called GPS Diagnostics 1.05 (there are many others but we have found this one to be excellent). As shown in the accompanying photos, the Deluxe GPS 1pps timebase is housed in a small plastic case. It can be powered via its USB port or from the 12-digit Frequency Counter. The latter approach is appropriate when you are not using your computer to monitor the GPS signal status. Circuit details Fig.1 shows the full circuit details of the Deluxe GPS 1pps Timebase. It’s still fairly simple but again that’s because all the complex circuitry needed to receive the signals from the GPS satellites and derive both the 1pps (1Hz) pulses and the NMEA 0183 data siliconchip.com.au stream from them is buried deep inside the GPS receiver module. We are again specifying either of two low-cost receiver modules which are currently available from various suppliers: the GlobalSat EM-406A module which is available for as little as $39.90, or the Fastrax UP501 module which is physically smaller but priced at $59.90. The project is also compatible with various other receiver modules, if you find the EM-406A or the UP501 hard to get. The type of GPS receiver module required is one that incorporates its own ceramic “patch” antenna for the UHF signals from the GPS satellites, while also providing an output for the 1pps (pulse per second) time pulses. It can operate from a DC supply of either 5.0V or 3.3V. A few currently available modules are listed in a panel elsewhere in this article. The EM-406A has its own built-on GPS patch antenna and operates directly from 5V DC. It features the SiRF Star III high-performance GPS chip set, very high sensitivity and a relatively fast time to first fix (from a cold start). The UP501 and other compatible GPS modules operate from 3.3V DC, so we have made provision for fitting a 3.3V LDO (low drop-out) regulator (REG1) to provide this lower voltage for modules that need it. In this case, we are using an LP2950-3.3 regulator, which comes in a TO-92 package. Apart from the power supply arrangements, there is a 40106B hex CMOS Schmitt inverter (IC1), used for buffering both the 1pps timebase pulses for the counter and the NMEA 1 UB3 jiffy box, 130 x 68 x 44mm 1 PCB, code 04104131, 121 x 57mm 1 GPS receiver module with in-built patch antenna & 1pps output 4 3-pin SIL pin headers (LK1LK4) 4 jumper shunts to match 1 12MHz crystal, HC-49US (X1) 1 5-pin DIN socket, PCB-mount (CON1) 1 DB9F socket, PCB-mount (CON2) 1 USB type B socket, PCBmount (CON3) 1 14-pin DIL IC socket 4 M3 x 10mm tapped metal spacers 4 self-adhesive rubber feet 8 M3 x 6mm machine screws 25 x 25mm double-sided adhesive foam (to secure GPS module) Semiconductors 1 40106B hex Schmitt inverter (IC1) 1 MCP2200 USB2.0 to serial converter (IC2) 1 LP2950-3.3 LDO regulator (REG1*) 1 NX2301P P-channel Mosfet (Q1) 1 2N7002 N-channel Mosfet (Q2) 1 3mm green LED (LED1) 1 3mm red LED (LED2) Capacitors 2 10µF 16V RB electrolytic 1 470nF MMC 2 100nF MMC or MKT 1 33pF NP0 ceramic 1 15pF NP0 ceramic Resistors (0.25W 1%) 1 47kΩ 3 470Ω 1 10kΩ 1 22Ω 1 1kΩ *Only required if you are using a GPS module which requires a 3.3V supply 0183 data stream. IC1c is the buffer for the NMEA data, with its output going to pin 2 of CON2. The other five inverters in IC1 are used for the 1pps pulse buffer and as a level translator, with IC1a used as an optional inverter to restore pulse polarity if necessary. As shown, IC1b, IC1d, IC1e & IC1f are connected in parallel and drive pin 3 of CON1, which goes to the counter’s external timebase input. April 2013  61 5V LK1 REG1 LP2950-3.3* 3.3V OUT GND GLOBALSAT EM406A GPS RECEIVER MODULE Vin Rx Tx GND 1PPS GND 10 F 1 +5V IN IN 10 F 100nF 3 14 5 4 5 6 NMEA 0183 OUT CON2 IC1: 40106B 1PPS POLARITY LK2 IC1b 1 2 IC1a LK3 FROM COUNTER 4 8 Tx GND Vin B/UV 1PPS 10 470 2 3 Q1 NX2301P +5V IN 3 Tx LED 4 LED2 5 A A K 6 Rx LED  LED1  6 5 13 12 LK4 10 11 2 X1 12MHz 33pF 47k MMC 1k 4 470 7 * ONLY REQUIRED FOR GPS RECEIVER MODULES REQUIRING 3.3V. 15pF 3 1 VDD RST D G USB TYPE B GP5 D– RxLED/GP6 D+ 19 TxLED/GP7 CTS RX TX RTS OSC1 IC2 MCP2200 GP4 GP3 GP2 GP1/USBCFG