Quectel L70 and L76 Breakout Board
I have been reviewing a range of GPSs to evaluate their performance for GPS tracker applications. The objective was to find out which GPSs had the lowest power consumption and thus would operate the longest from batteries, the detail of the review is at the link below;
The main objective of the study was to review GPSs for ground based trackers although some of the GPSs used had a high altitude balloon mode meaning that they operated above 18,000M.
The Quectel GPSs needed breakout boards so that the GPSs could be properly tested, so I designed two breakout boards one for the Quectel L70 and L76 and another for the L80 and L86. The boards are approximatly 25mm x 28mm and can be fitted either with an 8 way 0.1" pin connector on the bottom or as a plug in Mikrobus board compatible with the Mikroelectronica Click Boards;
The board schematic is;
One key point from the GPS review mentioned above is that for long term GPS tracker applications, say two week or more battery life, then access to the backup pin of the GPS is essential and this is rarely provided on a GPS breakout board. The pinout thus decided on was as below;
Pin1. VCC supply to GPS, normally 3.3V Pin2. Ground or 0V connection. Pin3. GPS RX data, commands sent to GPS. Pin4. GPX TX data, serial NMEA data from GPS. Pin5. VBKP. Voltage supply for GPS backup. Pin6. GPSPOWER. A logic low turns GPS power on, high for off. Pin7. PPS output from GPS, indicates fix status and provides time sync Pin8. RESET. Holds GPS in reset, not normally used.
A smaller GPS breakout board (28mm x 17mm) is planned primarily for high altitude balloon use, that board will use the layout above, omitting pin 8.
The breakout board can also be fitted with 2 x 0.1" 2 pin header and 1 x 0.1" 3 pin headers so the board can be used as a Mikrobus board, the default layout of a Mikrobus board is this;
The matching connections for the L70/L76 breakout as a Mikrobus board are;
AN Not used RST RESET. Holds GPS in reset, not normally used. CS GPSPOWER. A logic low turns GPS power on, high for off. SCK Not used SDO Not used SDI Not used 3V3 VCC to GPS GND Ground or 0V connection GND Not used 5V Not used SDA Not used SCL Not used RX GPS RX data, commands sent to GPS. TX GPX TX data, serial NMEA data from GPS. INT PPS output from GPS, indicates fix status and provides time sync PWM Not used
For the Mikrobus plug in format these connections are the MB1,MB2 and MB3 0.1" pin headers shown on the schematic and PCB layouts above. The pin headers can be seem in use in the picture below (L80 breakout board). When used as a Mikrobus board there is no access to the VBKP Voltage supply for the GPS backup.
Quectel L70 and L76 GPS
These GPS can be fitted with a choice of antennas, a simple wire of 1/4 wavelength at 1575Mhz, 4.85cm, there are holes to add 4 x 1/4 wavelength radials if you need an improved antenna.
The GPS breakout board can also be used by attaching different sizes of stick on ceramic patch antenna to the opposite side of the PCB to the GPS, there are various sizes of ceramic patch available, from 25mm square, 13mm square to 20mm x 6mm;
The L70 in particular proved to be a very low power GPS, even with a simple wire antenna when a tracker was programmed to use the GPS to send a location fix every 10 minutes and sleep in the gaps the signal performance and power consumption were;
- With simple horizontal wire antenna.
- Fix time from cold, 32 seconds.
- Current whilst acquiring fix, 14mA to 15mA
- Current after fix acquired, 15mA to 22mA.
- Total Tracker Power in 24 hours, 10 minute fixes, 17.6mAhr
- GPS Power in 24 hours, 10 minute fixes, 15.4mAhr
- Tracker battery life AA Alkalines 159 days
Signal Reception L70 - Wire Antenna
Hot Fix Performance L70
Quectel L70 - 25mm Ceramic Patch Antenna
- With 25mm ceramic patch on PCB rear.
- Fix time from cold, 30 seconds.
- Current whilst acquiring fix, 14mA to 15mA
- Current after fix acquired, 11.2 to 14.6mA
- Total Tracker Power in 24 hours, 10 minute fixes, 15.76mAhr
- GPS Power in 24 hours, 10 minute fixes, 13.56mAhr
- Tracker battery life AA Alkalines 178 days
Signal Reception L70 - 25mm Ceramic Patch Antenna
Hot Fix Performance L70 - 25mm Ceramic Patch Antenna
Quectel L76 - 13mm Ceramic Patch Antenna
- With 13mm ceramic patch on PCB rear.
- Fix time from cold, xx seconds.
- Current whilst acquiring fix, xxmA to 15mA
- Current after fix acquired, xx to xx
- Total Tracker Power in 24 hours, 10 minute fixes, 27.76mAhr
- GPS Power in 24 hours, 10 minute fixes, 25.56mAhr
- Tracker battery life AA Alkalines 101 days
Signal Reception L76 - 13mm Ceramic Patch Antenna
Quectel L76 with Wire versus 13mm Ceramic patch antenna
The Quectel L76 has a built in LNA and so should cope well with a simple wire antenna or a small ceramic patch antenna. The fix performance with a wire antenna was tested earlier, but below is the signal performance of the GPS first with a wire antenna (left) and a small 13mm ceramic patch antenna. The ceramic patch antennas are stick on an easy to fix in place on the rear side to the GPS. The Wire antenna provides a bit more signal gain but at 4.75cm long may not be suitable in some applications. The ceramic patch is much more compact yet still give adequate performance.
Breakout Board Build Options
You can use the breakout board with just the GPS and an antenna fitted with the 8 way pin header providing the connections. For this simple layout you will need to short link LK2, a solder bridge is enough. it would be a good idea to fit C1 and C6 as well.
GPS on and off
Fit C1, R1, R4 and Q1. Do not short link LK2 leave it open. With this option applying a logic 0 (LOW) to the GPSPOWER will turn the GPS on, applying a logic 1 (HIGH) or leaving the pin float will turn the GPS off.
GPS Backup with Logic pin
A backup supply is needed for the GPS VBCPK pin. A backup supply of at least 1.5V is needed to allow the GPS to backup the information on satellites that allows the hot fix to operate. If the GPS on and off option is in use then the backup pin just needs a supply of around 6uA, this can be a wired connection to VCC (3.3V) or a logic pin set to 1 (HIGH). Fit C4 and C2.
GPS Backup with battery or super capacitor
The backup power needed when using the GPS on and off option can be provided by fitting a super capacitor or small lithium battery. The supper capacitor is a DM53R3224 and the Lithium battery would be a MS621FE Seiko Lithium battery.
####Super Capacitor The DM53R3224 super capacitor is 0.22F and should have enough capacity to provide a backup voltage to a L70 for around 12 hours. However do remember that when used in hot fix mode the battery will not receive much in the way of charge (the GPS is not on very long) so do not expect extended hot fix operation. Fit C2, C4 and R2.
To keep the inrush current low (7mA) a 470R resistor was selected for R2, this will mean the capacitor will take 250 seconds to charge, see the link below for an example of a charge time calculator;
####Lithium battery The MS621FE has a charge current of 100uA max so at 3V input the value of R2 should be about 27K. Note that with a capacity of 5.5mA and a charge current of 100uA it will take a couple of days for the battery to charge. Fit C2, C4 and R2.
If you want a visual check of when the GPS has a fix fit R4 and L1. I would normally fit a RED LED.