GP0/SSPND VUSB OSC2 CON3 18 2 3 1 4 8 9 14 D 15 22 16 G Q2 2N7002 S 17 Vss 20 470nF MMC LEDS NX2301P, 2N7002 D DELUXE GPS 1PPS TIMEBASE S 100nF K IC1 PIN1 2013 5 1PPS OUT 1 470 SC  TO COUNTER 2 7 FASTRAX UP501 GPS RECEIVER MODULE 1 CON1 IC1e 11 5 12 IC1d 9 +3.3V 4 IC1f 13 ALTERNATIVES Rx 2 IC1c 6 3 (CERAMIC PATCH ANTENNA) DB9F SOCKET 10k 2 FROM USB (CERAMIC PATCH ANTENNA) G S LP2950-3.3 GND K A IN OUT Fig.1: the circuit consists of the GPS receiver module plus a hex CMOS Schmitt trigger inverter to buffer the 1pps (1Hz) pulses and NMEA data from the module. The NMEA data is also fed to IC2 which drives the USB serial port. As with the no-frills circuit, link LK2 is used to allow the 1pps pulses to be either inverted or not by the buffer, so that their leading edges are positivegoing regardless of their polarity out of the GPS module (some modules may output them as inverted). Basically, we need to ensure that the leading edges of the 1pps pulses fed to the 12-Digit Frequency Counter are positive-going. That’s because it’s the leading edges of the pulses that are locked closely to the “atomic time” provided by the GPS satellites. 62  Silicon Chip The remaining circuitry in Fig.1 is used to provide the USB serial port. Here we are using a Microchip MCP­ 2200, a dedicated USB2.0-to-UART Protocol Converter device. It appears to be similar to a PIC18F14K50 microcontroller chip but is “hard wired” to perform USB/serial and serial/USB conversion, so that when it’s linked to the USB port of a PC it behaves as a “virtual COM port device”. As a result, Windows will communicate with the MCP2200 via a virtual COM port (VCP) driver. In addition, Microchip has a freeware “Configuration Utility” program which can be used to configure the MCP2200 in terms of baud rate, data format and so on. We will describe this in greater detail later. The MCP2200 (IC2) needs a 12MHz crystal (X1) for its clock oscillator. This crystal is connected between pins 2 & 3, along with two small NP0 ceramic capacitors. It also needs a 470nF MMC bypass capacitor connected between its VUSB pin (pin 17) and ground, together with a 100nF MMC capacitor siliconchip.com.au siliconchip.com.au 1PPS LED2 X1 12MHz 10 LK2 1PPS OUT GND CTR Q1 NX2301P 47k 100nF 2 5 11 1 20 Q2 2N7002 4 1k 33pF 15pF Rx IC1 40106B 2 Tx NC 470 3 GND A CON2 5 100nF LED1 22 A 470 LK4 TX 1PPS 4 DB9F POLARITY MCP2200 IC2 RX CON1 LK3 +V 470 1 GPS/USB EMIT BSTIME U/SP32G RECEIVER REVIECER 4 13140140 5 04104131 (PATCH ANT) 6 102 C C 32013 + USB +5V +3.3V LK1 10k GLOBALSAT EM-406A GPS RX MODULE 10 F REG1 470nF + +5V IN LP2950-3.3 10 F 1 bypassing the +5V rail from the PC’s USB port (ie, pin 1 of CON3). The D- and D+ data lines from CON3 connect directly to pins 18 & 19 of IC2, while the NMEA data stream from the GPS receiver module is fed directly to pin 12 of IC2. IC2 converts this data stream into USB packets for transmission to the PC via CON3. NMEA commands are also sent back from the PC via the USB cable and these emerge from pin 10 of IC2. These can be fed back to the Rx input of the GPS receiver module when link LK4 is used to complete the circuit. In this application, we don’t need to send any commands to the GPS receiver module – we simply use its default operating configuration. However, we found that when this connection was made in addition to the main Tx-to-Rx connection to pin 12 of IC2, there could be a conflict whereby IC2 could prevent the GPS receiver module from finding a “fix”. In addition, the GPS receiver could prevent IC2 from configuring and enumerating correctly. So it seems best to leave LK4 in the “open” position, as shown in Fig.1 (and Fig.2). LED1 (receive) & LED2 (transmit) are driven from pins 6 & 5 of IC2. These LEDs flash when data is passing through IC2 in one direction or the other. The remaining part of the circuit involves Mosfets Q1 & Q2, which are used to allow IC2 to control the +5V power fed from USB socket CON3 to link LK3 (this link is used to select the power source for the GPS receiver module and IC1). This is done to conform to the USB 2.0 requirement that current drain from the PC’s USB port drops to less than 2.5mA when the PC’s USB host controller holds the device in “suspended” mode. IC2’s SSPND-bar output (pin 16) is connected to Q2’s gate via a 22Ω suppressor resistor, so that Q2 is only turned on when IC2 receives a “wake up from suspension” directive. Then when Q2 turns on, it turns on Q1 which makes the connection between pin 1 of CON3 and LK3. So if LK3 is in the power “From USB” position, (rather than “From Counter” position), the GPS receiver module will only receive power when (a) the project is connected to a USB port on a PC; (b) the PC is powered up; and (c) software is running on the PC and “listening” to the GPS data stream, so that IC2 is not CON3 USB TYPE B Fig.2: follow this layout diagram to build the unit. Omit REG1 and the 10μF capacitor to its left if you are using the Globalsat EM-406A module and install LK1 in the +5V position. Alternatively, install REG1 and the capacitor if your GPS module requires a 3.3V supply and fit LK1 to the +3.3V position. in suspended mode. Note that the GPS receiver module can take over a minute to get a “fix” after power is applied. Alternatively, be fitting LK3 to the “From Counter” position, the upper part of the circuit can be powered from either the counter or an external plugpack supply (via CON1). This means that you don’t have to connect the unit to a PC in order to simply derive 1pps pulses. Building it All the parts for the Deluxe GPS 1pps Timebase fit on a PCB coded 04104131 and measuring 122 x 57mm. Fig.2 shows the PCB parts layout diagram, while Fig.3 shows the pin connections for the GlobalSat EM406A and Fastrax UP501 GPS receiver modules. Note that almost half of the PCB is reserved for mounting the GPS module itself, which is held in place using double-sided adhesive foam. Begin by fitting SMD components IC2, Q1 & Q2 to the PCB, as it is much easier to do this before any other parts are fitted. Take the usual precautions when soldering these parts, ie, use an earthed soldering iron with a finetipped bit. Tack-solder one or two device leads first, so that the device is held in position while you solder the rest of the leads. You then re-solder the original tacked leads to ensure reliable joints. Don’t worry if you accidentally bridge two or more SMD device leads with solder during this procedure. These bridges can subsequently be removed quite easily by pressing solder wick braid against the bridged leads using the tip of your soldering iron. This sucks up the excess solder while leaving the solder joining the leads to the PCB pads underneath in place. Once the SMD parts have been installed, add the SIL pin headers for links LK1-LK4, followed by the resistors, capacitors and the 12MHz crystal. April 2013  63 1 Vin (+5V) 3 SERIAL Rx 4 (PATCH ANTENNA AT TOP) GND 2 SERIAL Tx 5 GND 6 1PPS OUT (PATCH ANTENNA AT TOP) 6 5 4 3 2 1 BACKUP V+ +3.3V GND SERIAL Tx SERIAL Rx FIX LED GLOBALSAT EM-406A 1PPS OUT FASTRAX UP501 Fig.3: the pin connections for the GlobalSat EM-406A and Fastrax UP501 GPS modules. Check the pin connections if you use a different module. A 14-pin socket for IC1 can then be fitted – make sure it’s orientated as shown. Connectors CON1-CON3 can then go in, followed by LED1 & LED2. The latter are mounted vertically above the PCB, with their leads left at full length so that they later protrude through their matching holes in the case (see Fig.4). Voltage regulator option Regulator REG1 and the 10µF electrolytic capacitor to its left are installed only if the GPS receiver module you are using requires a 3.3V DC supply rather than a 5V supply. This means that if you are using the EM-406A module, you won’t need to fit REG1 or that 10µF capacitor. By contrast, the regulator and the capacitor must be installed if you are using the UP501 receiver module, since this runs off 3.3V. The same goes for the Digilent PmodGPS and RF Solutions GPS-622R GPS modules. The GPS receiver module is installed last but before doing this, you need to make the connections between its output pads (or lead wires) and the relevant pads on the PCB (ie, just to the left of LK4). Fig.3 shows the outputs for the Globalsat EM-406A and Fastrax UP501 modules. Be sure to connect these to their matching pads on the PCB. The EM-406A module comes with a short 6-wire ribbon cable fitted with a sub-miniature 6-pin plug at each end. One of these plugs connects directly to the EM-406A’s output socket. The plug at the other end of the cable is cut off and the six wires stripped and tinned before soldering them to their PCB pads. By contrast, the UP-501 module just has a row of pads along one edge of its PCB. It’s connected by first cutting six 25mm-lengths of light-duty hookup wire (eg, from a ribbon cable), then carefully stripping and tinning all the wire ends before soldering the leads into place. Don’t forget to match the output leads from the GPS module to the PCB pads (see Figs.2 & 3), as the connec- Compatible GPS Receiver Modules The following GPS receiver modules should be compatible with this project • GlobalSat EM-406A: 30 x 30 x 10.5mm including patch antenna. Operates from 5V DC with a current drain of 44mA. Provides a 1pps output and a “fix” indicator LED. Rated sensitivity -159dBm. • Digilent PmodGPS: approximately 30 x 55 x 12mm including patch antenna. Operates from 3.3V DC with a current drain of 24/30mA. Provides a 1pps output and a “fix” indicator LED. Rated sensitivity -165dBm. • RF Solutions GPS-622R: 43 x 31 x 6mm including patch antenna. Operates from 3.3V DC with a current drain of 23/50mA. Provides a 1pps output and a “fix” indicator LED. Rated sensitivity -148dBm/-165dBm. • Fastrax UP501: 22 x 22 x 8mm including patch antenna. Operates from 3.3V DC with a current drain of 23mA. Provides a 1pps output. Rated sensitivity -165dBm. Note that for use in this project, the GPS receiver module should have a built-in ceramic patch antenna and also provide an output for the GPS-derived 1pps pulses. Not all GPS modules currently available provide both of these features. 64  Silicon Chip tions are not “straight through”. Once all the connections have been made, the GPS receiver module can be secured to the top of the PCB using a 25mm-square piece of double-sided adhesive foam – see Fig.4. Make sure you attach the module with its patch antenna facing upwards – it won’t work very well if it faces downwards! Fitting the links LK1’s shunt position depends on the supply voltage (5V or 3.3V) required for the GPS receiver module you’re using, while LK2’s position depends on the polarity of the 1pps output pulses from the GPS receiver. In most cases, LK2 will need to be to the lower position (ie, nearest Q1). LK3’s position depends on just how you plan to power the GPS receiver module and IC1 (ie, the 1pps timebase section of the circuit). If you only intend using this part of the circuit when the unit is connected to a PC via a USB cable, then LK3 can be fitted in the USB (lefthand) position (ie, the circuit is powered from the PC’s USB port). Alternatively, if you want to use this part of the circuit continuously (eg, whenever the 12-Digit Frequency Counter is on but without having to fire up the PC), you’ll need to fit LK3 in the righthand CTR (From Counter) position and power the unit either from the counter or an external 5V plugpack via CON1. Finally, LK4 should almost always be fitted to the upper position, to break the connection between pin 10 of IC2 and the Rx input of the GPS module. Preparing the box Fig.4 shows how the PCB assembly is fitted inside a standard UB-3 jiffy box. The completed unit can be mounted near a window to get a good “view” of the sky. As shown, the PCB is mounted on the lid of the box, which then becomes the base. The main part of the box then fits down over the lid/board assembly, to act as a dust cover. Fig.5 shows the drilling details for the box. Four mounting mounting holes have to be drilled in the lid to accept the PCB, while two holes must be drilled through the top of the main box section for the LEDs. In addition, you have to drill a hole in the rear side of the box and make cut-outs in the front side and righthand end. Use a small (eg, 1.5mm) pilot drill siliconchip.com.au HOLE FOR ACCESS TO CON1 (UB-3 JIFFY BOX) LED2 HOLE FOR ACCESS TO CON2 LED1 DOUBLE-SIDED ADHESIVE FOAM ATTACHING MODULE TO PCB EM-406A GPS Rx MODULE IC2 15p RECEIVER PCB CON3 LK2 LK4 Fig.4 here’s how the PCB assembly is fitted inside a standard UB-3 jiffy box. Be sure to install links LK1LK4 correctly (see text) before securing the top section of the case to the lid. The completed assembly should be mounted near a window to give the GPS module a good “view” of the available GPS satellites. CON2 IC1 M3 x 10mm TAPPED SPACERS UB-3 BOX LID M3 x 6mm SCREWS BOX ASSEMBLY SCREWS to start all the holes, then drill the 3mm holes out to the correct size. The hole in the rear side of the box can be enlarged to the correct size (16mm) using a tapered reamer. The two square cut-outs can be made by drilling a series of small holes around the inside perimeter, then knocking out the centre piece and carefully filing the inside edges. If you are using a GPS receiver module with a “fix” indicator LED, you might want to drill an additional hole in the adjacent side of the box, so that you can view this LED to confirm that the receiver does indeed have a fix. The prototype shown in the photos uses an EM-406A module, which does have such a LED in the lower righthand corner – see Fig.3. That’s the reason for the 5mm hole you can see in the front of the box, located 45mm from the lefthand end and 20mm up from the outer surface of the lid. The UP501 module doesn’t have a “fix” LED, so there’s no need to drill this hole. However, many other modules do have this LED and the hole location will depend on the LED’s location on your particular module. Once the box holes have been drilled, the PCB assembly can be mounted on the lid on four M3 x 10mm tapped spacers and secured using M3 x 6mm machine screws. That done, check that you’ve fitted the jumper shunts to each of the four SIL pin headers (for LK1-LK4) as required (see above). The box can then be lowsiliconchip.com.au Other Uses For This Project The NMEA output of this Deluxe GPS 1pps Timebase can be used with a range of navigation software and free Windows GPS-related software packages. • For nautical chart and navigation software that works with NMEA-compatible GPS units see: http://capcode.sourceforge.net/ • To show your position on Google Maps as you move (multiple options) see: http://mboffin.com/earthbridge/ http://download.cnet.com/Google-Maps-with-GPS-Tracker/3000-12940_410494227.html?tag=keyword.feed&part=rss&subj=dl.gps http://blog.geoblogspot.com/2008/09/navigator-101.html • For a GPS data logger: https://github.com/javarobots/GpsDataLogger • Many more here: http://www.maps-gps-info.com/fgpfw.html#Windows ered down onto the lid, taking care to ensure that LED1 and LED2 protrude through their respective holes at the top, and the assembly secured by fitting the four supplied self-tapping screws. Finally, fit four small adhesive rubber feet to the lid (which now becomes the base) to prevent scratches due to the protruding screw heads. Your Deluxe GPS 1pps Timebase is now complete. Counter connections As with the simpler GPS 1pps Timebase unit, only three connections have to be run to the 12-Digit Frequency Counter. These can all be made via a shielded stereo cable fitted with a 5-pin DIN plug which plugs into CON1 of the Deluxe GPS Timebase. Fig.6 shows the wiring details. One of the inner conductors of the stereo cable connects to pin 3 of the 5-pin DIN plug, to carry the 1pps output pulses, while the other inner conductor connects to pin 1 of the DIN plug, to carry the +5V supply rail for the timebase. The shield braids are both connected to pin 2 of the plug, to link the two grounds. At the other end of this cable, the 1pps signal lead and its shield braid should be fitted with a BNC plug, to connect to the counter’s external timebase input (CON3). The +5V/GND power lead can either be connected to a 5V DC plugpack or fitted with a 2.5mm concentric DC plug which mates with a matching DC power socket added to the rear of the frequency counter. In the latter case, you will also have to connect the +5V and ground lines inside the counter to the added DC April 2013  65 16mm DIAMETER 12.5 13 22 12 11 13 18.25 23.5 18.25 31 RIGHT-HAND END OF UB3 BOX RH END OF BOX FRONT SIDE 31 RH END OF BOX REAR SIDE RIGHT-HAND END 3mm DIAMETER HOLES CL 64 11 OUTSIDE OF UB3 BOX 4 x 3.0mm DIAMETER HOLES 49.5 97.5 INSIDE UB3 BOX LID Fig.5: the drilling details for the UB3 jiffy box. The rectangular cutouts can each be made by drilling a series of small holes around the inside perimeter, then knocking out the centre piece and filing to shape. power socket – see Fig.6. Make sure that LK3 on the timebase PCB is in the CTR (righthand) position if you are powering the timebase section (ie, the GPS module and IC1) from the counter 66  Silicon Chip or an external plugpack. Alternatively, if you intend running the entire unit exclusively from USB power, then you don’t need to install this separate supply cable. Instead, it’s simply a matter of connecting the Deluxe GPS 1pps Timebase to a USB port on a PC (or a downstream USB hub) using a standard USB cable. Don’t forget to set LK3 to the USB position siliconchip.com.au IC17 74AC74 IC13 74AC00 4518B IC7 74HC00 IC18 IC11 4012B IC12 74AC10 4518B IC9 4093B 100nF 100nF GROUND IC15 74AC00 1MHz TP2 74AC163 VC1 6-30pF 8.00MHz X1 TMR1 IN +5V SUPPLY PIC16F877A 100nF D7 GND +5V POWER IC23 CON4 9-12V DC IN 5819 1pps PULSES 100nF 1s 100s 74HC373 100nF 10s IC22 39pF IC24 100nF 74HC161 27pF 2.5mm PLUG 100nF SEL CHAN A FREQ*/PRD 74HC244 EXT/INT TB IC19 SEL CHAN B 74HC244 1000s Fig.6: a shielded stereo cable can be used to make the connections between the Deluxe 1pps GPS Timebase and the frequency counter. The 1pps pulses are fed in via the counter’s existing BNC socket on the rear panel, while a 2.5mm DC power socket can be added to accept a matching plug to pick up the counter’s +5V and GND connections. You can omit this DC socket and the supply connections if you don’t intend powering the timebase unit from the counter. TPG TP4 HIGH NORESOLUTION ITULOSER HGIH COUNTER RETNUOC MAIN C 2012 DRBOARD AOB NIAM tob0411 121111121 140top 2102 C TP1 TPG 4060B IC6 X2 32768Hz TP5 TPG IC8 4093B 220k 10M 39pF 6-30pF D6 VC2 1k 1PPS PULSES CON3 EXT TB IN IC10 100nF 100nF IC14 IC16 74HC160 BNC PLUG 100nF ADDED 2.5mm POWER SOCKET 100nF 100nF CENTRE PIN 100nF 100nF CRIMP SLEEVE 100nF 100nF siliconchip.com.au 100nF When you first connect the unit to a PC, Windows will respond by installing its standard “virtual COM port” driver. Once it’s done that, launch the Device Manager (eg, via Control Panel) and look under “Printers and Devices” to make sure that you now have a “USB serial port”. You can then also check its Properties to discover the COM port number and check that it’s working properly. You can also set the driver’s baud rate to match the GPS module’s rate, which is usually 4800bps. Assuming this checks out so far, the next step is to download and install Microchip’s custom MCP2200 Configuration Utility, available from: ww1.microchip.com/downloads/en/ DeviceDoc/MCP2200_Configuration_ Utility_v1.3.zip (5.13MB). Unzipping this provides a self-installing version of the MCP2200 Configuration Utility. When you run this and then fire it up, you should see a dialog window as shown in Fig.7 – although you won’t see any text as yet in the “Output” box. This box will be blank initially, while some of the smaller boxes will have different contents. Before clicking on the “Configure” button at lower left, you’ll need to ensure that the contents of all of the smaller boxes are as shown in Fig.7. You probably won’t need to change the contents of the Manufacturer, Product, Vendor ID or Product ID boxes, nor will you need to click on the “Update VID/PID” button. However, you may need to click on the check box next to “Enable TX/RX LEDs”, to display the tick as shown. Similarly you may need to click on the check box next to “Enable Suspend Pin”, to display its tick. If the “Baud Rate” box is not showing “4800”, click on the down arrow to its right and then select “4800” from the drop-down list. Then, if the “I/O Config” box is showing something other than “00000000”, click inside the box so that you can type in the correct “00000000” text string. Similarly, if the “Output Default” box is not showing “11111111”, enter in that text string yourself. Now turn your attention to the “LED Function” section at lower right and click on the “Blink LEDs” radio button if this isn’t already selected (ie, dis- 4148 Configuration D5 4148 22k if powering the entire unit from a USB port. April 2013  67 Fig.7: this is the dialog you will see when you launch Microchip’s MCP2200 Configuration Utility (except that the Output box will be blank). Configure it as described in the text. playing the central bullet). Similarly, click on the “200ms” radio button so that it too is selected. At this stage you should be seeing a display very much like that shown in Fig.7, except that the “Output” window should be blank. If so, you can now click on the “Configure” button at lower left. There should then be a brief pause while the configuration utility “does its thing” with the MCP2200 chip in your Deluxe GPS 1pps Timebase, then the text shown in Fig.7 should appear in the “Output” window. This indicates that the configuration routine has been completed and that the unit is now communicating with the the PC via the USB cable. Once it’s done that, you can then close the Configuration Utility. Installing the PC software The final step is to install a software application to allow your PC to analyse and display the useful information carried in the NMEA 0183 output data stream. There are many software apps capable of doing this but one that we particularly recommend is called “GPS Diagnostics V1.05”. Developed by CommLinx Solutions, this freeware program can be downloaded from download.cnet.com/windows The quickest way to get to the 68  Silicon Chip Fig.8: the GPS Diagnostics dialog displays a range of information from the analysed NMEA data, including UTC time, longitude, latitude, altitude, the number of satellites in “view” and the signal strength from each one. download page is to search for it by typing its full name in the search box at top right. Downloading the software is a 2step process. First, you have to download the customised installer program cbsidlm-tr1_10a-GPSDiag-ORG10055902.exe (620kB). You then run this installer to download and install the GPS Diagnostics program itself. Once it’s installed, launch the program to bring up a dialog window much like that shown in Fig.8. The only differences are that all of the text boxes and bargraphs will initially be blank – including the large box at the bottom labelled “Received data”. Earlier, when you first plugged the USB cable from the GPS Time Receiver into the PC’s USB port, Windows in- stalled it as a USB Serial COM port. The allocated port number could then be determined by going to Device Manager and checking under Ports (COM & LPT). Usually, this will be COM3, COM4 or COM5. Once you’ve determined the allocated port number, the next step is to select the corresponding port number in the GPS Diagnostics window. That’s dome by selecting the appropriate radio button at upper left. This tells the program which COM port the incoming NMEA 0183 data stream from the Deluxe GPS 1pps Timebase will be on (in our case, it’s COM5). Analysing NMEA data You should now find that GPS Diagnostics starts displaying all the siliconchip.com.au This photo demonstrates the accuracy of the counter when using the Deluxe GPS 1pps Timebase. Here we’re measuring a GPSderived 10MHz frequency and the counter shows 10MHz exactly. information coming into the PC via that COM port. You’ll see the NMEA sentences as they arrive in the large Received Data window at the bottom and within a few seconds, you’ll also see the UTC time and date, the longitude and latitude, the altitude of your GPS receiver module and a great deal of other interesting information (see upper right of Fig.8). It will also show the number of GPS satellites currently in “view”, plus a bar chart for each one indicating the approximate signal strength. Under each chart, you’ll also see its PRN number, its current elevation and azimuth, its signal-to-noise ratio (SNR) and whether or not it’s currently being used. For example, when the screen grab of Fig.8 was captured, our prototype Deluxe GPS 1pps Timebase was able to view and use the signals from no fewer than 12 satellites. That’s a bit unusual though. Most of the time, it will use anywhere between five and nine satellites, while at odd times there may be only three or four in view and usable. So how do you verify that the unit has a good “fix” and is delivering usable GPS-locked 1pps pulses to your 12-Digit Frequency Counter? That’s done in GPS Diagnostics by examining the “Mode” message box. This shows “Auto 3D” in Fig.8, which means that it was able to achieve the highest level of fix when this screen grab was captured. When you get this message, you can be satisfied that your counter is getting the best possible 1pps pulses. When the GPS receiver is able to see only a small number of satellites (eg, two or three), the Mode box dis- play can drop back to “Manual 2D”. This still indicates that the receiver has achieved a “fix”, although some of the navigation information won’t be of high quality. However, the 1pps pulses being fed to the counter should still be OK. It’s only time to worry if the Mode message box is blank or showing “No fix”, since that indicates that the unit will probably not be delivering any 1pps pulses at all. If that happens, the trick is to try moving the unit to a location where it can “view” more of the sky and therefore “see” more satellites so that it can get a good fix. In short, GPS Diagnostics is an excellent tool for optimising the position of your Deluxe GPS 1pps Timebase. It also allows you to then monitor the reception conditions on a day-to-day SC basis. Frequency Counter Measurement Accuracy I N THE FIRST article describing our 12-Digit Frequency Counter (SILICON CHIP, December 2012), we advised readers that by using a GPS-based external 1pps timebase, it should be possible to achieve measurement accuracy approaching that of an atomic clock. In the specifications panel, we also quoted measurement accuracy with a GPS 1Hz timebase of approximately ±1 part in 1011. Subsequent testing has quantified the accuracy that can be achieved. Over the last three months, Jim has made measurements using the set-up shown above, with the 12-Digit Frequency Counter fed with an external timebase (using the simpler February 2013 unit for the first five weeks and the deluxe unit described here for the remaining seven weeks). The counter was siliconchip.com.au measuring the 10MHz output from our GPS-based Frequency Reference (SILICON CHIP, March-May 2007) and was set for a gating time of 1000 seconds, so that each measurement took 16.66 minutes. This was done to provide the highest measurement resolution. The results from this extended testing are: the GPS-locked 10MHz signal from the 10MHz Frequency Reference gave readings of 10,000,000.000 ± 0.003Hz – with a roughly Gaussian or “bell shaped” distribution centred on 10,000,000.000Hz. In other words, a measurement accuracy of ±3 parts in 1010 can be achieved. Note that with this measurement set-up there are three sources of measurement jitter: (1) the GPS module in the 1pps timebase(s); (2) the GPS module in the GPS-Based 10MHz Frequency Reference and (3) the inevitable jitter in the PLL (phase-locked loop) inside the 10MHz Frequency Reference itself (used to lock the 10MHz output to the GPS 1pps pulses). Clearly it isn’t easy to separate these three sources of jitter, but with all three present they still allowed us to achieve a measurement accuracy of ±3 parts in 1010. So the true measurement accuracy of the 12-digit frequency counter with the GPS 1Hz timebase is somewhere between ±3 parts in 1010 and ±1 part in 1011 – still very impressive. Unless you are measuring an atomic frequency reference, your measurement accuracy is like to be far in excess of the drift and jitter of any source that is commonly available. April 2013  69